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PROFESSOR: OK,
let's get started.

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Everyone can go ahead and
take 10 more seconds on

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today's clicker question.
 

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This is about hybridization,
which we will go over more to

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start today, but just to see
where we are as a

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starting point.
 

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All right, great.
 

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So most of you got this.
 

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The question is what was the
hybridization of an atom

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if we know that it has
two hybrid orbitals.

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This is really, really
important, so I'm just going to

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write this out for that 20% of
you that didn't see

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this right away.
 

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So we know that when we have
hybrid orbitals that come from

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s and p orbitals, these are our
atomic orbitals we have to

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choose from to hybridize.
 

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So if we know, for example,
that we need two hybrid

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orbitals, that means we
needed to have started

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with two atomic orbitals.
 

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So that means what we would
form are two s p hybrid

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orbitals and we'd be left
with our other two p

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orbitals left over.
 

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So any time you figure out that
you're going to need two hybrid

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orbitals, and we'll go over
when you would find this out in

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just a minute, but any time
that happens you know it has to

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come from an s and a p, so
that's why you'll end up with

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s p hybrid orbitals there.
 

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And exam 2 is happening in
just one week from today,

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so next Wednesday.
 

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The semester moves really fast.
 

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So I can't believe we're
already up to exam

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2 talk, but we are.
 

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So, you should have all
received two handouts when you

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walked in today, your notes and
then an exam 2 info sheet.

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Did everybody get that?
 

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If you didn't get it, just
raise your hand and a TA

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will come to you with that.
 

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I see there's a few around,
if one of the TA's can

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jump up and fill those in.
 

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All right, so basically,
as with exam 1, all the

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information you need to know
about what's going to be on

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exam 2 is on this handout.
 

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Specifically exam 2 is going to
cover lectures 10 through 17.

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Today is lecture 16, so that
means it's going to cover

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today's lecture and then a bit
into Friday's lecture as well,

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so I'll be really clear on
Friday where exam 2 material

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ends, where you can kind
of switch off the exam 2

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part of your studying.
 

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And it will also be on
problem-sets 4 and 5.

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So it's going to be on two
problem-sets instead of three

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problem-sets, but as you know,
there are a lot of different

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concepts covered in
these two problem-sets.

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So it's still a lot to be
thinking about and you need to

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make sure that you really
understand all of these

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different concepts.
 

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So this is explicitly listed
in this sheet here, so make

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sure you look it over.
 

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But I just want to say is well,
the concepts that we've covered

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since exam 1 have been talking
about covalent bonds, going

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along with that are all the
Lewis structures

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that we've done.
 

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Then we talked about ionic
bonds, so you need to make

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sure you can solve those
ionic bond problems.

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And then we started talking
about different types of

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bonding in terms of thinking
about MO theory, and also

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vesper theory and
hybridization.

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And then today, and a little
bit on Friday, you'll be

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responsible for some of this
new material, which is

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going to be a little
bit of thermochemistry.

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Not too much on exam 2, most of
that will be saved for exam 3

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material, but an intro to
thermochemistry, that will also

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be on the exam, just as far
as these two lectures.

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So, basically this is all
written out here, and I also

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want to mention in terms of
your MO diagrams and MO theory,

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I sent out an email about this,
but I also wrote it out here so

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you have it written down in
terms of what you are

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responsible for in terms of
ordering the energy levels

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of molecular orbitals.
 

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So that's all written down
explicitly right here, this

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is what you need to
know for the exam.

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Also it's helpful for the
problem-set as well.

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And also, in terms of MO
diagrams, I think that you will

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already gone over this pretty
explicitly either last week in

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recitation or yesterday, in
terms of what you need to do to

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get full credit when you're
actually drawing an MO diagram.

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Just in case you don't have
that information or you didn't

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write it down, I wrote this
down for you here as well in

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terms of things like making
sure to draw your energy

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arrows, or remembering to label
all of your atomic orbitals.

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We don't want anyone getting
points off on the exam for

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doing something silly where
they do understand what's going

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on, but they're just not
writing it in the correct form.

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So all this information
is on your sheet here.

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And also on this sheet
mentions that you will be

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getting optional extra
problems on Friday.

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This is just like for exam 1.
 

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The way that you should
approach these extra problems,

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you don't need to turn them in,
you're not required to do them,

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but if you want to do well on
the exam we really, really

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encourage that you
do these problems.

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And in terms of having it be
worth your time when you're

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solving these problems, what
you want to do is make sure

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you're at the point where you
can put your notes away, where

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you can sit alone without your
friends, and really approach

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these extra problems as if
they're exam problems.

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This way if there's some
conceptual leap that you

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haven't quite made yet, but you
don't realize it because you've

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been using your notes or using
the conversations with your

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friends, you want to make that
leap and run into those

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problems alone in your room
when you have two hours

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to figure it out.
 

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You don't want to do
this leap on an exam.

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That's not the ideal situation.
 

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So especially with Lewis
structures where sometimes you

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can run into confusions that
you need to work through and

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make sure it all makes sense in
terms of your head how to do

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this, make sure you take the
time to do that before the exam

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and don't have to end up
struggling with that on the

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exam in a time situation.
 

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All right, so the extra
problem solutions will

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be posted on Sunday.
 

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And in terms of office hours
for next week, my office hours

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are going to be on Monday from
2 to 4, I'm moving them up, and

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your TAs will move up their
office hours as well, so check

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with them in recitation
or on the website.

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So that's pretty
much it for exam 2.

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Are there any questions about
what you're responsible for or

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anything regarding exam 2?
 

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OK, and in terms of the
room it's in Walker again,

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so at least this part
should be familiar.

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You'll be used to going to your
row, it'll be the same row,

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it's all written here again,
but you can just do exactly

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what you did for exam 1.
 

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All right, so one last class
announcement, and this is that

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if you've been looking at our
first sheet of the whole year,

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sort of the course overview
sheet, you know that our class

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in terms of grading is out of
750 points, and 50 of those

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points are for attendance and
something called occasional

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in-class quizzes.
 

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So what we're going to do is
start filling in some of those

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points because we only have 36
classes worth of attendance.

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So we have a couple of points
to make up in terms of quizzes,

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and these are going to be
clicker quizzes, and let

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me explain how this
is going to work.

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Basically, in at least for my
part of the class, what we're

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going to do, because what I
really care about is that

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people are answering the
clicker questions, it's

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really valuable for me.
 

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I don't necessarily care if you
get them right or wrong during

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class, because this is the
point at which you're

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still learning them.
 

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So these quiz points, which
could be up to 2 points per

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class, will be in addition to
attendance points, and you'll

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get full credit if you
answer the quiz question.

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So you need to be answering the
questions because the quizzes

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will not be announced.
 

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So this is another reward
for people that are

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always participating.
 

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These clicker questions are
really valuable to us in terms

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of the feedback, it really
gives me an idea of where

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the class is as a whole.
 

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So really, it's important that
you are answering these

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questions and this is a way to
get your quiz points up, so in

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addition to attendance,
you'll get 2 quiz points.

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We'll have our first
unannounced quiz today, so you

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won't know what problem it is
beforehand, so I suggest you

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make sure you're answering all
the clicker questions, which

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most of you do anyway.
 

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And I will just say I recognize
that some of you are just

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scurrying across campus
to get here on time.

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So I'll never make that first
question a quiz question,

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because I know how hard it is
to get from one place to

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another on this campus, and I
know most of you do a great job

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of sneaking in right at the
last minute when you

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have that long run.
 

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So we won't make the first
question a quiz question,

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but be on the look out for
a quiz question today.

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All right.
 

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So let's get to today's topic.
 

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We're going to finish talking
about valence bond theory

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and hybridization.
 

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So what we've already done is
really cover all the theory

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that we're going to cover
behind how hybridization

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works, and behind how
valence bond theory works.

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But what we haven't covered
is how do we actually solve

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problems doing this in
the really quick way.

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So we're going to do that.
 

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This is going to be a
more practical lesson

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in hybridization.
 

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And once we do that, we'll
move on to now talking about

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energies and enthalpies
of chemical reactions.

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So this is a real shift.
 

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We spent a while talking about
single atoms, and then we've

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spent most of the material,
since exam 1, talking

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about bonding.
 

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So now we're taking it one step
further and we're going to

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actually get to talk about
some chemical reactions.

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So we'll do that once
we finish up with our

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hybridization unit here.
 

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All right, so how do we go
about determining hybridization

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in complex molecules?
 

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It's actually incredibly
simple, and all that you need

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to do is think about for the
given atom that you're looking

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at, what you want to do is add
up the number of atoms that are

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bonded to your central atom, or
the atom you're considering,

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and add to that the
number of lone pairs.

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And what you end up with
is the number of hybrid

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orbitals that you need.
 

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All right, so the theory behind
hybridization and picturing

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everything, that can be pretty
complicated, and I do want

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you to understand that.
 

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But if, for example, on an
exam situation, you don't

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necessarily have to think
through everything, you can

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just use this very quick way to
figure out the number of hybrid

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orbitals that you need.
 

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And what we just said is that
any time we need two hybrid

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orbitals, we know that the
hybridization of that atom is

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going to be s p, because
it's made up of one

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s and one p orbital.
 

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So let's say we need
three hybrid orbitals.

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What is our hybridization
of our atom in this case?

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Yeah, it's s p 2.
 

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We want to take three atomic
orbitals to make three

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hybrid orbitals, so
it's going to be s p 2.

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So what if we need four
hybrid orbitals, what's

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the hybridization going
to be in this case?

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STUDENT: S p 3.
 

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PROFESSOR: S p 3.
 

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All right, great.
 

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So, really that's it.
 

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Hopefully hybridization just
got a lot simpler for any of

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you that have been struggling
a little bit with it.

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But let's go ahead and do
an example to make sure

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that we can all do this.
 

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And let's take a look --
first let me mention

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an exception here.
 

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The one exception to thinking
about how many hybrid orbitals

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that you have are in the
case where you have single

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bonded terminal atoms.
 

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So in the case of a terminal
atom that is a single bond,

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you're not going
to hybridize it.

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So in that case, just don't
even change anything it's just

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going to be one of the p
orbitals, or unless you're

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talking about hydrogen in
which case will be an s

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orbital that overlaps.
 

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All right, so we can illustrate
this with an example

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of formal chloride.
 

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So this is a good example
because we're just dealing with

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our carbon as our central atom
here, and we have three

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terminal atoms, two of which
are single bonded, so we're not

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going to hybridize those, and
one which is double bonded,

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the oxygen, so we will
hybridize that one.

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So in terms of thinking about
the carbon, what is a

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hybridization around
the carbon atom?

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STUDENT: [INAUDIBLE]
 

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PROFESSOR: All right, I'm
hearing some mixed answers.

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So let's think.
 

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So the carbon atom is bonded
to three different atoms,

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and it has no lone pairs.
 

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So what's the hybridization?
 

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STUDENT: S p 2.
 

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PROFESSOR: Good s p 2.
 

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00:11:47 --> 00:11:50
So, if we're talking about the
c h bond, we're going to say

272
00:11:50 --> 00:11:54
that it's carbon, it's a sigma
bond because it's a single

273
00:11:54 --> 00:11:57
bond, and then it's carbon
2 s p 2 bonded to a

274
00:11:57 --> 00:11:59
hydrogen 1 s orbital.
 

275
00:11:59 --> 00:12:02
All right, so let's take
a look at the carbon

276
00:12:02 --> 00:12:03
chlorine bond here.
 

277
00:12:03 --> 00:12:09
Again, carbon is still 2 s p 2,
it's the same carbon atom, and

278
00:12:09 --> 00:12:12
it's going to be a sigma bond
because it's a single bond, but

279
00:12:12 --> 00:12:14
what about the chlorine, what
atomic orbital is going to

280
00:12:14 --> 00:12:15
be bonding in the chlorine?
 

281
00:12:15 --> 00:12:16
STUDENT: [INAUDIBLE]
 

282
00:12:16 --> 00:12:17
PROFESSOR: All right.
 

283
00:12:17 --> 00:12:21
I'm hearing a little bit of
mixes, they're all p's,

284
00:12:21 --> 00:12:22
which is a good start.
 

285
00:12:22 --> 00:12:26
It turns out that it's going
to be the chlorine 3 p z.

286
00:12:26 --> 00:12:29
So the reason that it's p is
because we're not hybridizing

287
00:12:29 --> 00:12:32
it, and the reason that I
specifically say that it's the

288
00:12:32 --> 00:12:37
p z instead of the p x or the p
y, is remember that the z-axis,

289
00:12:37 --> 00:12:41
that's our bonding axis,
that's our internuclear axis.

290
00:12:41 --> 00:12:44
So any time we have a p orbital
that's involved in a sigma

291
00:12:44 --> 00:12:47
bond, it has to be the z
orbital because that's the only

292
00:12:47 --> 00:12:50
one that has the right
orientation to overlap

293
00:12:50 --> 00:12:52
along that z-axis.
 

294
00:12:52 --> 00:12:55
So we're going to say
it's carbon s p 2, and

295
00:12:55 --> 00:12:57
then chlorine 3 p z.
 

296
00:12:57 --> 00:13:02
All right, so let's look at the
last bond here, which is a

297
00:13:02 --> 00:13:04
carbon oxygen double bond.
 

298
00:13:04 --> 00:13:08
We know that any time we have a
double bond it's made up of one

299
00:13:08 --> 00:13:10
sigma bond plus one pi bond.
 

300
00:13:10 --> 00:13:14
So, when we look at the carbon
again, that's 2 s p 2.

301
00:13:14 --> 00:13:17
But what about the oxygen,
what's the hybridization

302
00:13:17 --> 00:13:20
of this oxygen atom?
 

303
00:13:20 --> 00:13:22
S p 2.
 

304
00:13:22 --> 00:13:28
So we're going to have carbon 2
s p 2, and then oxygen 2 s p 2.

305
00:13:28 --> 00:13:32
It's s p 2 because the oxygen
is bonded to one atom plus two

306
00:13:32 --> 00:13:34
lone pairs, so we're going to
have a total of three

307
00:13:34 --> 00:13:35
hybrid orbitals.
 

308
00:13:35 --> 00:13:38
All right, so this is not our
only bond, we have a double

309
00:13:38 --> 00:13:40
bond, so we also need to
talk about the pi bond.

310
00:13:40 --> 00:13:44
So if we talk about the pi
bond, we can say that's carbon

311
00:13:44 --> 00:13:47
2 p y, then the oxygen 2 p y.
 

312
00:13:47 --> 00:13:49
We could alternatively say,
and be correct, that it

313
00:13:49 --> 00:13:52
was the carbon 2 p x
and the oxygen 2 p x.

314
00:13:52 --> 00:13:54
Either one of those is fine.
 

315
00:13:54 --> 00:13:58
All right, so that's an example
of how we can assign very

316
00:13:58 --> 00:14:00
quickly and very easily what
the hybridization is

317
00:14:00 --> 00:14:02
of a given atom.
 

318
00:14:02 --> 00:14:05
And then also fully describe
the symmetry and the

319
00:14:05 --> 00:14:08
atomic or hybrid orbitals
that make up bonds.

320
00:14:08 --> 00:14:10
So this is a lot of your
problems on the p-set, so if

321
00:14:10 --> 00:14:12
you haven't finished that
section, hopefully you'll be

322
00:14:12 --> 00:14:14
able to get through that
section pretty quickly

323
00:14:14 --> 00:14:16
when you go back to it.
 

324
00:14:16 --> 00:14:19
And let's take a look at a
little bit of a more complex

325
00:14:19 --> 00:14:23
molecule here, which is
ascorbic acid or vitamin C.

326
00:14:23 --> 00:14:25
And this is a good example
because it's starting to look a

327
00:14:25 --> 00:14:28
little more complicated, but
not too much more complicated.

328
00:14:28 --> 00:14:32
But the reality is if you know
how to do assigning the

329
00:14:32 --> 00:14:35
hybridization this way, even if
I gave you a 1,000 atom

330
00:14:35 --> 00:14:37
protein, you should
still be able to get it

331
00:14:37 --> 00:14:39
completely correct.
 

332
00:14:39 --> 00:14:42
Vitamin C is a really important
molecule to actually think

333
00:14:42 --> 00:14:44
about in terms of thinking
about its shape and

334
00:14:44 --> 00:14:45
its hybridization.
 

335
00:14:45 --> 00:14:48
Vitamin C is an antoxidant.
 

336
00:14:48 --> 00:14:50
So we might remember that
from when we were talking

337
00:14:50 --> 00:14:52
about free radicals.
 

338
00:14:52 --> 00:14:54
Free radicals can cause
oxidative damage.

339
00:14:54 --> 00:14:57
These antioxidants, some of our
vitamins, including vitamin

340
00:14:57 --> 00:14:59
C are antioxidants.
 

341
00:14:59 --> 00:15:01
So that's something that a lot
of people in the general

342
00:15:01 --> 00:15:04
population know if they're
into their health.

343
00:15:04 --> 00:15:07
But another really important
thing about ascorbic acid is

344
00:15:07 --> 00:15:10
that it's an enzyme cofactor.
 

345
00:15:10 --> 00:15:15
And a cofactor just means that
it's some type of molecule or

346
00:15:15 --> 00:15:18
atom, in this case it's a
molecule, that's required by an

347
00:15:18 --> 00:15:21
enzyme in order for the enzyme
to carry out its chemistry.

348
00:15:21 --> 00:15:24
And the chemistry that the
enzymes that vitamin C are a

349
00:15:24 --> 00:15:29
cofactor for are responsible
for putting oxygen, o h groups,

350
00:15:29 --> 00:15:32
onto collagen molecules in
order to form what is the

351
00:15:32 --> 00:15:35
collagen triple helix.
 

352
00:15:35 --> 00:15:38
And collagen is the main
constituent of bones, of

353
00:15:38 --> 00:15:44
joints, of connective tissue,
of many important structural

354
00:15:44 --> 00:15:45
parts of our bodies.
 

355
00:15:45 --> 00:15:48
So you can imagine that vitamin
C is very important because we

356
00:15:48 --> 00:15:49
need to keep our
collagen intact.

357
00:15:49 --> 00:15:52
Collogen is actually one of the
most prevalent proteins in our

358
00:15:52 --> 00:15:56
entire body, and it makes up
a large part of our cells.

359
00:15:56 --> 00:15:59
So you can imagine if we don't
have collagen that is in this

360
00:15:59 --> 00:16:02
triple helix, we're going
to run into some problems.

361
00:16:02 --> 00:16:05
Does anyone, or I'm sure many
of you do know, but can someone

362
00:16:05 --> 00:16:07
tell me what the name is of
the disease if you have a

363
00:16:07 --> 00:16:09
deficiency in vitamin C?
 

364
00:16:09 --> 00:16:10
STUDENT: Scurvy.
 

365
00:16:10 --> 00:16:12
PROFESSOR: Yeah, scurvy.
 

366
00:16:12 --> 00:16:17
So scurvy, most often
associated with ships because

367
00:16:17 --> 00:16:23
of the 1500's and 1600's and
1700's when boating technology

368
00:16:23 --> 00:16:27
was far above biochemistry
technology, so people could

369
00:16:27 --> 00:16:30
make ships and go on these long
journeys, but people did not

370
00:16:30 --> 00:16:33
yet understand they needed to
keep certain vitamins in their

371
00:16:33 --> 00:16:35
bodies, such as vitamin C.
 

372
00:16:35 --> 00:16:37
Maybe also, they didn't know
about -- well they didn't even

373
00:16:37 --> 00:16:39
have the structure, so they
couldn't have realized that

374
00:16:39 --> 00:16:42
vitamin C is polar and won't
stay in their bodies for very

375
00:16:42 --> 00:16:45
long, it will get excreted
out through their urine.

376
00:16:45 --> 00:16:48
So they were not thinking about
how quickly they could run

377
00:16:48 --> 00:16:50
into trouble in the
long sea voyages.

378
00:16:50 --> 00:16:54
And I don't know how many of
you had a unit on explorers in

379
00:16:54 --> 00:16:57
elementary school -- I did in
fifth grade, and one that

380
00:16:57 --> 00:17:00
always sticks in my head is
Magellan's trip around the

381
00:17:00 --> 00:17:03
world, who led the first fleet
of ships around the globe.

382
00:17:03 --> 00:17:07
What they did not mention to us
in fifth grade was that 90% of

383
00:17:07 --> 00:17:10
his men died on that journey.
 

384
00:17:10 --> 00:17:14
And I understand we were young
kids, I didn't want to know

385
00:17:14 --> 00:17:16
that at the time, but it's
interesting and it's really

386
00:17:16 --> 00:17:18
important, and the reason
for many of these

387
00:17:18 --> 00:17:19
deaths was scurvy.
 

388
00:17:19 --> 00:17:22
And it's totally preventable.
 

389
00:17:22 --> 00:17:25
And, in fact, the cure for
scurvy, which is as simple as

390
00:17:25 --> 00:17:29
taking vitamin C in any form,
was known for -- since the

391
00:17:29 --> 00:17:32
fifth century people started
figuring this out, but the

392
00:17:32 --> 00:17:36
problem was there was so much
false information as well that

393
00:17:36 --> 00:17:39
it was not a general practice
on ships to be treating men

394
00:17:39 --> 00:17:42
that were suffering with
vitamin C or using it as any

395
00:17:42 --> 00:17:43
kind of preventative measure.
 

396
00:17:43 --> 00:17:45
The other somewhat interesting
thing I want to say about

397
00:17:45 --> 00:17:48
vitamin C is that it was
part of the first ever

398
00:17:48 --> 00:17:50
published clinical study.
 

399
00:17:50 --> 00:17:53
And this was a controlled
clinical study that was done in

400
00:17:53 --> 00:17:56
the 1700's, the first one
that's ever been reported, and

401
00:17:56 --> 00:18:00
not surprisingly it was done on
a ship and it was done by James

402
00:18:00 --> 00:18:02
Lind who was a Scottish
naval surgeon.

403
00:18:02 --> 00:18:05
And basically, he took 12 of
his men, so this was not a very

404
00:18:05 --> 00:18:08
large clinical study like they
do today, but he took 12 men

405
00:18:08 --> 00:18:12
that were suffering from
scurvy, and he gave them a diet

406
00:18:12 --> 00:18:15
mostly of just carbohydrate
mush, because when you have

407
00:18:15 --> 00:18:18
scurvy, one of the first signs
is bleeding gums and teeth

408
00:18:18 --> 00:18:20
falling out, they couldn't
really eat too much.

409
00:18:20 --> 00:18:22
But he supplemented their diet.
 

410
00:18:22 --> 00:18:25
He put them in pairs of
two and supplemented

411
00:18:25 --> 00:18:26
with 1 of 6 things.
 

412
00:18:26 --> 00:18:30
So two of them got a
pint of cider a day to

413
00:18:30 --> 00:18:31
see if that helped.
 

414
00:18:31 --> 00:18:34
Two of them got half a pint
of seawater -- maybe not

415
00:18:34 --> 00:18:36
the best group to be in.
 

416
00:18:36 --> 00:18:38
Even worse group to be
in is two of them got

417
00:18:38 --> 00:18:40
diluted sulfuric acid.
 

418
00:18:40 --> 00:18:43
Tuned out that didn't
help so well.

419
00:18:43 --> 00:18:47
Other things where some
got spices, not so bad.

420
00:18:47 --> 00:18:50
And then two of them
actually got citrus fruit.

421
00:18:50 --> 00:18:54
So no big surprise ending to
this clinical study, the people

422
00:18:54 --> 00:18:57
with citrus fruit were
completely cured and back to

423
00:18:57 --> 00:19:00
duty within a week, and I'm not
sure exactly what happened to

424
00:19:00 --> 00:19:05
everyone else, but let's hope
they got switched over as well.

425
00:19:05 --> 00:19:07
But it's just neat to think
about even back then, people

426
00:19:07 --> 00:19:10
were doing these controlled
scientific studies, this was

427
00:19:10 --> 00:19:12
one of the first cases.
 

428
00:19:12 --> 00:19:16
Unfortunately, it was another
40 years between this study

429
00:19:16 --> 00:19:19
being published and the British
Navy requiring supplements

430
00:19:19 --> 00:19:20
of citric acid.
 

431
00:19:20 --> 00:19:24
So, really, we've come a long
way in terms of disseminating

432
00:19:24 --> 00:19:26
scientific knowledge to the
community, and that's a really

433
00:19:26 --> 00:19:29
important part of being
scientists -- not just making

434
00:19:29 --> 00:19:32
these discoveries, but also
passing along, for example, to

435
00:19:32 --> 00:19:36
these ships full of people
that need to know it.

436
00:19:36 --> 00:19:39
So, in terms of scurvy, we
don't see too much of it today,

437
00:19:39 --> 00:19:43
but who should be concerned
about suffering from scurvy?

438
00:19:43 --> 00:19:46
Do we have to worry about our
cats or dogs at home, are they

439
00:19:46 --> 00:19:49
getting enough of their
vegetables and vitamin C?

440
00:19:49 --> 00:19:51
It turns out we don't
have to worry.

441
00:19:51 --> 00:19:54
Primates are who
should be concerned.

442
00:19:54 --> 00:19:56
So we need to be concerned,
other primates need

443
00:19:56 --> 00:19:57
to be concerned.
 

444
00:19:57 --> 00:20:01
It turns out that most other
mammals actually biosynthesize

445
00:20:01 --> 00:20:03
vitamin C, so we don't have to
worry about most of

446
00:20:03 --> 00:20:07
our pets at home.
 

447
00:20:07 --> 00:20:10
Unless you have a guinea pig
and they don't, in fact,

448
00:20:10 --> 00:20:13
biosynthesize vitamin C, so I
think they supplement most

449
00:20:13 --> 00:20:16
guinea pig pellets with
vitamin C, but maybe keep an

450
00:20:16 --> 00:20:18
eye on your guinea pig.
 

451
00:20:18 --> 00:20:21
We primates and guinea pigs are
the only types of mammals that

452
00:20:21 --> 00:20:24
don't actually biosynthesize
their vitamin C.

453
00:20:24 --> 00:20:27
So again, vitamin C is a
cofactor, that means it needs

454
00:20:27 --> 00:20:31
to bind to an enzyme, it means
that the shape of vitamin C and

455
00:20:31 --> 00:20:34
the hybridization are going
to be really important.

456
00:20:34 --> 00:20:37
So let's take a look and
talk about some of these

457
00:20:37 --> 00:20:38
carbon atoms here.
 

458
00:20:38 --> 00:20:41
And you can look at your
structure of vitamin C in your

459
00:20:41 --> 00:20:44
class notes while you're doing
this, and first I want you to

460
00:20:44 --> 00:20:48
tell me what the hybridization
is of that carbon a in

461
00:20:48 --> 00:20:59
the vitamin C molecule.
 

462
00:20:59 --> 00:21:14
And let's take 10 more
seconds on that.

463
00:21:14 --> 00:21:16
OK, 78%.
 

464
00:21:16 --> 00:21:18
We'll do another question
like this in a minute.

465
00:21:18 --> 00:21:21
So if we can switch back
to the notes we'll

466
00:21:21 --> 00:21:23
take a look at that.
 

467
00:21:23 --> 00:21:25
I think we can get better, I
think we can get to the 90's

468
00:21:25 --> 00:21:28
with these hybridization, we
just need to follow the rules.

469
00:21:28 --> 00:21:30
So if we're thinking about
vitamin C and we're talking

470
00:21:30 --> 00:21:34
about carbon a, how many
things is carbon a bonded to?

471
00:21:34 --> 00:21:36
STUDENT: [INAUDIBLE]
 

472
00:21:36 --> 00:21:36
PROFESSOR: four things.
 

473
00:21:36 --> 00:21:39
We have two hydrogen atoms,
an oxygen, and then

474
00:21:39 --> 00:21:40
another carbon.
 

475
00:21:40 --> 00:21:44
So that means we need, if it's
bonded to four things, we need

476
00:21:44 --> 00:21:47
four different hybrid orbitals,
so it needs to be

477
00:21:47 --> 00:21:51
s p 3 hybridized.
 

478
00:21:51 --> 00:21:54
All right, let's try this again
but just shouting out, what is

479
00:21:54 --> 00:21:57
the hybridization of carbon b?
 

480
00:21:57 --> 00:21:59
Good.
 

481
00:21:59 --> 00:22:01
OK, I'm going to say that was
about 90% of you I heard,

482
00:22:01 --> 00:22:03
which is excellent.
 

483
00:22:03 --> 00:22:04
S p 3.
 

484
00:22:04 --> 00:22:06
What about carbon c here?
 

485
00:22:06 --> 00:22:11
Yup, s p 3 again.
 

486
00:22:11 --> 00:22:14
Bonded to four
things it's s p 3.

487
00:22:14 --> 00:22:17
What about carbon d?
 

488
00:22:17 --> 00:22:20
S p 2, great.
 

489
00:22:20 --> 00:22:21
Carbon e?
 

490
00:22:21 --> 00:22:24
STUDENT: S p 2.
 

491
00:22:24 --> 00:22:25
PROFESSOR: All right, and
this last one, the last

492
00:22:25 --> 00:22:27
10% of you can join in.
 

493
00:22:27 --> 00:22:28
Carbon f?
 

494
00:22:28 --> 00:22:31
Great, s p 2.
 

495
00:22:31 --> 00:22:36
All right, so we can also think
about shape once we know

496
00:22:36 --> 00:22:38
hybridization and how many
atoms a carbon atom

497
00:22:38 --> 00:22:39
is bonded to.
 

498
00:22:39 --> 00:22:43
What would we say that the
shape is of these s p 3

499
00:22:43 --> 00:22:47
orbitals -- or the shape
of the carbon atom.

500
00:22:47 --> 00:22:52
Yeah, it's tetrahedral.
 

501
00:22:52 --> 00:22:57
So this is going to
be tetrahedral.

502
00:22:57 --> 00:23:00
And what about
carbon d, e, and f?

503
00:23:00 --> 00:23:04
What is the geometry of
carbon d, e, and f?

504
00:23:04 --> 00:23:06
Good, trigonal planar.
 

505
00:23:06 --> 00:23:14
All right.
 

506
00:23:14 --> 00:23:17
So let's actually take this one
step further from just talking

507
00:23:17 --> 00:23:20
about the hybridization
of the individual atom.

508
00:23:20 --> 00:23:22
Let's talk about a
couple of these bonds.

509
00:23:22 --> 00:23:24
I won't go through all of them.
 

510
00:23:24 --> 00:23:27
I think you can get the idea
with just a few, but you can go

511
00:23:27 --> 00:23:30
back and try all of them later.
 

512
00:23:30 --> 00:23:34
So let's start with thinking
about carbon b hydrogen bond.

513
00:23:34 --> 00:23:38
Is this going to be a
sigma or a pi bond?

514
00:23:38 --> 00:23:38
Sigma.
 

515
00:23:38 --> 00:23:42
We know it's sigma, because
any time we have a single

516
00:23:42 --> 00:23:45
bond it's a sigma bond.
 

517
00:23:45 --> 00:23:48
So carbon b, we just
said, is carbon, it's

518
00:23:48 --> 00:23:51
going to be 2 s p 3.
 

519
00:23:51 --> 00:23:53
And then hydrogen, what's
the atomic orbital we're

520
00:23:53 --> 00:23:55
talking about here?
 

521
00:23:55 --> 00:23:57
1 s.
 

522
00:23:57 --> 00:23:58
All right, great.
 

523
00:23:58 --> 00:24:01
So let's take a look at another
one, let's look again at carbon

524
00:24:01 --> 00:24:06
b, but now let's talk about it
bonding to its oxygen there.

525
00:24:06 --> 00:24:08
So again, are we going
to be talking about a

526
00:24:08 --> 00:24:11
sigma or a pi bond?
 

527
00:24:11 --> 00:24:16
A sigma bond, because again,
it's a single bond here.

528
00:24:16 --> 00:24:21
So what we'll say again is
carbon 2 s p 3, and let's

529
00:24:21 --> 00:24:24
take a look at this oxygen
that it's bound to.

530
00:24:24 --> 00:24:27
Is this oxygen -- what
is the hybridization of

531
00:24:27 --> 00:24:30
this oxygen right here?
 

532
00:24:30 --> 00:24:32
What is it?
 

533
00:24:32 --> 00:24:35
OK, good, it's s p 3.
 

534
00:24:35 --> 00:24:38
It's bound to two atoms, plus
it has two lone pairs, that's

535
00:24:38 --> 00:24:39
a total of four things.
 

536
00:24:39 --> 00:24:42
So it needs to have three
different hybrid orbitals, so

537
00:24:42 --> 00:24:51
we'll say it's oxygen 2 s p 3.
 

538
00:24:51 --> 00:24:53
So let's skip to a different
carbon now, let's talk about

539
00:24:53 --> 00:24:58
carbon d, specifically the
carbon d oxygen bond.

540
00:24:58 --> 00:25:01
So let's do a clicker
question for this one.

541
00:25:01 --> 00:25:03
Let's get into the 90's,
if you don't mind, this

542
00:25:03 --> 00:25:06
would be very good to do.
 

543
00:25:06 --> 00:25:08
It'll make me feel better
about you finishing your

544
00:25:08 --> 00:25:10
problem-sets tonight.
 

545
00:25:10 --> 00:25:15
So if you talk about the
symmetry and the hybrid

546
00:25:15 --> 00:25:18
or atomic orbitals that
contribute, we're talking

547
00:25:18 --> 00:25:20
about the c d oxygen bond.
 

548
00:25:20 --> 00:25:22
So go ahead and look
at your notes and see

549
00:25:22 --> 00:25:24
what that bond is.
 

550
00:25:24 --> 00:25:27
We already identified the
hybridization of carbon d, so

551
00:25:27 --> 00:25:30
we really just need to think
about the oxygen here.

552
00:25:30 --> 00:26:04
All right, let's take
10 more seconds.

553
00:26:04 --> 00:26:07
OK, I'll take that, 84%.
 

554
00:26:07 --> 00:26:10
That's not bad.
 

555
00:26:10 --> 00:26:12
So again, we're looking
at a sigma bond here.

556
00:26:12 --> 00:26:15
It looks like the one people
got it confused with was

557
00:26:15 --> 00:26:18
talking about oxygen
s p 2 versus s p 3.

558
00:26:18 --> 00:26:21
Remember, you just need to look
at what it is actually bound

559
00:26:21 --> 00:26:24
to, so for carbon d, the
oxygen's bound to two atoms

560
00:26:24 --> 00:26:27
plus two lone pairs, so it's
going to be s p 3 hybridized.

561
00:26:27 --> 00:26:33
So we'll call this a sigma bond
where we have carbon 2 s p, now

562
00:26:33 --> 00:26:37
it's s p 2, and oxygen 2 s p 3.
 

563
00:26:37 --> 00:26:41
All right, let's take a look
at just one more here.

564
00:26:41 --> 00:26:45
So, let's look at the carbon
d bond with carbon e.

565
00:26:45 --> 00:26:48
So if we can switch back to the
class notes just to see that,

566
00:26:48 --> 00:26:51
though you guys can see it
on your notes as well.

567
00:26:51 --> 00:26:54
So in terms of that, we're
talking about a double bond

568
00:26:54 --> 00:26:57
now, so we know that we have
to have one sigma bond and

569
00:26:57 --> 00:27:01
one pi bond to completely
describe our double bond.

570
00:27:01 --> 00:27:03
So for the sigma bond,
again, it's going to

571
00:27:03 --> 00:27:07
be carbon 2 s p 2.
 

572
00:27:07 --> 00:27:09
And now we're talking
about the other carbon.

573
00:27:09 --> 00:27:13
What is the hybridization
of the other carbon?

574
00:27:13 --> 00:27:14
Yup, s p 2.
 

575
00:27:14 --> 00:27:17
So carbon 2 s p 2.
 

576
00:27:17 --> 00:27:19
All right, but we're
not done there.

577
00:27:19 --> 00:27:22
We also need to talk
about the pi bond.

578
00:27:22 --> 00:27:26
So for the pi bond, we'll talk
about carbon 2 p y, and then

579
00:27:26 --> 00:27:29
the second carbon 2 p y.
 

580
00:27:29 --> 00:27:34
All right, so I filled in a few
more in your notes, so you can

581
00:27:34 --> 00:27:36
maybe cover those up and make
sure that you get all of those

582
00:27:36 --> 00:27:40
correct is a good self test
before you finish off any

583
00:27:40 --> 00:27:41
in your problem-set.
 

584
00:27:41 --> 00:27:43
Does anyone have any
questions about figuring

585
00:27:43 --> 00:27:44
out hybridization?
 

586
00:27:44 --> 00:27:44
Yes?
 

587
00:27:44 --> 00:27:48
STUDENT: [INAUDIBLE]
 

588
00:27:48 --> 00:27:52
PROFESSOR: I'm sorry,
what is that?

589
00:27:52 --> 00:27:54
In this bond here?
 

590
00:27:54 --> 00:27:55
Oh, OK, that's a good question.
 

591
00:27:55 --> 00:27:57
So the question was why is
there not hybridization in

592
00:27:57 --> 00:27:59
these p orbitals here.
 

593
00:27:59 --> 00:28:03
So in order to form a double
bond, we need to have a pi bond

594
00:28:03 --> 00:28:06
forms, and a pi bond forms from
two unhybridized p orbitals.

595
00:28:06 --> 00:28:09
And if you can kind of picture
those p orbitals, if our

596
00:28:09 --> 00:28:12
molecules are this way and
they're coming up here, we need

597
00:28:12 --> 00:28:15
to have electron density above
the bond and below the bond.

598
00:28:15 --> 00:28:18
So if they're hybrid, then
they're going to be spread out

599
00:28:18 --> 00:28:20
into that tetrahedral geometry.
 

600
00:28:20 --> 00:28:23
We want to have them parallel
to each other and they need to

601
00:28:23 --> 00:28:25
be the p bonds, they need
to be the p orbitals.

602
00:28:25 --> 00:28:28
Does that makes sense?
 

603
00:28:28 --> 00:28:33
So any time we have a double
bond, the pi part of the bond

604
00:28:33 --> 00:28:34
is going to be p orbitals.
 

605
00:28:34 --> 00:28:34
Yes?
 

606
00:28:34 --> 00:28:34
STUDENT: [INAUDIBLE]
 

607
00:28:34 --> 00:28:38
PROFESSOR: No, absolutely not.
 

608
00:28:38 --> 00:28:42
It would be absolutely correct
to put 2 p x and 2 p x here,

609
00:28:42 --> 00:28:45
you just can't put z, because
that's the one that's going to

610
00:28:45 --> 00:28:46
be involved in the sigma bond.
 

611
00:28:46 --> 00:28:49
And the other important thing
is if you do put x for one, you

612
00:28:49 --> 00:28:52
have to make sure you put x in
the other, because they do need

613
00:28:52 --> 00:28:54
to be able to interact
with each other.

614
00:28:54 --> 00:28:57
You can't have one x and
one y, but sure, you can

615
00:28:57 --> 00:29:02
have both x or both y.
 

616
00:29:02 --> 00:29:05
OK, so let's shift gears a
little bit and start talking

617
00:29:05 --> 00:29:11
about bonding in terms
of chemical reactions.

618
00:29:11 --> 00:29:13
So we're going to start today
with talking about bond

619
00:29:13 --> 00:29:19
energies, and also something
called bond enthalpies.

620
00:29:19 --> 00:29:22
And the first point is just to
bring us back to something

621
00:29:22 --> 00:29:23
we're very familiar with.
 

622
00:29:23 --> 00:29:26
When we talked about covalent
bonding, a concept that we

623
00:29:26 --> 00:29:29
have been discussing is
bond dissociation energy.

624
00:29:29 --> 00:29:33
That's just the energy that's
required to put into a molecule

625
00:29:33 --> 00:29:35
in order to break that bond.
 

626
00:29:35 --> 00:29:38
This is something that we saw
when we were talking just at

627
00:29:38 --> 00:29:41
the very beginning of our
unit on covalent bonds.

628
00:29:41 --> 00:29:45
That energy difference is the
energy difference between when

629
00:29:45 --> 00:29:49
we have the c h 4, if we're
talking about methane, versus

630
00:29:49 --> 00:29:51
one of those hydrogen bonds
breaking, one of those

631
00:29:51 --> 00:29:53
c h bonds breaking.
 

632
00:29:53 --> 00:29:58
So we've talked in the past
about bond energy, but what I

633
00:29:58 --> 00:30:01
want to introduce to you today
is a very related concept,

634
00:30:01 --> 00:30:03
which is called bond enthalpy.
 

635
00:30:03 --> 00:30:08
So that's delta h here is
what we call bond enthalpy.

636
00:30:08 --> 00:30:11
So, this is talking about
instead of the change of energy

637
00:30:11 --> 00:30:14
accompanied with a bond
breaking, we're talking about a

638
00:30:14 --> 00:30:17
change in heat accompanied
with a bond breaking.

639
00:30:17 --> 00:30:20
So whether when that bond
breaks it requires heat, or

640
00:30:20 --> 00:30:23
whether it gives off heat
when you break that bond or

641
00:30:23 --> 00:30:25
talking about a reaction.
 

642
00:30:25 --> 00:30:26
That's what we're talking
about when we're

643
00:30:26 --> 00:30:28
talking about enthalpy.
 

644
00:30:28 --> 00:30:31
It turns out that enthalpy is,
in fact, very related to

645
00:30:31 --> 00:30:35
energy, and we can relate it
with this equation here that

646
00:30:35 --> 00:30:39
bond enthalpy is equal to bond
energy plus the change in

647
00:30:39 --> 00:30:41
pressure times volume.
 

648
00:30:41 --> 00:30:44
So we could, in fact, go back
and forth between bond energies

649
00:30:44 --> 00:30:48
and bond enthalpies, but the
reality is if we're talking

650
00:30:48 --> 00:30:51
about gases, which we are in
many cases, then what we find

651
00:30:51 --> 00:30:54
is the difference between bond
enthalpy and bond energy

652
00:30:54 --> 00:30:57
is only 1% to 2%.
 

653
00:30:57 --> 00:30:58
So it's not very significant.
 

654
00:30:58 --> 00:31:02
And, in fact, if we're talking
about solids or liquids, now we

655
00:31:02 --> 00:31:05
can say that the difference
between bond energy and bond

656
00:31:05 --> 00:31:07
enthalpy is going
to be negligible.

657
00:31:07 --> 00:31:11
So, for the rest of this class
today, we're going to stop

658
00:31:11 --> 00:31:13
talking about bond energy,
which I did just for a moment,

659
00:31:13 --> 00:31:16
to orient you back to a
discussion that was familiar,

660
00:31:16 --> 00:31:19
we're going to switch our
discussion to bond enthalpies.

661
00:31:19 --> 00:31:23
One reason that we like to talk
about enthalpies is unlike

662
00:31:23 --> 00:31:26
energy, which can be a little
bit more tricky, bond

663
00:31:26 --> 00:31:27
enthalpies are easy to measure.
 

664
00:31:27 --> 00:31:31
It's easy to measure how much
heat a reaction gives off

665
00:31:31 --> 00:31:35
or how much heat a
reaction takes in.

666
00:31:35 --> 00:31:38
So we can also talk about
something, which is called the

667
00:31:38 --> 00:31:42
standard bond enthalpy, so any
time you see this symbol here,

668
00:31:42 --> 00:31:45
which looks like a knot, so it
looks like a delta h knot, that

669
00:31:45 --> 00:31:48
refers to the standard
bond enthalpy.

670
00:31:48 --> 00:31:50
Any time we we're talking about
a standard, whether it's

671
00:31:50 --> 00:31:53
enthalpy or, as we'll see in
the next lecture, standard

672
00:31:53 --> 00:31:57
entropy or a standard free
energy, we're just saying that

673
00:31:57 --> 00:32:00
the molecules that we're
talking about are in their

674
00:32:00 --> 00:32:03
standard states, which means
they're in their pure form.

675
00:32:03 --> 00:32:05
If they're a gas it means
they're at one bar.

676
00:32:05 --> 00:32:08
Usually this is referring
to room temperature,

677
00:32:08 --> 00:32:11
so 298 kelvin.
 

678
00:32:11 --> 00:32:14
It's often that when you look
up tables of different bond

679
00:32:14 --> 00:32:17
enthalpies, you'll see them in
their standard state and it

680
00:32:17 --> 00:32:18
will be at room temperature.
 

681
00:32:18 --> 00:32:22
So if we talk about standard
bond enthalpy, let's talk about

682
00:32:22 --> 00:32:24
what this is for these c h
bonds, which we just

683
00:32:24 --> 00:32:26
saw in methane.
 

684
00:32:26 --> 00:32:29
So if we're going from c h 4
to breaking one of these c h

685
00:32:29 --> 00:32:32
bonds, what we see is that the
bond enthalpy is 438

686
00:32:32 --> 00:32:33
kilojoules per mole.
 

687
00:32:33 --> 00:32:38
The fact that it's positive
tells us that we need to put in

688
00:32:38 --> 00:32:42
that much heat into the system
in order to break that bond.

689
00:32:42 --> 00:32:45
So we now know the enthalpy of
a c h bond, but, of course,

690
00:32:45 --> 00:32:48
methane is not the only kind of
c h bond that you can envision.

691
00:32:48 --> 00:32:52
We could talk about the c h
bond, for example, in ethane or

692
00:32:52 --> 00:32:57
trifluoromethane or trichloro
or tribromomethane, and what

693
00:32:57 --> 00:33:00
you'll find is that the amount
of enthalpy that that reaction

694
00:33:00 --> 00:33:03
is associated with depends on
the exact type of c h

695
00:33:03 --> 00:33:05
bond that you have.
 

696
00:33:05 --> 00:33:07
So, for example, we can see
that we have slightly

697
00:33:07 --> 00:33:09
different, not vastly
different, but slightly

698
00:33:09 --> 00:33:13
different changes in enthalpy
depending on which kind of c h

699
00:33:13 --> 00:33:17
bond that were breaking.
 

700
00:33:17 --> 00:33:19
And what I want to point out
also is that all of these delta

701
00:33:19 --> 00:33:23
h's, all of these standard
bond enthalpies are positive.

702
00:33:23 --> 00:33:27
Any time we have a positive
delta h, we call this an

703
00:33:27 --> 00:33:29
endothermic reaction.
 

704
00:33:29 --> 00:33:32
And again, endothermic just
means that the reaction

705
00:33:32 --> 00:33:34
takes in energy.
 

706
00:33:34 --> 00:33:37
It's very similar to what when
we're talking about energy

707
00:33:37 --> 00:33:40
whether something's taken in or
released, whether it's

708
00:33:40 --> 00:33:41
positive or negative.
 

709
00:33:41 --> 00:33:44
If you have positive delta h,
it's endothermic, it takes

710
00:33:44 --> 00:33:48
in heat in order for
the reaction to go.

711
00:33:48 --> 00:33:50
So we can think about instead
of talking about all of these

712
00:33:50 --> 00:33:54
individual c h bonds, instead
we can talk about what the

713
00:33:54 --> 00:33:57
average value would be
for all c h bonds.

714
00:33:57 --> 00:34:00
So we can say that the average
of any c h bond is 412

715
00:34:00 --> 00:34:02
kilojoules per mole.
 

716
00:34:02 --> 00:34:04
So you can imagine if you're
looking these things up in

717
00:34:04 --> 00:34:06
terms of a reference table or
maybe in the appendix of your

718
00:34:06 --> 00:34:10
textbook, it would be an
impossibly huge number of

719
00:34:10 --> 00:34:13
pages to write every single
different kinds of c h bond.

720
00:34:13 --> 00:34:16
So that's why instead they
talk about the average

721
00:34:16 --> 00:34:17
bond enthalpies.
 

722
00:34:17 --> 00:34:21
So for c h, again, that's 412,
and all of the enthalpies we

723
00:34:21 --> 00:34:24
just saw fall within
about 8% of that.

724
00:34:24 --> 00:34:27
And in your book you can
look up any type of bond

725
00:34:27 --> 00:34:28
that's listed here.
 

726
00:34:28 --> 00:34:30
So we saw c h is 412.
 

727
00:34:30 --> 00:34:34
You can look up a c c bond, you
can look up a c c double bond,

728
00:34:34 --> 00:34:36
a c c triple bond, and so on.
 

729
00:34:36 --> 00:34:39
And you'll note that what these
are are mean bond enthalpies,

730
00:34:39 --> 00:34:43
so they're the average of all
different types of c h bonds or

731
00:34:43 --> 00:34:47
c c bonds that you can imagine.
 

732
00:34:47 --> 00:34:50
So, you might be asking what's
so important about being able

733
00:34:50 --> 00:34:52
to look up and think
about these different

734
00:34:52 --> 00:34:53
bond enthalpies.
 

735
00:34:53 --> 00:34:55
And the reason that it's
important is because if you're

736
00:34:55 --> 00:34:58
looking at a reaction, no
matter how complicated that

737
00:34:58 --> 00:35:02
reaction is, you can actually
figure out what the enthalpy of

738
00:35:02 --> 00:35:06
the entire reaction is by
adding up all the individual

739
00:35:06 --> 00:35:09
mean bond enthalpies of the
products, and all the

740
00:35:09 --> 00:35:13
individual mean bond enthalpies
of their reactants and thinking

741
00:35:13 --> 00:35:15
about the difference
between those two.

742
00:35:15 --> 00:35:18
So we'll do that in just a
second, and the reaction that

743
00:35:18 --> 00:35:21
we'll do it with is the
oxidation of glucose here.

744
00:35:21 --> 00:35:26
So, c 6 h 12 o 6, one mole of
glucose plus six moles of

745
00:35:26 --> 00:35:29
oxygen, gives us six moles
of carbon dioxide and

746
00:35:29 --> 00:35:31
six moles of water.
 

747
00:35:31 --> 00:35:34
So what we can find out, and
hopefully what we will match

748
00:35:34 --> 00:35:37
up when we look at using the
different bond enthalpies, is

749
00:35:37 --> 00:35:41
that the enthalpy of this
entire reaction is negative

750
00:35:41 --> 00:35:44
2816 kilojoules per mole.
 

751
00:35:44 --> 00:35:47
So in this case we're saying
that delta h is negative.

752
00:35:47 --> 00:35:48
Does anyone know what
it's called when

753
00:35:48 --> 00:35:51
delta h is negative?
 

754
00:35:51 --> 00:35:52
Everyone knows, great.
 

755
00:35:52 --> 00:35:52
So it's exothermic.
 

756
00:35:52 --> 00:35:59
This is an exothermic reaction,
the reaction releases heat.

757
00:35:59 --> 00:36:02
So I just want to mention
before we go on, if you look at

758
00:36:02 --> 00:36:06
almost any freshman chemistry
textbook, what you'll find is

759
00:36:06 --> 00:36:10
this oxidation of glucose
reaction is used a lot in

760
00:36:10 --> 00:36:12
talking about thermochemistry.
 

761
00:36:12 --> 00:36:14
And one reason it's talked
about is because it's very

762
00:36:14 --> 00:36:17
convenient to talk about
something where we start with

763
00:36:17 --> 00:36:21
one mole of glucose and end up
with 12 moles of products.

764
00:36:21 --> 00:36:23
That's going to be helpful what
we're doing a practice problem

765
00:36:23 --> 00:36:25
to see exactly how you
deal with it when there's

766
00:36:25 --> 00:36:27
different numbers of moles.
 

767
00:36:27 --> 00:36:29
But the other reason you
always see this reaction is

768
00:36:29 --> 00:36:32
because it's an incredibly
important reaction.

769
00:36:32 --> 00:36:34
The oxidation of glucose is
going on all the time in our

770
00:36:34 --> 00:36:39
body, this is our main source
of energy for all animals.

771
00:36:39 --> 00:36:42
So let's think a little bit
about why this reaction is so

772
00:36:42 --> 00:36:44
important and so prevalent.
 

773
00:36:44 --> 00:36:46
It turns out that if we're
talking about plants, plants

774
00:36:46 --> 00:36:48
do the reverse reaction.
 

775
00:36:48 --> 00:36:52
So plants take carbon dioxide
and water and they turn it into

776
00:36:52 --> 00:36:55
glucose or energy for us,
energy stored in the bonds

777
00:36:55 --> 00:36:58
of glucose plus oxygen.
 

778
00:36:58 --> 00:37:01
So if we do the reverse
reaction, this is actually

779
00:37:01 --> 00:37:02
going to require energy.
 

780
00:37:02 --> 00:37:06
Where do plants
get this energy?

781
00:37:06 --> 00:37:07
Yeah, this is just
photosynthesis here.

782
00:37:07 --> 00:37:10
This is the photosynthesis
where plants are turning

783
00:37:10 --> 00:37:15
carbon dioxide and water
into sugar and into oxygen.

784
00:37:15 --> 00:37:18
So what happens when we eat the
plants or when we eat animals

785
00:37:18 --> 00:37:21
that have eaten the plants is
that we perform the reverse

786
00:37:21 --> 00:37:23
reaction now, which is what
I just showed you, the

787
00:37:23 --> 00:37:24
oxidation of glucose.
 

788
00:37:24 --> 00:37:29
And even though it's not the
products of the reaction that

789
00:37:29 --> 00:37:32
are particularly valuable to
us, we just breathe out the c o

790
00:37:32 --> 00:37:36
2, and actually we'd rather
have less of that than more in

791
00:37:36 --> 00:37:40
our environment, but what's
important here is instead the

792
00:37:40 --> 00:37:43
energy or the enthalpy that's
given often in this reaction.

793
00:37:43 --> 00:37:47
So as I said, this reaction
has a negative enthalpy of

794
00:37:47 --> 00:37:49
2816 kilojoules per mole.
 

795
00:37:49 --> 00:37:52
That's a lot of enthalpy
and a lot of energy.

796
00:37:52 --> 00:37:56
So we actually end up using
that energy to fuel most of

797
00:37:56 --> 00:37:57
what is going on in our bodies.
 

798
00:37:57 --> 00:38:01
So instead of storing it as
sugar, once we oxidize the

799
00:38:01 --> 00:38:04
sugar, now we just store it
as ATP, and as you know,

800
00:38:04 --> 00:38:07
ATP is the currency of
energy in the cells.

801
00:38:07 --> 00:38:10
So this is why you see this
reaction again and again and

802
00:38:10 --> 00:38:12
again in just about any
chemistry textbook that you

803
00:38:12 --> 00:38:15
open up as sort of a
general reaction.

804
00:38:15 --> 00:38:17
The reason it's used is it's
just so important and so

805
00:38:17 --> 00:38:21
prevalent in terms of thinking
about our bodies and how we're

806
00:38:21 --> 00:38:24
staying alive and using energy.
 

807
00:38:24 --> 00:38:27
All right, so let's go ahead
and use this as an example of

808
00:38:27 --> 00:38:30
what we just said, which is
using the bond enthalpies of

809
00:38:30 --> 00:38:34
the products and the reactants
to figure out the enthalpy

810
00:38:34 --> 00:38:36
of the entire reaction.
 

811
00:38:36 --> 00:38:40
So the way that we do this is
we add up all of the individual

812
00:38:40 --> 00:38:44
mean bond enthalpies of the
reactants, and we subtract from

813
00:38:44 --> 00:38:49
that all of the individual bond
enthalpies of the products.

814
00:38:49 --> 00:38:51
So we can think about
what this will tell us.

815
00:38:51 --> 00:38:55
If you think about the fact if
the bonds are stronger in the

816
00:38:55 --> 00:38:59
products than they were in the
reactants, you can go ahead and

817
00:38:59 --> 00:39:02
click in and tell me if you
think we'll have a negative

818
00:39:02 --> 00:39:04
or a positive delta h here.
 

819
00:39:04 --> 00:39:31
All right, let's take 10
more seconds on this.

820
00:39:31 --> 00:39:38
OK, great, so negative is
correct and some people

821
00:39:38 --> 00:39:39
got totally mixed up.
 

822
00:39:39 --> 00:39:40
Didn't just get one thing --
so let's just focus on the

823
00:39:40 --> 00:39:42
right answer to start with.
 

824
00:39:42 --> 00:39:45
The correct answer is that
it's negative, it's an

825
00:39:45 --> 00:39:47
exothermic reaction.
 

826
00:39:47 --> 00:39:50
And actually, lets switch to
our class notes to explain why.

827
00:39:50 --> 00:39:53
And also, this was the quiz
question for today, so whether

828
00:39:53 --> 00:39:56
you got the answer correct or
incorrect, you get full quiz

829
00:39:56 --> 00:39:57
points if you did,
in fact, answer.

830
00:39:57 --> 00:40:00
But let's still focus on
the right answer here and

831
00:40:00 --> 00:40:01
see why it's correct.
 

832
00:40:01 --> 00:40:04
So if we're talking about the
bonds being stronger in the

833
00:40:04 --> 00:40:09
products, that basically means
that we ended up releasing a

834
00:40:09 --> 00:40:12
lot of heat when we made those
bonds and the products, and we

835
00:40:12 --> 00:40:15
didn't have to use up too much
of that heat to break all the

836
00:40:15 --> 00:40:16
bonds and the reactants.
 

837
00:40:16 --> 00:40:19
So that's why we say that delta
h is going to be negative.

838
00:40:19 --> 00:40:23
You could also just do it not
conceptually, just plugging it

839
00:40:23 --> 00:40:25
into the equation, but it's
better to kind of understand

840
00:40:25 --> 00:40:27
exactly why that is.
 

841
00:40:27 --> 00:40:30
It's because it takes more
energy -- you gain more energy

842
00:40:30 --> 00:40:34
forming the products than you
take breaking up the reactants.

843
00:40:34 --> 00:40:37
So if we have the opposite case
here, if the bonds are stronger

844
00:40:37 --> 00:40:40
in the reactants, now what
we're going to find is that the

845
00:40:40 --> 00:40:43
delta h of the reaction is
positive and what you're

846
00:40:43 --> 00:40:49
dealing with is an
endothermic reaction.

847
00:40:49 --> 00:40:52
So let's go ahead and do this
with our example of the

848
00:40:52 --> 00:40:54
oxidation of glucose.
 

849
00:40:54 --> 00:40:58
So, I've just written out the
glucose molecule so you can see

850
00:40:58 --> 00:41:01
all of its individual bonds
here since we're going to

851
00:41:01 --> 00:41:04
be using that information.
 

852
00:41:04 --> 00:41:07
So let's start by talking about
the bonds that are broken in

853
00:41:07 --> 00:41:11
terms of thinking about all
of the reactants here.

854
00:41:11 --> 00:41:14
So if we're talking about the
sugar molecule itself, what

855
00:41:14 --> 00:41:18
we have is we have
seven c h bonds.

856
00:41:18 --> 00:41:22
And if we add up all the o h
bonds, we have five of those.

857
00:41:22 --> 00:41:27
How many c o single
bonds do we have?

858
00:41:27 --> 00:41:29
What do people think?
 

859
00:41:29 --> 00:41:32
Yeah. five c o single bonds.
 

860
00:41:32 --> 00:41:36
What about c c single bonds?
five of those as well.

861
00:41:36 --> 00:41:39
C o double bonds?
 

862
00:41:39 --> 00:41:41
Just one c o double bond.
 

863
00:41:41 --> 00:41:44
And then we have our oxygen
molecule to worry about and we

864
00:41:44 --> 00:41:47
have six o o double bonds.
 

865
00:41:47 --> 00:41:49
So if we add up all of the
bonds are broken, and we

866
00:41:49 --> 00:41:52
subtract from that the bonds
that are formed, those

867
00:41:52 --> 00:41:57
strengths, the c double bond
o, we have 12 of those.

868
00:41:57 --> 00:42:01
And how many o h
bonds do we have?

869
00:42:01 --> 00:42:02
Right, 12 as well.
 

870
00:42:02 --> 00:42:07
So if we add up all the bonds
broken, what we end up with is

871
00:42:07 --> 00:42:12
12,452 kilojoules per mole, and
it's talking about per mole

872
00:42:12 --> 00:42:14
of glucose that's oxidized.
 

873
00:42:14 --> 00:42:17
And in terms of bonds formed,
what we see is 15

874
00:42:17 --> 00:42:21
kilojoules per mole.
 

875
00:42:21 --> 00:42:27
So all we need to do to figure
out the change in enthalpy, and

876
00:42:27 --> 00:42:30
when it has this subscript r
here, I meant to mention, that

877
00:42:30 --> 00:42:32
means the enthalpy
for the reaction.

878
00:42:32 --> 00:42:38
We just subtract these bonds
here from the ones that

879
00:42:38 --> 00:42:40
we ended up forming.
 

880
00:42:40 --> 00:42:44
So basically, what we're saying
-- excuse me, these are

881
00:42:44 --> 00:42:45
the ones that we broke.
 

882
00:42:45 --> 00:42:51
It's 12,452 minus 15,192,
which we formed.

883
00:42:51 --> 00:42:55
So we end up with an enthalpy
of reaction of negative 2

884
00:42:55 --> 00:43:01
kilojoules per mole of
glucose that's oxidized.

885
00:43:01 --> 00:43:03
All right, so if you remember
the number that I told you

886
00:43:03 --> 00:43:07
before, it doesn't exactly
match up with what we had

887
00:43:07 --> 00:43:09
said is the exact number.
 

888
00:43:09 --> 00:43:12
The exact number is 2,816.
 

889
00:43:12 --> 00:43:15
It is within 3% though,
that's pretty good.

890
00:43:15 --> 00:43:18
Because remember, we're not
using exact bond enthalpies

891
00:43:18 --> 00:43:21
here, what we were using is
average bond enthalpies.

892
00:43:21 --> 00:43:24
So it makes sense that we're
going to be a little bit off.

893
00:43:24 --> 00:43:27
But if all you have in front of
you is the information on

894
00:43:27 --> 00:43:30
bond enthalpies, mean bond
enthalpies, this is a great way

895
00:43:30 --> 00:43:33
to figure out the enthalpy
of an overall reaction.

896
00:43:33 --> 00:43:35
We can think of a different
way, however, to think about

897
00:43:35 --> 00:43:38
the enthalpy of an overall
reaction, and this is talking

898
00:43:38 --> 00:43:41
about the heat of formation.
 

899
00:43:41 --> 00:43:46
And the heat of formation or
delta h formation here is equal

900
00:43:46 --> 00:43:49
to the heat of -- or the
enthalpy of reaction, if we're

901
00:43:49 --> 00:43:54
talking about forming one mole
of compound from its pure

902
00:43:54 --> 00:43:56
elements, which are in
their standard states.

903
00:43:56 --> 00:43:59
So basically, there's a table
that you can look up which

904
00:43:59 --> 00:44:02
tells you the enthalpy of
formation for any compound that

905
00:44:02 --> 00:44:05
you're interested in, and this
is actually an appendix to of

906
00:44:05 --> 00:44:08
your textbooks, and you will
need to use this to solve

907
00:44:08 --> 00:44:10
your problems on p-set.
 

908
00:44:10 --> 00:44:14
But let me say that, for
example, if we're talking about

909
00:44:14 --> 00:44:18
water here, that's formed from
hydrogen and oxygen, if we're

910
00:44:18 --> 00:44:23
talking about the elements in
their pure forms at one bar,

911
00:44:23 --> 00:44:26
standard states, and
room temperature.

912
00:44:26 --> 00:44:28
And if we look this up in our
textbook, we would find that

913
00:44:28 --> 00:44:32
the delta h of formation for
water is negative 286

914
00:44:32 --> 00:44:35
kilojoules per mole.
 

915
00:44:35 --> 00:44:38
Similarly we can look up the
same thing, for example, for

916
00:44:38 --> 00:44:42
carbon dioxide, and what we'll
find here is that our delta h

917
00:44:42 --> 00:44:46
of formation that we look
up is negative 393 .

918
00:44:46 --> 00:44:48
5 kilojoules per mole.
 

919
00:44:48 --> 00:44:51
We can also look up or think
about what our delta h of

920
00:44:51 --> 00:44:53
formation would be for oxygen.
 

921
00:44:53 --> 00:44:57
Does anyone have a guess here?
 

922
00:44:57 --> 00:44:58
Yup, it's actually
going to be zero.

923
00:44:58 --> 00:45:02
Oxygen already is in its most
stable state, so any time we're

924
00:45:02 --> 00:45:05
talking about an element in
their most stable state,

925
00:45:05 --> 00:45:06
it's going to be zero.
 

926
00:45:06 --> 00:45:09
That's going to be the delta
h of formation, there

927
00:45:09 --> 00:45:11
is none, it's zero.
 

928
00:45:11 --> 00:45:15
We can also look up what
it was for sugar, for

929
00:45:15 --> 00:45:18
glucose, c 6 h 12 o 6.
 

930
00:45:18 --> 00:45:22
So that coming from its most
stable forms, it was the most

931
00:45:22 --> 00:45:25
pure form of the elements, is
going to be negative 1260

932
00:45:25 --> 00:45:27
kilojoules per mole.
 

933
00:45:27 --> 00:45:31
All of this information has
been tabulated and is in

934
00:45:31 --> 00:45:33
tables, you can refer to them,
and they're also in the back of

935
00:45:33 --> 00:45:35
your textbook, so you can refer
to those in terms of

936
00:45:35 --> 00:45:37
your problem-set.
 

937
00:45:37 --> 00:45:40
So let's go ahead and solve a
problem actually using the

938
00:45:40 --> 00:45:42
delta h's of formation.
 

939
00:45:42 --> 00:45:44
Let's see if we can do any
better to coming close

940
00:45:44 --> 00:45:47
to the reality for the
oxidation of glucose.

941
00:45:47 --> 00:45:50
So if we're going to calculate
the delta h for the reaction

942
00:45:50 --> 00:45:54
for the oxidation of glucose,
or actually for any reaction at

943
00:45:54 --> 00:45:58
all, what we want to do is take
the delta h of formation of the

944
00:45:58 --> 00:46:02
products and subtract from that
the delta h of formation

945
00:46:02 --> 00:46:03
of the reactants.
 

946
00:46:03 --> 00:46:08
So let's go ahead and do that.
 

947
00:46:08 --> 00:46:13
And in terms of talking about
the oxidation of glucose, if we

948
00:46:13 --> 00:46:19
talk about delta h of this
reaction, what we need to take

949
00:46:19 --> 00:46:26
is 6 times delta h of formation
of c o 2, since we're forming,

950
00:46:26 --> 00:46:30
that's in our products, we're
forming six moles of c o 2.

951
00:46:30 --> 00:46:37
Plus 6 times delta h formation
of h 2 o, since we're

952
00:46:37 --> 00:46:41
forming six moles of h 2 o.
 

953
00:46:41 --> 00:46:45
And we subtract from that
the heat of formation of

954
00:46:45 --> 00:46:47
all of our reactants.
 

955
00:46:47 --> 00:46:52
So we only have one mole, so we
just say delta h formation of c

956
00:46:52 --> 00:46:59
6 h 12 o 6, and then in
addition to that we have six

957
00:46:59 --> 00:47:07
moles delta h formation
of oxygen, of o 2.

958
00:47:07 --> 00:47:10
All right, so this is our
equation here and at this point

959
00:47:10 --> 00:47:14
what we would do is we would
look up what all of the delta h

960
00:47:14 --> 00:47:21
formation values are for c o 2,
h 2 o, sugar, and oxygen.

961
00:47:21 --> 00:47:26
So what we would find is
that we end up having

962
00:47:26 --> 00:47:30
6 times negative 393 .
 

963
00:47:30 --> 00:47:32
5 -- that's what we had
just looked up and

964
00:47:32 --> 00:47:35
told you for c o 2.
 

965
00:47:35 --> 00:47:39
Plus 6 times negative 285 .
 

966
00:47:39 --> 00:47:41
8 for the water.
 

967
00:47:41 --> 00:47:51
Minus 1 times 1260, so
it's negative 1260

968
00:47:51 --> 00:47:54
for our sugar here.
 

969
00:47:54 --> 00:47:57
And then what we're going to
end up with is having minus,

970
00:47:57 --> 00:48:00
and what is it for
oxygen again?

971
00:48:00 --> 00:48:03
So minus 0.
 

972
00:48:03 --> 00:48:07
So what we end up with in terms
of the delta h for the entire

973
00:48:07 --> 00:48:12
reaction here, is we end up
with negative 2816, and it's

974
00:48:12 --> 00:48:21
going to be kilojoules
per mole of glucose.

975
00:48:21 --> 00:48:25
All right, so let's see how
this matches up, and hopefully

976
00:48:25 --> 00:48:28
you can actually remember that
this matches up actually

977
00:48:28 --> 00:48:30
perfectly here.
 

978
00:48:30 --> 00:48:34
So what we are going to see
is that, in fact, what we

979
00:48:34 --> 00:48:36
calculated versus what
is experimental is

980
00:48:36 --> 00:48:38
dead-on the same.
 

981
00:48:38 --> 00:48:40
So there's actually one more
way to figure out the

982
00:48:40 --> 00:48:42
enthalpies of a reaction, I'm
going to go over it

983
00:48:42 --> 00:48:44
just very briefly.
 

984
00:48:44 --> 00:48:47
And that's based on the fact
that enthalpy is a state

985
00:48:47 --> 00:48:52
function, and by state function
what I mean is that it doesn't

986
00:48:52 --> 00:48:56
matter how you got from point a
to point b, all that matters is

987
00:48:56 --> 00:48:58
the difference between the two.
 

988
00:48:58 --> 00:49:02
So another example of a state
function is altitude, for

989
00:49:02 --> 00:49:03
example, on a mountain.
 

990
00:49:03 --> 00:49:06
So if you're talking about
altitude, you go from point a

991
00:49:06 --> 00:49:11
to point b on the mountain, and
it doesn't matter how you got

992
00:49:11 --> 00:49:14
there -- you could have climbed
all the way up the top of the

993
00:49:14 --> 00:49:16
mountain, then went back
down and eventually

994
00:49:16 --> 00:49:17
landed on point b.
 

995
00:49:17 --> 00:49:20
Or you could have gone straight
from point a to point b.

996
00:49:20 --> 00:49:24
The change in altitude between
point a and point b do not

997
00:49:24 --> 00:49:26
depend on the path, they're
independent of path.

998
00:49:26 --> 00:49:29
So this difference right here
does not matter on how you got

999
00:49:29 --> 00:49:32
there, it does not make a
difference how you got there,

1000
00:49:32 --> 00:49:35
altitude is a state function.
 

1001
00:49:35 --> 00:49:39
So similarly, we can say that
enthalpy is a state function as

1002
00:49:39 --> 00:49:43
well, if we're talking about
part reaction a here, which has

1003
00:49:43 --> 00:49:47
our reactants going to our
products in 2 b here.

1004
00:49:47 --> 00:49:49
It doesn't matter
how we got there.

1005
00:49:49 --> 00:49:52
We can actually calculate the
change in enthalpy at all

1006
00:49:52 --> 00:49:53
different points here.
 

1007
00:49:53 --> 00:49:56
All that we actually have to
worry about at the end of the

1008
00:49:56 --> 00:49:59
day is the difference
between a and b.

1009
00:49:59 --> 00:50:02
So that's what we mean
by state function.

1010
00:50:02 --> 00:50:04
Actually, we're not going to
have a chance to get to fully

1011
00:50:04 --> 00:50:07
explaining the consequence of
this, which is Hess's law,

1012
00:50:07 --> 00:50:11
which allows us to add and
subtract different reactions.

1013
00:50:11 --> 00:50:14
So what I'm going to say is the
last two problems on your

1014
00:50:14 --> 00:50:17
problem-set you won't have to
do because we're not going to

1015
00:50:17 --> 00:50:20
get a chance to cover
in class on Friday.

1016
00:50:20 --> 00:50:22
So I'll send out an email
about this as well, so

1017
00:50:22 --> 00:50:25
that'll be pushed back.
 

1018
00:50:25 --> 00:50:29
So, it will be on the exam, so
you want to do them at some

1019
00:50:29 --> 00:50:31
point, but you don't need
to actually turn those in.

1020
00:50:31 --> 00:50:34
So those are the last two
problems, I believe.