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PROFESSOR: OK, so today we're
going to start talking about

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acids and bases, and this is
acid-base equilibrium, so you

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can't forget anything that
you've learned from the last

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two lectures about equilibrium.
 

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And then we're going to talk
about, after this, oxidation

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reduction equilibrium,
and so there's a lot of

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equilibrium going on.
 

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So today we're going to
give you some definitions.

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We're going to talk about
autoionization of water.

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We're going to talk about the
p h function, which most

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people are familiar with.
 

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We may think about what the p
h's are of some commonly found

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ingredients around campus.
 

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And talk about the strengths
of those acids and bases.

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And then if we have time, we'll
start thinking about how to

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work a problem associated
with a weak acid.

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So actually, I guess, do
we want to do that other

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clicker question, maybe,
before we get started?

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At some point, we're going
to ask you a question about

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when you want forums.
 

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So, I'm not sure if we're going
to have that for you today or

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not, but we need to -- we want
to have these pizza forums

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where you can come, and the
time isn't always working, so

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we thought we could use the
clickers to figure out what

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the best time is for people.
 

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So we may have that
for you later.

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So this is the narrowest
definition of an

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acid and a base.
 

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00:01:48 --> 00:01:51
So the narrowest definition of
an acid and a base is that an

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acid is a substance when you
dissolve it in water, it

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increases the concentration of
hydrogen ions, or h plus.

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Whereas a base is a substance
that when dissolved in water,

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increases the concentration of
hydroxide ions, or o h minus.

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So that's pretty narrow.
 

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We can be broader.
 

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We can talk about
Bronsted-Lowry, and here,

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an acid is described as
a substance that can

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donate a hydrogen ion.
 

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And a base is described
as a substance that can

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accept a hydrogen ion.
 

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So let's look at some examples
using that definition.

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So let's look at an example, so
c h 3, c o o h plus water, and

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I guess I should put that in
aqueous, going to hydronium ion

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plus c h 3, c o o
minus aqueous.

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All right, so what
is going on here?

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So if we look at the things on
one side of the equation and

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the other, you can see that
this is an acid that has lost

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its hydrogen ion, so the
hydrogen is gone, whereas the

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water molecule has gained a
hydrogen ion, and so

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now it's h 3 o plus.
 

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So we have an acid here, it's
acting as an acid, and acid

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acts as a substance that gives
up a hydrogen ion, and the

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water is acting as a base, it's
accepting that hydrogen ion.

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And when it accepts the
hydrogen ion, it becomes an

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acid in the reverse direction,
whereas when the acid gives off

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the hydrogen ion, it becomes a
base in the reverse direction.

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So in the reverse direction,
hydronium ions is giving off a

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hydrogen ion to this base,
reforming the acid, and after a

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hydronium ion gives off
its hydrogen ion, it

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forms water again.
 

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So that would be an example of
Bronsted-Lowry talking about

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substances as acids and bases
whether they accept or

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donate a hydrogen ion.
 

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And this is the definition
we're going to be using

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mostly throughout this unit.
 

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So, let's look at a little
movie of this going on.

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So, in this movie, we have our
water molecules with red and

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the white dots here are their
hydrogen atoms, and now we're

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going to come in and have an
acid come in, there's the acid,

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it has the hydrogen
ion on it in white.

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It meets up with a water
molecule, and now you

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formed hydronium ion.
 

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And that forms another water
molecule and it passes it

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along, so there's a different
molecule of hydronium ion.

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So that's what's going
on in this definition.

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All right.
 

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So this brings us to another
term, which is conjugate acid

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base pairs, so you can talk
about something being a

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conjugate base of a particular
acid, and so a conjugate base

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of an acid is a base that's
formed after the acid has

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donated its hydrogen ion.
 

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A conjugate acid of a base is
the acids that its formed

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00:06:00 --> 00:06:04
when the base accepts
the hydrogen ion.

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So you can look at
those examples here.

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So we have a pair that we've
drawn here with this red line,

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an acid base pair, and the
other pair is the water

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and the hydronium ion.
 

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00:06:21 --> 00:06:24
All right, let's look at a
couple more examples to get the

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sense of this and figure out
what are the acid base pairs.

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00:06:34 --> 00:06:39
So, one more example.
 

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So now let's look at h c o 3
minus an aqueous solution and

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water, going to, again,
hydronium ion, and c o 3 minus

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2, also an aqueous solution.
 

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So, what is h c o 3
minus acting as here?

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As an acid.
 

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00:07:14 --> 00:07:18
And so what does
that make water?

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A base.
 

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And so the conjugate acid of
that base, again, is the

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hydronium ion concentration.
 

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00:07:29 --> 00:07:34
And the conjugate base
of the acid is c o 3

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00:07:34 --> 00:07:38
minus 2, over here.
 

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00:07:38 --> 00:07:41
And so in the reverse
direction, this base will be

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accepting a hydrogen ion from
the acid, forming the

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conjugates on the other side.
 

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OK, now you can do an example.
 

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Let's have a clicker question.
 

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So identify what the acid and
the base pairs are here.

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All right, let's
take 10 seconds.

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00:08:57 --> 00:08:59
That's quite good actually.
 

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So, that's correct and you can
write it in your notes that

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here we see that there's a
little bit of change, water's

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doing something different.
 

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So instead of the conjugate of
water being the hydronium ion,

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00:09:12 --> 00:09:15
we see it's a hydroxide.
 

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So here, the water is acting as
an acid giving off a hydrogen

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00:09:19 --> 00:09:25
ion to this h c o 3 minus.
 

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00:09:25 --> 00:09:28
And so now we have a second
hydrogen ion over here, we have

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00:09:28 --> 00:09:33
the h 2 species, and that the
conjugate of water is o h.

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00:09:33 --> 00:09:36
So this is acting as a base,
it's accepting a hydrogen ion,

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00:09:36 --> 00:09:39
this is donating it, it's an
acid, this is the conjugate

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00:09:39 --> 00:09:42
acid of that base, and this is
the conjugate base

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00:09:42 --> 00:09:43
of that acid.
 

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00:09:43 --> 00:09:46
So in the reverse direction,
this is an acid giving

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00:09:46 --> 00:09:51
off a hydrogen ion to the
hydroxide forming water.

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00:09:51 --> 00:09:53
So one thing that you'll notice
about these examples that we've

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00:09:53 --> 00:09:57
written up is that when you see
water in the equation, you

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00:09:57 --> 00:10:00
don't really know what it's
going to be doing until you

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00:10:00 --> 00:10:02
actually look at what the
products are and then

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00:10:02 --> 00:10:03
you can figure it out.
 

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00:10:03 --> 00:10:07
So water can act as
either an acid or a base

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00:10:07 --> 00:10:09
in these equations.
 

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00:10:09 --> 00:10:16
And if we go to the lecture
notes, the term is amphoteric,

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which is a molecule that can
act as either an acid or a

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00:10:19 --> 00:10:22
base, depending on the
reaction conditions.

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00:10:22 --> 00:10:24
So depending on if it's
mixed with something that's

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a stronger acid or a
stronger base than it is.

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00:10:27 --> 00:10:34
And an example, one of the most
common examples is water.

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00:10:34 --> 00:10:41
So now let's consider a broader
example of acids and bases, and

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these are Lewis acids and
bases, and we're going to

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00:10:43 --> 00:10:46
actually come back to this
around Thanksgiving time

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when we talk about
transition metals.

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And so here it's really broad
-- we're not actually even

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going to talk about a
hydrogen ion at all.

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So in this case, we're talking
about a Lewis base as a species

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00:11:00 --> 00:11:04
that donates lone pair
electrons, and a Lewis acid is

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00:11:04 --> 00:11:09
a species that accepts
such electrons.

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So here would be an example.
 

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So we can think about forming a
complex, and which thing is

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00:11:16 --> 00:11:20
going to act as an
acid or a base.

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One will be donating its
lone pair electrons and the

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00:11:24 --> 00:11:26
other will be accepting.
 

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So this is a very broad, a much
broader definition, and so when

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you talk about acid base here,
so always say as a Lewis acid

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or Lewis base to make it
clear what's going on.

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So again we have our base
donating its lone pair

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00:11:41 --> 00:11:44
electrons and the
acid accepting.

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All right, so those are our
definitions of acids and bases.

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So now let's come back to this
issue of water and how water

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can act as an acid or a base.
 

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So if it can act as an acid or
a base, it seems like it can

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react with itself to do some
chemistry, and it can.

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So up here, you could have one
water molecule acting as an

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acid, giving up its hydrogen
ion to another water acting as

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a base, forming hydronium ion
and also forming hydroxide ion.

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So then you can ask the
question, well, how much h 2 o

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00:12:26 --> 00:12:28
is in a typical glass of water.
 

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How much, so you don't like the
idea that I'm drinking

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00:12:31 --> 00:12:34
hydroxide ions, how much
hydronium ion and how much

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hydroxide ion are in this glass
of water, how much h 2 o

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is in a glass of water?
 

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00:12:44 --> 00:12:47
So that's the question.
 

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00:12:47 --> 00:12:52
So here's the equation again,
and we can think about

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00:12:52 --> 00:12:53
how to calculate it.
 

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00:12:53 --> 00:12:58
What do we really want to
know in this question?

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00:12:58 --> 00:13:00
What are we really asking?
 

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00:13:00 --> 00:13:05
How much, at an equilibrium
situation, how much are

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00:13:05 --> 00:13:09
products and how much
reactants do you have?

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00:13:09 --> 00:13:12
What can you tell me about
ratios of products and

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00:13:12 --> 00:13:15
reactants at equilibrium?
 

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00:13:15 --> 00:13:16
K.
 

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00:13:16 --> 00:13:20
And how are some ways
you can calculate k's?

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00:13:20 --> 00:13:29
Different terms, but, right,
it's what about -- what is k?

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So we have the equilibrium
constant, and there a couple

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different ways one might
calculate k -- you might be

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given concentrations at
equilibrium, or you might be

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given information
about delta g.

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00:13:45 --> 00:13:51
So you can calculate k's from
delta g nought, and this is an

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equation that you'll use pretty
often -- delta g nought equals

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00:13:54 --> 00:13:57
minus r t natural log of k.
 

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So if we want to know
k, we need to find out

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00:14:00 --> 00:14:02
what delta g nought is.
 

200
00:14:02 --> 00:14:06
What are ways to
calculate delta g?

201
00:14:06 --> 00:14:12
So you've seen some of those --
oh, you're probably recognizing

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00:14:12 --> 00:14:15
these already, temperature
and our gas constant.

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00:14:15 --> 00:14:19
So we can solve for k
in terms of delta g.

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00:14:19 --> 00:14:23
And how do we calculate
that delta g.

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00:14:23 --> 00:14:25
Well, there are a
couple of ways.

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00:14:25 --> 00:14:32
So you can think about the
delta g's of formations, and

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00:14:32 --> 00:14:37
we can think about one of my
personal favorites, which

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00:14:37 --> 00:14:41
is relationship between
delta h and t delta s.

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00:14:41 --> 00:14:45
So you can think about your
enthalpies and your entropies

210
00:14:45 --> 00:14:48
at a certain temperature, and
you can calculate delta g, and

211
00:14:48 --> 00:14:50
from that you can get the
equilibrium constant.

212
00:14:50 --> 00:14:54
So this is just a little review
showing the relevance of

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00:14:54 --> 00:14:57
material you've learned
before to the material

214
00:14:57 --> 00:14:59
we're covering now.
 

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00:14:59 --> 00:15:04
So we're going to pick one and
just calculate the delta g.

216
00:15:04 --> 00:15:09
So we can look up these values
of formation for our products

217
00:15:09 --> 00:15:13
and our reactants and plug them
in, and we get a value for

218
00:15:13 --> 00:15:16
delta g nought of plus 79 .
 

219
00:15:16 --> 00:15:20
89 kilojoules per mole.
 

220
00:15:20 --> 00:15:23
So we have a positive
value here.

221
00:15:23 --> 00:15:28
So without doing any more math,
which we'll do it a minute, do

222
00:15:28 --> 00:15:33
you expect a large or small
value for k for the equilibrium

223
00:15:33 --> 00:15:36
constant if delta g
nought is positive 79 .

224
00:15:36 --> 00:15:37
89 kilojoules per mole.
 

225
00:15:37 --> 00:15:39
STUDENT: Small.
 

226
00:15:39 --> 00:15:42
PROFESSOR: We would
expect it to be small.

227
00:15:42 --> 00:15:45
And you probably already knew
that that there's a lot of

228
00:15:45 --> 00:15:47
h 2 o in a glass of water.
 

229
00:15:47 --> 00:15:50
So again, from that
perspective, we'd also

230
00:15:50 --> 00:15:51
expect it to be small.
 

231
00:15:51 --> 00:15:57
So now we can plug those values
in, we calculated this delta g,

232
00:15:57 --> 00:16:01
we know the gas constant, we're
at room temperature, and so

233
00:16:01 --> 00:16:04
we get a value of k as 1 .
 

234
00:16:04 --> 00:16:11
0 times 10 to the minus 14
at room temperature, and

235
00:16:11 --> 00:16:14
that's a small number.
 

236
00:16:14 --> 00:16:19
So, the very small value
indicates that only a small

237
00:16:19 --> 00:16:25
percentage all your h 2 0 has
ionized, and that mostly,

238
00:16:25 --> 00:16:29
there's h 2 o in a glass of
water, not so many ions.

239
00:16:29 --> 00:16:33
Not many of the molecules have
ionized, because k is a small

240
00:16:33 --> 00:16:37
number, not a lot of products
at this equilibrium.

241
00:16:37 --> 00:16:41
So there's a lot of h 2
o in a glass of water.

242
00:16:41 --> 00:16:45
So, this particular k
has a special name, and

243
00:16:45 --> 00:16:48
it's k w, w for water.
 

244
00:16:48 --> 00:16:52
And this term and this number,
if you haven't memorized it in

245
00:16:52 --> 00:16:55
high school, you probably will
by the time you're done

246
00:16:55 --> 00:16:56
with problem-sets.
 

247
00:16:56 --> 00:17:00
This is a very valuable number,
you'll be using it a lot in

248
00:17:00 --> 00:17:04
calculating acid base problems,
and you will end up memorizing

249
00:17:04 --> 00:17:08
it whether you want to or not.
 

250
00:17:08 --> 00:17:16
So, then k w equals your
hydronium ion concentration

251
00:17:16 --> 00:17:21
times your hydroxide
ion concentration.

252
00:17:21 --> 00:17:25
Now for a minute, let's
consider why that's true.

253
00:17:25 --> 00:17:30
So our reaction, it's products
over reactants for an

254
00:17:30 --> 00:17:33
equilibrium constant, but now,
all of a sudden, I don't have

255
00:17:33 --> 00:17:36
my reactants going on in here.
 

256
00:17:36 --> 00:17:39
So the k w is expressed in
terms of the concentration a

257
00:17:39 --> 00:17:43
hydronium ions times the
concentration of hydroxide,

258
00:17:43 --> 00:17:46
and it doesn't have this
water term at the bottom.

259
00:17:46 --> 00:17:50
And that'll be true for
any problem in which

260
00:17:50 --> 00:17:52
the water is a solvent.
 

261
00:17:52 --> 00:17:56
And so, it's really not going
to be changing very much.

262
00:17:56 --> 00:18:00
A solvent is nearly pure, and
when you have in a nearly pure

263
00:18:00 --> 00:18:04
solvent or solid, it's
not included in the

264
00:18:04 --> 00:18:05
equilibrium expression.
 

265
00:18:05 --> 00:18:09
So we'll see other examples
of this as we go along.

266
00:18:09 --> 00:18:13
So you always want to ask
yourself is this the solvent.

267
00:18:13 --> 00:18:19
If so, we're talking about very
dilute things going on in

268
00:18:19 --> 00:18:21
solvent, the solvent
concentration isn't changing

269
00:18:21 --> 00:18:26
very much, so it drops
out of our term.

270
00:18:26 --> 00:18:30
So, because k w is an
equilibrium constant, the

271
00:18:30 --> 00:18:33
products are always going to
be equal to the same thing

272
00:18:33 --> 00:18:35
at the same temperature.
 

273
00:18:35 --> 00:18:39
So, at room temperature, or
298 kelvin, it's always

274
00:18:39 --> 00:18:40
going to be equal to 1 .
 

275
00:18:40 --> 00:18:43
0 times 10 to the minus
14, and that's why it's

276
00:18:43 --> 00:18:45
such a valuable number.
 

277
00:18:45 --> 00:18:49
And when we're talking about
acid base problems, you're

278
00:18:49 --> 00:18:52
almost always going to be at
room temperature, just

279
00:18:52 --> 00:18:55
to not make life more
complicated for you.

280
00:18:55 --> 00:18:59
So, you can pretty much assume,
it should be in big bold

281
00:18:59 --> 00:19:02
letters if the temperature is
not room temperature, so that

282
00:19:02 --> 00:19:06
you can use these values.
 

283
00:19:06 --> 00:19:14
All right, so let's look
at the p h function now.

284
00:19:14 --> 00:19:18
So what does p h equal?
 

285
00:19:18 --> 00:19:24
So p h is equal to the minus
the log of the hydronium ion

286
00:19:24 --> 00:19:30
concentration, and let's also
talk about p o h, and that's

287
00:19:30 --> 00:19:39
equal to minus log of the
hydroxide ion concentration.

288
00:19:39 --> 00:19:45
I just told you that k w is
equal to the concentration of

289
00:19:45 --> 00:19:48
hydronium ions times the
concentration of

290
00:19:48 --> 00:19:51
hydroxide ions.
 

291
00:19:51 --> 00:19:54
And now we can express
this in another very

292
00:19:54 --> 00:19:56
useful way for you.
 

293
00:19:56 --> 00:20:02
If we take the log of all
sides, actually let's take the

294
00:20:02 --> 00:20:13
minus log of everything, so
minus log of k w equals the

295
00:20:13 --> 00:20:23
minus log of hydronium ion
concentration, minus the log of

296
00:20:23 --> 00:20:32
hydroxide ion concentration,
and we end up with terms of p k

297
00:20:32 --> 00:20:37
w being equal to p h, because
minus log of the hydronium ion

298
00:20:37 --> 00:20:42
concentration is p h, and minus
the log of the hydroxide

299
00:20:42 --> 00:20:45
concentration is p o h.
 

300
00:20:45 --> 00:20:47
So plus p o h.
 

301
00:20:47 --> 00:20:51
And we know that this term
at room temperature is 1 .

302
00:20:51 --> 00:20:56
0 times 10 to the minus 14.
 

303
00:20:56 --> 00:21:00
So this term at room
temperature is 14 .

304
00:21:00 --> 00:21:09
0 0, again at 25 degrees
c or 298 kelvin.

305
00:21:09 --> 00:21:12
So this is also a
useful expression.

306
00:21:12 --> 00:21:19
If you know the p h, you can
calculate the p o h if you're

307
00:21:19 --> 00:21:24
at room temperature,
remembering this number of 14.

308
00:21:24 --> 00:21:26
So these are things that you
will be doing a lot in the

309
00:21:26 --> 00:21:28
problems and you will start
remembering all of these

310
00:21:28 --> 00:21:31
numbers really well.
 

311
00:21:31 --> 00:21:36
So p h and p o h, what
does p h do for you?

312
00:21:36 --> 00:21:39
Well, the p h tells you about
the strength of the acid.

313
00:21:39 --> 00:21:48
So the p h of pure water should
be neutral, which is 7.

314
00:21:48 --> 00:21:51
And now, tell me what the
p h of an acid is and

315
00:21:51 --> 00:22:09
the p h of a base.
 

316
00:22:09 --> 00:22:11
Let's just do 10 seconds
on this, this is pretty

317
00:22:11 --> 00:22:12
straightforward.
 

318
00:22:12 --> 00:22:27
So this tests previous
knowledge on this topic,

319
00:22:27 --> 00:22:30
and it is very good.
 

320
00:22:30 --> 00:22:34
People know about
what the p h's are.

321
00:22:34 --> 00:22:36
So that's right.
 

322
00:22:36 --> 00:22:40
So the p h of an acid solution
is less than 7, and the p h of

323
00:22:40 --> 00:22:42
a base solution is
greater than 7.

324
00:22:42 --> 00:22:48
And the EPA defines corrosive
as something where the p h

325
00:22:48 --> 00:22:52
is lower than 3 or
greater than 12 .

326
00:22:52 --> 00:22:54
5.
 

327
00:22:54 --> 00:23:01
So, if here is our scale of p
h, we're neutral at 7, we're

328
00:23:01 --> 00:23:05
acidic below 7, and we're
corrosive below 3.

329
00:23:05 --> 00:23:08
We're basic above 7, and
corrosive above 12 .

330
00:23:08 --> 00:23:09
5.
 

331
00:23:09 --> 00:23:14
So now what I want to do is ask
Dr. Taylor to come up and we're

332
00:23:14 --> 00:23:18
going to measure some p h's of
things, so we're going to be

333
00:23:18 --> 00:23:24
interested in knowing how much
danger you're in around MIT.

334
00:23:24 --> 00:23:35
So we're going to have
you measure them.

335
00:23:35 --> 00:23:38
We have these little strips on
it, and so someone will come

336
00:23:38 --> 00:23:42
around and help you read it,
so you can read off the

337
00:23:42 --> 00:23:46
strip of what you have.
 

338
00:23:46 --> 00:23:49
Should we start with water?
 

339
00:23:49 --> 00:23:51
This is random MIT water.
 

340
00:23:51 --> 00:24:00
Let's start there.
 

341
00:24:00 --> 00:24:02
So, just pick a volunteer.
 

342
00:24:02 --> 00:24:22
PROFESSOR: OK, so what we're
going to do is have the TA's go

343
00:24:22 --> 00:24:25
in and ask you to read off of a
p h strip, what the p h of

344
00:24:25 --> 00:24:27
various things are, and
actually Marcus, if you could

345
00:24:27 --> 00:24:30
write on the board
what these are.

346
00:24:30 --> 00:24:33
So we'll start with MIT water.
 

347
00:24:33 --> 00:24:37
So, we know if it's 7 it's
neutral, if it's below 7 we're

348
00:24:37 --> 00:24:40
talking about acidic, and
above that it's basic.

349
00:24:40 --> 00:24:44
We actually, also for you to be
able to visualize as well, what

350
00:24:44 --> 00:24:47
I did is I just boiled up some
cabbage last night and brought

351
00:24:47 --> 00:24:49
in the extract with me.
 

352
00:24:49 --> 00:24:54
And cabbage actually has
anthocyanins in it, which is a

353
00:24:54 --> 00:24:58
color indicator, and it changes
color based on whether it's in

354
00:24:58 --> 00:25:01
an acidic or a basic solution.
 

355
00:25:01 --> 00:25:03
So we'll let you see this here.
 

356
00:25:03 --> 00:25:06
It looks like MIT water,
pretty safe to drink,

357
00:25:06 --> 00:25:08
which is good news.
 

358
00:25:08 --> 00:25:18
We can go ahead and -- so it
looks like if we add MIT water

359
00:25:18 --> 00:25:21
to cabbage solution, what do
you think's going to happen --

360
00:25:21 --> 00:25:24
this is neutral right now.
 

361
00:25:24 --> 00:25:25
Hopefully not much.
 

362
00:25:25 --> 00:25:29
We either have invalid
strips or we'll see

363
00:25:29 --> 00:25:31
nothing happen here.
 

364
00:25:31 --> 00:25:32
All right, so you can see
we still have the purple

365
00:25:32 --> 00:25:34
color for MIT water.
 

366
00:25:34 --> 00:25:38
Two confirmations that it's
safe to drink right out of

367
00:25:38 --> 00:25:39
the tap when you get home.
 

368
00:25:39 --> 00:25:43
So, the next thing is
vinegar did someone

369
00:25:43 --> 00:25:44
take the, the strip?
 

370
00:25:44 --> 00:25:46
STUDENT: We have a 2 and a 1/2.
 

371
00:25:46 --> 00:25:47
PROFESSOR: OK, so are
we drinking vinegar?

372
00:25:47 --> 00:25:49
STUDENT: No.
 

373
00:25:49 --> 00:25:50
PROFESSOR: Probably
not straight.

374
00:25:50 --> 00:25:58
All right, so we've got
our cabbage extract

375
00:25:58 --> 00:25:59
here, it's purple.
 

376
00:25:59 --> 00:26:01
Does anyone have a guess as to
what color it's going to turn

377
00:26:01 --> 00:26:03
if we pour in vinegar,
very acidic.

378
00:26:03 --> 00:26:07
All right, a couple guesses
I hear, blue and pink.

379
00:26:07 --> 00:26:09
They're both good guesses
because different

380
00:26:09 --> 00:26:12
color indicators turn
different colors.

381
00:26:12 --> 00:26:21
But, all right, looks like a
dramatic difference here.

382
00:26:21 --> 00:26:24
What do we have next out there?
 

383
00:26:24 --> 00:26:29
Baking soda.
 

384
00:26:29 --> 00:26:31
Seven for the baking soda.
 

385
00:26:31 --> 00:26:33
I'm going to guess we did
not pour in enough or it

386
00:26:33 --> 00:26:35
is not dissolved here.
 

387
00:26:35 --> 00:26:38
All right, so let's do our
secondary test here and see

388
00:26:38 --> 00:26:39
what happens with
the baking soda.

389
00:26:39 --> 00:26:48
PROFESSOR: [UNINTELLIGIBLE]
 

390
00:26:48 --> 00:26:49
the water we added it to
was also not neutral.

391
00:26:49 --> 00:26:50
PROFESSOR: All right, so
we're actually pretty basic

392
00:26:50 --> 00:26:51
with the baking soda.
 

393
00:26:51 --> 00:26:53
We won't give it a number
because it was just

394
00:26:53 --> 00:26:54
dissolved in water.
 

395
00:26:54 --> 00:27:04
But we'll remember baking
soda, blue here is basic.

396
00:27:04 --> 00:27:06
So the next thing we're
going to test is soda that

397
00:27:06 --> 00:27:07
you drink all the time.
 

398
00:27:07 --> 00:27:08
I brought Sprite.
 

399
00:27:08 --> 00:27:13
Coke probably would have been a
good pick as well or Diet Coke.

400
00:27:13 --> 00:27:15
So, we'll test what that is.
 

401
00:27:15 --> 00:27:26
Hopefully it comes
out neutral, right?

402
00:27:26 --> 00:27:27
So while we're waiting,
we'll start taking a

403
00:27:27 --> 00:27:30
little bit of a look here.
 

404
00:27:30 --> 00:27:34
All right, we've got a 3.
 

405
00:27:34 --> 00:27:34
Soda, corrosive.
 

406
00:27:34 --> 00:27:41
It's not just the sugar
that's bad for your teeth.

407
00:27:41 --> 00:27:46
Luckily we see here it is not
quite as bad as vinegar in

408
00:27:46 --> 00:27:48
terms of how acidic it is, but
we definitely have a

409
00:27:48 --> 00:27:50
color change here.
 

410
00:27:50 --> 00:28:00
PROFESSOR: Has anyone
used soda for something

411
00:28:00 --> 00:28:01
other than drinking?
 

412
00:28:01 --> 00:28:03
What did you use it for?
 

413
00:28:03 --> 00:28:04
STUDENT: Cleaning pennies.
 

414
00:28:04 --> 00:28:07
PROFESSOR: Cleaning
pennies, what else?

415
00:28:07 --> 00:28:11
STUDENT: When you have stuff
all over your car battery

416
00:28:11 --> 00:28:13
you can use Coke.
 

417
00:28:13 --> 00:28:17
PROFESSOR: And some of those
other uses make sense.

418
00:28:17 --> 00:28:17
[UNINTELLIGIBLE]
 

419
00:28:17 --> 00:28:17
What else?
 

420
00:28:17 --> 00:28:18
STUDENT: Taking the
galvanization off

421
00:28:18 --> 00:28:18
of a steel wire.
 

422
00:28:18 --> 00:28:18
PROFESSOR: OK, so
cleaning steel wire.

423
00:28:18 --> 00:28:19
How many of you still drink
soda knowing this information?

424
00:28:19 --> 00:28:40
PROFESSOR: All right, so the
next thing we put out there was

425
00:28:40 --> 00:28:42
aspirin dissolved in water, and
it's going to depend what

426
00:28:42 --> 00:28:45
concentration we did, but we
put aspirin in water

427
00:28:45 --> 00:28:47
and we got a 3 here.
 

428
00:28:47 --> 00:28:53
So, aspirin sometimes gives you
an upset stomach, and that's

429
00:28:53 --> 00:28:58
why Tylenol's an improvement in
some ways -- obviously, that

430
00:28:58 --> 00:28:59
has its own drawbacks, too.
 

431
00:28:59 --> 00:29:02
But you can see what your
stomach on aspirin might

432
00:29:02 --> 00:29:04
be feeling like here.
 

433
00:29:04 --> 00:29:07
So if you're having an upset
stomach, something you might

434
00:29:07 --> 00:29:12
do is take Tums or Mylanta or
some other kind of a -- yeah,

435
00:29:12 --> 00:29:20
let's measure the p h here.
 

436
00:29:20 --> 00:29:23
So if you decide to take some
Milk of Magnesia after an upset

437
00:29:23 --> 00:29:27
stomach, are you hoping it
will be acidic or basic here?

438
00:29:27 --> 00:29:28
STUDENT: Basic.
 

439
00:29:28 --> 00:29:36
PROFESSOR: All right,
let's see what we get.

440
00:29:36 --> 00:29:38
All right, so this
is kind of thicker.

441
00:29:38 --> 00:29:41
Let's see how this works.
 

442
00:29:41 --> 00:29:46
I think you can start to see
the green at the bottom.

443
00:29:46 --> 00:29:50
So, it's white in here, so
this is not just color.

444
00:29:50 --> 00:29:53
So we'll let that
slowly mix in.

445
00:29:53 --> 00:29:56
What did we get for a p h, if
you can read it on there.

446
00:29:56 --> 00:29:57
This might be another no-go.
 

447
00:29:57 --> 00:29:59
STUDENT: It says 7, but--
 

448
00:29:59 --> 00:30:00
PROFESSOR: It's probably blue--
 

449
00:30:00 --> 00:30:26
PROFESSOR: So, how many people
have had lunch yet today?

450
00:30:26 --> 00:30:26
How many are going
to have lunch soon?

451
00:30:26 --> 00:30:27
How many are reconsidering
what they're going to eat for

452
00:30:27 --> 00:30:27
lunch based on this demo?
 

453
00:30:27 --> 00:30:34
PROFESSOR: Al right, we'll
take a look at one last thing

454
00:30:34 --> 00:30:34
that you might be consuming.
 

455
00:30:34 --> 00:30:37
Lemon juice, or actually
this is lime juice here.

456
00:30:37 --> 00:30:39
All right, we're probably
ending with an easy one here.

457
00:30:39 --> 00:30:43
What do you think, acidic or
basic for the lime juice.

458
00:30:43 --> 00:30:47
So, really, the question is
probably just the shade that

459
00:30:47 --> 00:30:50
we're going to get from
going from a purple here.

460
00:30:50 --> 00:31:05
All right, so what are we
reading for the lime juice?

461
00:31:05 --> 00:31:24
A 2, OK.
 

462
00:31:24 --> 00:31:29
PROFESSOR: OK, so that's the
end of our demo here, providing

463
00:31:29 --> 00:31:33
you with some tips about what
is corrosive and what is

464
00:31:33 --> 00:31:38
not corrosive around MIT.
 

465
00:31:38 --> 00:31:42
And I think the title of this
lecture on the syllabus is "is

466
00:31:42 --> 00:31:48
it safe to drink the water at
MIT," and the answer is a lot

467
00:31:48 --> 00:31:50
safer than drinking soda.
 

468
00:31:50 --> 00:31:58
So let's talk about some acids
in water some more, and

469
00:31:58 --> 00:32:03
introduce something you'll use
a lot, which is an acid

470
00:32:03 --> 00:32:08
ionization constant.
 

471
00:32:08 --> 00:32:18
So let's look at an example
of an acid in water.

472
00:32:18 --> 00:32:28
So say we have c h 3, c o o h
aqueous, so it's in water,

473
00:32:28 --> 00:32:30
which is our solvent.
 

474
00:32:30 --> 00:32:35
It's acting as an acid, so it's
giving off a hydrogen ion to

475
00:32:35 --> 00:32:42
water forming hydronium ion,
and forming its conjugate,

476
00:32:42 --> 00:32:47
which is missing
its hydrogen ion.

477
00:32:47 --> 00:32:51
All right, so here we have an
equation, and now we're going

478
00:32:51 --> 00:32:55
to introduce something which is
the acid ionization constant,

479
00:32:55 --> 00:32:59
or k a, and you'll be using k a
a lot in this course,

480
00:32:59 --> 00:33:02
we're also going to have
some k b's for bases.

481
00:33:02 --> 00:33:05
So the acid
ionization constant.

482
00:33:05 --> 00:33:08
And it's an equilibrium
constant, so you all know how

483
00:33:08 --> 00:33:11
to write equilibrium constants.
 

484
00:33:11 --> 00:33:14
So the equilibrium constant is
going to be products, which in

485
00:33:14 --> 00:33:21
this case is hydronium ions
times the concentration of the

486
00:33:21 --> 00:33:32
conjugate base of the acid
over the conjugate acid.

487
00:33:32 --> 00:33:36
And there is no water in that
equation, because here the

488
00:33:36 --> 00:33:39
water is pretty much pure.
 

489
00:33:39 --> 00:33:42
It's the solvent so its
concentration is not going

490
00:33:42 --> 00:33:45
to change very much,
so it is left off.

491
00:33:45 --> 00:33:50
And I can tell you that the
ionization constant here is 1 .

492
00:33:50 --> 00:33:53
76 times 10 to the minus 5.
 

493
00:33:53 --> 00:33:55
Again, that's temperature
dependent, so that's

494
00:33:55 --> 00:33:57
at 25 degrees.
 

495
00:33:57 --> 00:34:01
This is a small number,
so that tells us this is

496
00:34:01 --> 00:34:04
not a very strong acid.
 

497
00:34:04 --> 00:34:08
So it's not ionizing
very much in solution.

498
00:34:08 --> 00:34:12
That is a definition of a
weak acid, something that

499
00:34:12 --> 00:34:14
doesn't ionize very much.
 

500
00:34:14 --> 00:34:17
The definition of a strong
acid is something that

501
00:34:17 --> 00:34:19
does ionize quite a bit.
 

502
00:34:19 --> 00:34:26
So here then, we can write
equations generically

503
00:34:26 --> 00:34:28
for acids and bases.
 

504
00:34:28 --> 00:34:31
You can write an equation
generically, h a being as your

505
00:34:31 --> 00:34:35
acid, plus water, goes to
hydronium ions, and your

506
00:34:35 --> 00:34:39
conjugate of the acid,
which is a minus.

507
00:34:39 --> 00:34:42
So this is an acid,
h a, in water.

508
00:34:42 --> 00:34:47
We can also write it as h b
plus as an acid in water going

509
00:34:47 --> 00:34:51
to hydronium ion concentrations
and the conjugate, which

510
00:34:51 --> 00:34:54
is base, it's lost its
hydrogen ion as well.

511
00:34:54 --> 00:34:58
A strong acid is something that
has a k a greater than one.

512
00:34:58 --> 00:35:02
More products than
reactant at equilibrium.

513
00:35:02 --> 00:35:06
So that means it ionizes almost
completely, so it goes far

514
00:35:06 --> 00:35:10
toward products, ionizes
almost completely.

515
00:35:10 --> 00:35:14
A weak acid is something with a
k a of less than one, which

516
00:35:14 --> 00:35:17
means that when you put this
acid in water, it doesn't

517
00:35:17 --> 00:35:23
ionize very much, when
you have equilibrium.

518
00:35:23 --> 00:35:28
So you can tell if something is
a strong acid or not by looking

519
00:35:28 --> 00:35:32
at it's k a value, or
alternatively you can consider

520
00:35:32 --> 00:35:34
something called p k a.
 

521
00:35:34 --> 00:35:40
So the p k a is minus log of
the k a, and if you have a low

522
00:35:40 --> 00:35:44
value of k a, you'll have a
higher p k a, and the higher

523
00:35:44 --> 00:35:47
the p k a then, the
weaker the acid.

524
00:35:47 --> 00:35:52
So you can look it k a or you
can think about p k a in terms

525
00:35:52 --> 00:35:55
of whether something is
a strong acid or not.

526
00:35:55 --> 00:35:59
And so we'll just finish
up with this slide.

527
00:35:59 --> 00:36:02
So, up here we have some
very strong acids.

528
00:36:02 --> 00:36:07
These k a's are much greater
than one, and you have

529
00:36:07 --> 00:36:10
extremely low values for p k a.
 

530
00:36:10 --> 00:36:13
And if you keep going from
strong acids, again, a strong

531
00:36:13 --> 00:36:17
acid has a k a greater than
one, so these are all strong.

532
00:36:17 --> 00:36:19
And so, then weak acids
are less than one.

533
00:36:19 --> 00:36:24
And so down here you'd have
small numbers for k a, and so

534
00:36:24 --> 00:36:31
-- up here we have big k a's,
here we have smaller k a's, and

535
00:36:31 --> 00:36:35
the corresponding p k a's are
going up, and if we keep going,

536
00:36:35 --> 00:36:40
this table is very long, we're
going to get some very high p k

537
00:36:40 --> 00:36:44
a values when you have some
very, very small numbers.

538
00:36:44 --> 00:36:47
OK, we'll stop there
for today and continue

539
00:36:47 --> 00:36:49
acid base next time.
 

540
00:36:49 --> 00:36:50