148

CHEMICAL EQUILIBRIUM

Acid.	-£i(ro°)	-£2(40°)	q
Acetic	.	.	.	0-0000179	0-0000187	^157
Propionic	.	.	.	138	141	126
Butyric	.	.	-	166	162	—144
Here q is the heat that would be developed—no external
work being done—if a/ kilogram-molecule of acid were
jo + 40 «,        ..    .
formed la solution at
_= 	j— from its ions.
%
Calorimetrically this quantity may be found as the
difference between the heats of neutralization JV^ and N% of
dilute nitric and, say, acetic acid by a dilute strong base.
For
HNO_8  +NaOH = NaNO_8      4-H_2O   .    .    .    .    N^

and remembering that nitric acid and the sodium saJts are
almost completely dissociated, but acetic acid hardly at all,,
in solutions such, as Thomsen used in his calorimetrie
researches ( i gram-molecule of acid or base in 200 gram-
molecules of water), these equations may lie written in the
form
= E_20
H + OH
Difference"H4 C_2H_30_2 = C_2H_4O_2    .    .    .    JV^- JT_2 = q.
The values of -ZV, — J7"_2 have "been found to be very small,
in accordance with tlie preceding theory :

Acetic acid
Propionie acid
Butyric acid

n! — N"_a ~ 13680 — 13400 « a8o
N"_L — N"_a = 13680 — 13480 « 200
N\ — N_2 « 13680 — 13800 m —120

and have the required positive sign in the case of acetic
and propioaie acids, and tke negative one for butyric,
IB.   Heterogeneous Equilibrium.
i. Eelations at Constant Temperature.
(a) Ju presence of gases.    Tke equation which served as
basis for dealing with homogeneous equilibrium,