H. A. Neidig and R. T. Yingling
Lebanon Valley College
Annville, Pennsylvania
Thermodynamics of the Ionization of
Acetic and Chloroacetic Acids
This research-centered investigation of
a series of acid-base equilibria involves the use of some
of the important basic equations and concepts of tberr n o d y ~ ~ : i ~ ~Sillre
~ . r ~ .this
~ . stndy rorr14ntv. .:r\.ernl : ~ r w s
of hr+vwr ~ ~ h r n ~" i,~i r riur vi~lrtillv
.
.+nitt,~l
ior nsc 3s one
of the concluding laboratory investigations in a firstyear course. A basic thermodynamic study of the
acetic acid-water, chloroacetic acid-water, dichloroacetic acid-water, and trichloroacetic-water system
has been described,' and can be modified to meet the
requirements of a first-year college chemistry course.
The ionization of acetic acid, chloroacetic acid,
dichloroacetic acid, and trichloroacetic acid in aqueous
solutions can be represented by the equilibria:
HOA(aq)
= OA-(ilq) + Ht(aq)
The students are asked to determine the effect of the
structure of the acid on the equilibria and to discuss
the observed effects in terms of standard free energy,
enthalpy, and entropy changes.
Data for calculating the equilibrium constants are
ohtained by titrating an aliquot of each of the acids
with a sodium hydroxide solution. The pH of the
solution of the acid is measured with a pH meter after
the addition of each portion of titrant. Each of the
equilibrium constants is then calculated from a graph of
pH versus volume of sodium hydroxide added to the
acid solution using the half-titration m e t h ~ d . ~The
equilibrium constants are used to calculate the standard
free energy of ionization (AGIO)of each of the four acids:
AGIO
=
-RT In K
The second phase of the investigation involves the
calcnlation of the enthalpies of neutralization of each of
the four acids from experimental data. Timetemperature data are obtained on stoichiometric quantities of
1.00 M sodium hydroxide and 1.00 A t solutions of each
of the acids (0.50 M sodium hydroxide and 0.50 M
dichloroacetic acid are used because of the limited solubility of dichloroacetic acid in water) every 30 sec for a
period of ahout 5 min. The sodium hydroxide solution
and the acid solution are mixed together in a Thermolcup which serves as a calorimeter, and time-temperature data are collected for the reaction mixture for a 10
min post-mixing interval. Thermometers graduated
Teaoher's Guide, "Investigating Chemical Systems," Webster
Division, MeGrera-Hill Book Co.,Inc., New York, 1963, p. 3 2 1 .
' REILLEY,C. N., AND SAWYER,
D. T., "Experimentsfor Imtrumental Methods,"McGraw-HillBook, Co.,Inc., New York, 1961,
p. 23.
484
/ lournal of Chemical Education
in O.l°C units are used for all temperature measurements. The temperature changes for the neutralization
reactions are found from a plot of the time-temperature
data using the extrapolation method. The heat
transfer (Q) and the enthalpy of neutralization ( A H N )
are calculated. For the purposes of this investigation,
the enthalpies of dilution of the reactants and the
products are assumed to he negligible or to cancel.
On this basis, it is assumed that the experimentally
determined enthalpy of neutralization is equal to t,he
standard enthalpy of neutralization (AHNO).
Since the enthalpy change for a reaction depends only
upon the initial and final states of the system, the
neutralization of each of the acid solutions can he
divided into two steps. First, any molecules of undissociated acid which exist in the solution of the acid a t
the time of mixing become dissociated. Second, the
hydrogen ions resulting from the dissociation of the
acid are neutralized by the hydroxide ions. The
standard enthalpy of neutralization of hydrogen ions
by hydroxide ions can be ohtained experimentally by
studying the neutralization of hydrochloric acid with
sodium hydroxide a t different concentrations. For
this investigation a literature value of -13.34 kcal/
mole3 was used as the standard enthalpy of neutralization of hydrogen ions. The standard enthalpy of
ionization for each of the acids can he calculated using
Hess' Law. If the assumption is made that none of the
acid is ionized before neutralization, the standard
enthalpy of ionization (AHI'') is given by:
AHr"' = AHN'
+ 13.34 kcal/mole
However, the fraction of each acid (x) that was not
ionized before neutralization can be calculated from the
equilibrium constant. The standard enthalpy of
ionization ( a H l O )calculated on the basis of the actual
number of molecules of acid which dissociated at the
time of neutralization is given by:
Since the standard free energy of ionization has been
calculated for each of the acids and the standard
enthalpy of ionization has been calculated, the sbndard
entropy of ionization of each of the acids can be calculated :
Representative student data for AGIO, AHI", and
HALE,J. D., IZATT, R. M.,
Chem., 67,2605 (1963).
AND
CHRISTENSBN,
J. J., .I. Phys.
TASlo obtained for each of the acids are given in the
table. These thermodynamic data may be used to
discuss changes in bonding and in structure which occur
in the aqueous solutions of the acids as a result of the
presence of carbon-to-chlorine bonds in the substituted
acetic acids.
AGro, AHI", and TASIo for the Ionization of Acetic and Chloroacetic Acids
-
K
% Ionized
AGI (kcal/mole)
AHxD(kcal/mole)
AH1"'- (kcal/mole)
AHroL(kcnl/rnole)
TASlo (kcd/mole)
6
1.00M
Acetic
acid
1.00M
Chloroacetic
acid
0.50M
Diehloroacetic
acid
1 .OOM
Trichloroeoetic
acid
2.2 X lo-"
0.47
6.36
-13.9
-0.6
-0.6
-7.0
1.6 X
3.9
3.82
-15.6
-2.3
-2.4
-6.2
3.6 X
24
1.96
-15.4
-2.1
-2.8
-4.8
2.0 X lo-'
36
0.94
-15.4
-2.1
-3.3
-4.2
B a e d on the assumption that none of the aoidvias dissooi&d beforeneutralization.
Based on the calculated fraction of aoid that was not dissociated before neutralization.
Volume 42, Number 9, September 1965
/
485