Page 70 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

Phase and Chemical Equilibria 55

c) Also, consider the equilibrium constant for the reaction

+
−
2H 2 O(l) = H 3 O (aq) + OH (aq) (6.23)
A neutral system in equilibrium has

+
[H 3 O ]= [OH ]= 10 −7 mol/L (6.24)
−

The activity of the pure liquid is essentially constant, yielding an
equilibrium constant, denoted as K w ,equal to

K w =10 −14 (6.25)

Example 6.4. The standard enthalpies of formation of C 6 H 5 COOH(s),
CO 2 (g) and H 2 O(l) are respectively −386.0 kJ, −285.8kJ and −393.5kJ.
Calculate

o
(a) the standard enthalpy, ∆H comb ,and
(b) the standard energy, ∆U o ,
comb

of combustion of [C 6 H 5 COOH(s)].

The combustion reaction is

15
C 6 H 5 COOH(s) + O 2 (g) → 7CO 2 (g) + 3H 2 O(l)
2

and so

o
o
o
(a) ∆H o =7∆H [CO 2 (g)] + 3∆H [H 2 O(l)] − ∆H [C 6 H 5 COOH(s)]
comb f f f
=7(−285.8) + 3(−393.5) − (−386.0) = −2795.1kJ
o o
(b) ∆U =∆H − ∆(PV )
comb comb
∆(PV )= 7PV m [CO 2 (g)] + 3PV m [H 2 O(l)] − PV m [C 6 H 5 COOH(s)]
15
− PV m [O 2(g)]
2