Page 71 - Bruno Linder Elementary Physical Chemistry

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

56 Elementary Physical Chemistry

V m are the molar volumes. Neglecting the molar volumes of the solid and
liquid and replacing PV by RT (assuming ideal gas behavior), we get
15 1
∆[PV (g)] = (7 − )RT = − RT .
2 2
Thus,
1
o
∆U = −2795.1kJ + RT
comb
2
1 −1
= −2795.1kJ + × 8.3145 J K × 298 K
2
=[−2795.1+ 1.24] kJ = −2793.9kJ
Example 6.5. The standard enthalpy change of formation in the reaction
o
H 2 (g) + I 2(s) → 2HI(g) is ∆H =52.96 kJ.
f
o −1
The standard entropy change is ∆S = 166.36J K .
f
(a) Calculate the equilibrium constant K at T = 298 K.
o o o
∆G =∆ H − T ∆S =[52.96 − 298 × 0.166.36] kJ = 3.38 kJ
o −1
K =exp (−∆G /RT )= exp [−3380 J/(8.3145 JK × 298 K)]
=e −1.366 =0.255

(b) If T < 298 K, will the equilibrium constant, K , be greater or smaller

than K?
o

ln K (T )= ln K(T )+ [∆H (T )/R] × (1/T − 1/T )
o
∆ H> 0; If T <T the quantity (1/T − 1/T ) will be negative.

Thus, the second term in the formula will be negative and ln K < ln K.

Thus, K <K.

6.2.7. Elements of Electrochemistry. Electrochemical Cells
Much of electrochemistry is covered in other courses, and we will only touch
on the highlights which are relevant to thermodynamics.
There are two types:
(a) Electrolytic Cells, which produce non-spontaneous reactions by an
external electrical current. Example: the production of H 2 and O 2 from
H 2 O.