Solutions to exercises
Solution 5.1 – Determination of standard free enthalpy
of formation for AgCl
1. Expressions for equilibria at the various interfaces
Interface α
e(C 1 ) m e(Ag (s) )
e,Ag(s)
e,C l
μ
μ
=
α
α
u
u
Interface β
Ag (s) m Ag
+ + e
Ag(s)
Ag
e,Ag(s)
μ
μ
μ
=
+
β
β
α
+
u
u
Homogeneous AgCl (s) phase a
a
Ag
Ag
=
β
γ
+
+
Ag
Ag
μ
μ
=
β
γ
+
+
u
u
Interface γ
Ag
+ + 2
1
Cl 2(g) + e(C 2 ) m AgCl (s)
Ag
2
1
Cl
e,C
2(g)
2
AgCl(s)
μ
μ
μ
μ
+
+
=
γ
γ
γ
γ
+
u
u
Combining the various equations gives
E
F
F
1
2
1
e,Cl
e,C
Ag(s)
Cl (g)
AgCl(s)
2
2
μ
μ
μ
μ
μ
Δ =
−
=
+
−
α
γ
β
γ
γ
u
u
`
j
Ag(s)
Cl (g)
°
°
E
F
1
2
1
2F
RT
ln P°
P
AgCl(s)
Cl
2
2
μ
μ
Δ =
+
−
+
β
γ
°
μ
γ
`
j
The formation reaction for AgCl (s) is
Ag (s) + 2
1
Cl 2(g) m AgCl (s)
The corresponding standard free enthalpy is
G°
2
1
f
AgCl(s)
Ag(s)
C l (g)
2
μ
μ
μ
Δ
=
−
−
γ
°
°
which gives
E
F
G°
2F
RT
ln P°
P
f
Cl 2
Δ
Δ
= −
+
where P° is the standard pressure.
2. The standard free enthalpy of formation of AgCl is
G°
F E
2
RT
ln P°
P
f
Cl 2
Δ
Δ
= −
+
G°
(0.924 96 480)
2
8.314 623
ln 10
5 10
f
5
4
#
Δ
= −
+
#
#