Solutions to exercises
243
or
G°
90.943 kJ mol
f
1
Δ
= −
−
3. The expression for the standard free enthalpy of formation of AgCl in terms
of thermodynamic quantities is
G°
H° T S°
f
f
f
Δ
Δ
Δ
=
−
with
H°
127.1kJ mol
f
1
Δ
= −
−
and
S° S° (AgCl ) S° (Ag ) 2
1
S° (Cl )
f
( s)
(s)
2 (g)
Δ
=
−
−
S° 96.2 42.5
2
223
57.8 J mol K
f
1
1
Δ
=
−
−
= −
−
−
We find
G°
91.09 kJ mol
f
1
Δ
= −
−
which is quite close to the value obtained from the electrochemical cell.
Conclusion – An electrochemical cell may be used to determine precisely the
standard enthalpy of formation of certain chemical compounds.
Solution 5.2 – Measurement of thermodynamic quantities
of metal fluorides
1. Reaction associated with anionic Frenkel disorder in CaF 2
F
#
F m F ′
i + V
•
F
2. Reaction for doping CaF 2 with
2 NaF
NaF $ Na ′
Ca + F
#
F + V
•
F
2 YF 3
YF 3 $ Y
•
Ca + 2F
#
F + F ′
i
3. Reaction of CaF 2 with water vapor
2F
#
F + H 2 O (g) $ O ′
F + V
•
F + 2HF (g)
or
CaF 2 + H 2 O (g) $ Ca
#
Ca + O ′
F + V
•
F + 2HF (g)
4. Examples of complexes
Complexes form from structure defects of opposite effective charge
2 (Na Ca V F )
#
Na ′
Ca + V
•
F m (Na Ca V F )
#
effective charge = 0
2 (Y Ca F i )
#
Y
•
Ca + F ′
i m (Y Ca F i )
#
……