Exercises
Exercise 5.1 – Determination of standard free enthalpy
of formation for AgCl
To determine the standard free enthalpy Δ f G° of formation of solid silver chloride AgCl, we use the following electrochemical chain:
C 1 / Ag / AgCl / C 2 ,Cl 2
α β
γ
where C 1 and C 2 represent the graphite electrodes, which are purely electronic
conductors.
1. Use the electrochemical potential of the species to write the expression
relating the standard free enthalpy Δ f G° for the formation reaction of AgCl,
with a potential difference ΔE measured between the terminals of the electrochemical chain. Assume that all thermodynamic equilibria hold.
2. A potential difference of 924 mV is measured between the terminals of
the electrochemical chain at 350 °C and under a chlorine partial pressure
of 5 # 10
4
Pa. Deduce the standard free enthalpy of formation of AgCl in
these conditions.
3. Compare the result with that calculated by using the thermodynamic data.
Data
Ag (s)
Cl 2(g)
AgCl (s)
Standard enthalpy of formation Δ f H° [kJ mol
−1
]
−
−
− 127.1
Absolute standard entropy S° [J mol
−1
K
−1
]
42.5
223
96.2
Exercise 5.2 – Measurement of thermodynamic quantities
of metal fluorides
We want to prepare a solid electrolyte based on calcium fluoride CaF 2 . The
dominant disorder in this ionic compound is anionic Frenkel disorder.