Question

Calculate ΔG° for the reaction in Example 1, the decomposition of calcium carbonate, from ΔH° and ΔS° values. 

Answer 1

We have already calculated ΔS° for the reaction CaCO₃(s) → CaO (s) + CO₂ (g) as 160.6 J/K. We can find ΔH° for the reaction in a similar manner: 

ΔH° = (1 mol)[ΔH° for CaO(s)] + (1 mol)[ΔH° for CO2(g)] – (1 mol)[ΔH° for CaCO3(s)] = 

(1 mol) (–635.3 kJ/mol) + (1 mol) (–393.5 kJ/mol) – (1 mol) (–1207.0 kJ/mol) 

= 178.2 kJ 

Now we use Equation 23.6 to find the value of ΔG°: 

ΔG° = ΔH° – TΔS° = 178.2 kJ – 298.15 K ×160.6 JK×1 kJx1000 J 

= 130.3 kJ 

Notice that we used 298.15 K, or 25°C as the value of T. This is thermodynamic standard temperature. Also note how the ΔS° value was in joules per kelvin, while the ΔH° value was in kJ.