1. PROBLEM: Heating some metal carbonates, among them magnesium carbonate, leads to their decomposition. MgCO3(g) → MgO(s) + CO2(g) (a) Calculate DrH° and DrS° for the reaction. (b) Is the reaction spontaneous under standard conditions at 298 K? (c) Is the reaction predicted to be spontaneous at higher temperatures?
rHº = fHº[MgO(s)] + fHº[CO2(g)] – fHº[MgCO3(s)] rHº = 1 mol (– kJ/mol) + 1 mol (– kJ/mol)
– 1 mol (– kJ/mol) rHº = kJ/mol-rxn
rSº = Sº[MgO(s)] + Sº[CO2(g)] – Sº[MgCO3(s)] rSº = 1 mol (26.85 J/K·mol) + 1 mol ( J/K·mol) – 1 mol (65.84 J/K·mol) rSº = J/K·mol-rxn

2. (b) rGº = rHº – TrSº = 116.94 kJ/mol-rxn
– (298 K)( J/K·mol-rxn)(1 kJ/103 J) rGº = kJ/mol-rxn The reaction is predicted to be non-spontaneous at 298 K (rGº > 0)
(c) The reaction is predicted to be spontaneous at higher temperatures.