Topic 4.4: Entropy and spontaneity Questions

Dissolving ammonium chloride in water is endothermic (ΔH = +14.8 kJ mol⁻¹) yet the solid dissolves readily at room temperature (298 K).

Using the value ΔS = +75.0 J K⁻¹ mol⁻¹, explain, with a calculation of ΔG, why the dissolving is spontaneous. [3]

Consider the reaction below, used to make quicklime:

CaCO₃(s) → CaO(s) + CO₂(g)

Using the standard molar entropies S°: CaCO₃(s) = 93, CaO(s) = 40, CO₂(g) = 214 (all in J K⁻¹ mol⁻¹), calculate ΔS° for the reaction and state, with a reason, whether the sign is expected.

Without using any data, predict and explain the sign of ΔS for each of the following changes:

(a) the sublimation of solid iodine, I₂(s) → I₂(g)

(b) 2NO(g) + O₂

(g) → 2NO₂(g)

For the reaction 2SO₂(g) + O₂(g) → 2SO₃(g), ΔH = −198 kJ mol⁻¹ and ΔS = −188 J K⁻¹ mol⁻¹.

(a) Calculate ΔG at 298 K and state whether the reaction is spontaneous at this temperature.

(b) Calculate the temperature above which the reaction becomes non-spontaneous. [4]