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Question

For CaF2(s) ⇌ Ca2+ (aq) + 2 F− (aq), K = 3.9 ×10−11 at 25 °C and the standard Gibbs energy of formation of CaF2(s) is −1167 kJ mol−1. Calculate the standard Gibbs energy of formation of CaF2 (aq) [so, Ca2+ (aq) + 2 F− (aq)].

Answer 1

Ca (s) + F₂ (g) ---> CaF₂ (s) ΔG⁰_f = -1167 kj/mole

CaF₂ (s) ---->Ca²⁺(aq) + 2F^1- (aq) ΔG⁰ = -RT LnK = - 8.314 J/mole x 298 K x Ln(3.9 x 0^-11) = 59.4 kJ/mole

Add the 2 reactions (CaF₂ (s) cancels.) Net reaction is

Ca (s) + F₂ (g) -----> Ca²⁺(aq) + 2F^1- (aq) ΔG⁰_f = -1167 kj/mole + 59.4 kj/mole = -1108 kj/mole