Q13. ΔfG° of M³⁺ and E° of M³⁺/M from thermal data
Given the reaction 2M + 6H⁺ → 2M³⁺ + 3H₂, for which ΔH°298 = −3.00 kcal, the entropies are 6.5 cal/K for M, −22.2 cal/K for M³⁺, 31.2 cal/K for H₂ and −10.0 cal/K for H⁺; ΔfG° for H⁺ is 0.00. Calculate (a) the standard free energy of formation of M³⁺; (b) E° for the half-reaction M³⁺ + 3e⁻ → M.
Answer in brief
(a) ΔfG°(M³⁺) = −15.83 kcal mol⁻¹ = −66.2 kJ mol⁻¹; (b) E°(M³⁺/M) = −0.229 V.
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Coversstandard free energy of formationstandard electrode potentialΔG° = ΔH° − TΔS°ΔG° = −nFE°entropy change