Calculate the Gibbs free energy and equilibrium constant

Question

Calculate the Gibbs free energy and equilibrium constant

The given reaction is: Ca/Ca+2//Cr+2/Cr

Should I reverse the reaction since the electromotive force would be negative or should I just continue calculating it? Or maybe I'm not understanding this question correctly?

Answer 1

You didn't supply the Eº for the cells, but upon looking them up I find them to be:

Ca²⁺ + 2e- ==> Ca Eº = -2.76 V

Cr²⁺ + 2e- ==> Cr Eº = -0.91 V

And since you wrote the cell as Ca|Ca²⁺ || Cr²⁺|Cr, if one follows convention, the cathode would be the chromium electrode and Ecell = -0.91 - (-2.76) = +1.85 and this is positive and hence spontaneous.

Now, to calculate the Gibbs energy...

∆G = -nFEº = -(2 moles e-)(96458 C/mol e-)(1.85 joules/C) = -356,895 J = -357 kJ