Question

Calculate the standard reaction Gibbs free energy for the following cell reactions:
a.)3Cr3+(aq) + Bi(s)= 3Cr2+(aq) + Bi3+(aq)
with Ecell=-.61V
b.)Mg(s) + 2H20(l)=Mg2+(aq)+H2(g)+2OH-(aq) with Ecell=2.36 V

I know the formula. I just need help figuring out n (change in number of moles of electrons). Is it 0 for a and 1 for b?

Answer 1

No. Each Cr changes from +3 to +2 which is a change of 1e per mol and you have 3 mols so that's 3 electrons. Or you can do it from Bi. Bi changes from 0 on the left to 3+ on the right or a change of 3 e per mol.
b. 2e. Mg on the left to Mg^2+ on the right is 2e/mol.