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?

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. anges 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.

To calculate the standard reaction Gibbs free energy (∆G°) for each cell reaction, you are correct in using the formula:

∆G° = -nFE°

Where:
∆G° is the standard reaction Gibbs free energy,
n is the change in the number of moles of electrons,
F is the Faraday constant (96,485 C/mol),
and E° is the standard cell potential (given in the problem).

To determine the change in the number of moles of electrons (n) for each reaction, you need to examine the balanced equation for that reaction. The stoichiometric coefficients in front of the species participating in the redox reaction will help you determine the change in the number of moles of electrons.

a.) In the reaction: 3Cr3+(aq) + Bi(s) = 3Cr2+(aq) + Bi3+(aq)
The balanced equation shows that 3 electrons are transferred from the bismuth (Bi) to the chromium ions (Cr^3+). Therefore, n = 3 for this reaction.

b.) In the reaction: Mg(s) + 2H2O(l) = Mg2+(aq) + H2(g) + 2OH-(aq)
The balanced equation indicates that 2 electrons are transferred from magnesium (Mg) to form magnesium ions (Mg^2+). Therefore, n = 2 for this reaction.

Now that you have determined the values of n for each reaction, you can substitute these values along with the given E° values into the ∆G° formula to calculate the standard reaction Gibbs free energy for each reaction.