Problem: A voltaic cell with Ni/Ni 2+ and Co/Co2+ half-cells has the following initial concentrations: [Ni2+] = 0.80 M; [Co 2+] = 0.20 M.(a) What is the initial Ecell?

Step 1. Write the two half-cell reactions and determine the half-cell potentials (refer to the Standard Reduction Potential Table)

Ni²⁺_(aq) + 2 e^-→ Ni_(s)                        E° = -0.230 V
Co²⁺_(aq) + 2 e^- → Co_(s)                     E° = -0.277 V                

Step 2. Identify the reduction half-reaction (cathode) and the anode half-reaction (anode)

Co²⁺_(aq) + 2 e^- → Co_(s)                     E° = -0.277 V               ↓ E° → oxidation → anode
Ni²⁺_(aq) + 2 e^-→ Ni_(s)                        E° = -0.230 V                ↑ E° → reduction → cathode

Step 3. Get the overall reaction by balancing the number of electrons transferred then adding the oxidation half-reaction and reduction half-reaction.

lose electrons → oxidation → anode

gain electrons → reduction → cathode