In a galvanic cell, how is E°cell calculated from the standard reduction potentials?

• A. E°cell = E°reduction at cathode.
• B. E°cell = E°anode − E°cathode.
• C. E°cell = E°cathode − E°anode.
• D. E°cell = E°cathode + E°anode.

Answer: (C)

The key idea is that the cell’s standard emf comes from the difference between the potential drops at the cathode and the anode. The cathode is the site of reduction, so you take its standard reduction potential as-is. The anode, where oxidation happens, effectively contributes the negative of its reduction potential. Putting that together gives E°cell = E°red(cathode) − E°red(anode). This difference represents the overall driving force for electrons to flow from the anode to the cathode under standard conditions, and it will be positive for a spontaneous galva nic cell. For example, if the cathode’s species has E°red = +0.80 V and the anode’s species has E°red = +0.30 V, the standard cell potential is 0.80 − 0.30 = +0.50 V. The other formulations would either ignore the anode’s contribution or misplace signs, leading to an incorrect value or a negative sign.