III. Electrochemical Cells (continued):Calculating Cell Voltages

The standard voltage for any electrochemical cell can be calculated from the theoretical standard voltage of each of the “half-cells”.

eg: Calculate the standard cell voltage for a zinc // copper cell.

E°cell = E°oxid + E°red

Standard cell

potential

Standard oxidation potential

Standard reduction potential

= +

Note: values on SRP table are for reduction half-reactions.To get standard oxidation potential, change sign of SRP.

E°cell = 0.76V + 0.34V = 1.10V

The Hydrogen Reference Electrode:

All half cell potentials are measured relative to H2.

eg: H2 + Cu2+ Cu + 2H+

If measured voltage under standard conditions is 0.34V, how do we know how much voltage comes from each half-reaction?

H2 2H+ + 2e-

Cu2+ + 2e- Cuor

In reality neither half-reaction alone provides any voltage.

We arbitrarily define the H2 half-reaction to be 0.0V.

E°cell = E°oxid + E°red

0.34V = 0.0V + E°red

The voltage of the reduction of Cu2+ is 0.34V.

A substance with a positive SRP is more easily reduced than H+ and a substance with a negative SRP is not as easily reduced H+.

Notes: Voltage is independent of the number of e-’s

transferred.

A negative value for E°cell means the forward reaction is not spontaneous (the reverse reaction is.)

Increasing reactant concentration or decreasing product concentration will increase the voltage of the cell.

If a cell reaches equilibrium E°cell = 0V (dead battery)