Calculating Voltages of Electrochemical Cells

1. Calculating Voltages of Electrochemical Cells Mr. Chapman Chemistry 30

2. Understanding the Standard Reduction Potentials Table • The table of Standard Reduction Potentials indicates which substances are the best at competing for electrons. • The higher the value for E°, the better that half-reaction is at competing for electrons. • In an electrochemical cell, the half-reaction with the highest value of E° will be reduced, and hence will be the cathode in the cell.

3. Example: • Which of the following will be reduced if an electrochemical cell is made between the two reactions? E° Cu2+(aq) + 2e- → Cu(s)  0.34 V Zn2+(aq) + 2e- → Zn(s) -0.76 V Answer: Copper will be reduced. This is because copper is better at competing for electrons, so it will force zinc to be oxidized. Notice that above we wrote both of the reactions as reductions.

4. Electrochemical Cell Notation • When we create an electrochemical cell, we have a special notation that we use to show the formation of the cell. • For example, to write down in short form the cell formed between the Zn and Cu half-cells in the previous examples, we write this: Cu|Cu2+|| Zn|Zn2+

5. Calculating Voltage • Since we now know whether it is copper or zinc being oxidized (zinc is being oxidized), we can calculate the voltage for the cell by reversing the zinc half-reaction. E° Cu2+(aq) + 2e- → Cu(s) 0.34 V Zn(s)  → Zn2+(aq) + 2e- +0.76 V ---------- Cu2+(aq) +  Zn(s)  → Zn2+(aq) + Cu(s) = 1.10 V

6. Notice the Steps Involved... • Write down both of the half-reactions involved in the electrochemical cells. • Use the standard potential reduction table to determine the voltage of each half-reaction. • Determine which half-reaction will undergo oxidation and which will undergo reduction. • Switch the direction of the equation that undergoes oxidation, as well as the sign of the voltage. Add the voltages together.

7. Important Point • The voltage of an electrochemical cell will always be positive. Reactions with a positive E° are always spontaneous, and all electrochemical cells are spontaneous reactions. • This makes it easy to see which equation to flip before you add them together. Just flip the equation that will give you a positive voltage. • Important: Before you add voltages, you need to balance the equation. This does not, however, changes values for E°.

8. Example Questions • Using your Standard Reduction Potential Table, determine the voltage of the following circuits: • Al|Al3+ and Pb|Pb2+ • Na|Na+ and Mg|Mg2+ • Cu|Cu2+ and Cd|Cd2+