Redox and Electrode Potentials: Quick Revision

A brief note on Redox and Electrode Potentials. Read it before a test. Try each question at the end before you open its answer.

Half-cells and the Daniell cell

• A half-cell is a metal in a solution of its ions. A couple is the oxidised and reduced forms together, such as $\text{Zn}^{2+}/\text{Zn}$.
• Anode: oxidation, negative. Cathode: reduction, positive.
• Electrons flow from anode to cathode in the wire. Ions carry the charge through the salt bridge.
• Cell diagram: $\text{Zn}(s) \mid \text{Zn}^{2+}(aq) \parallel \text{Cu}^{2+}(aq) \mid \text{Cu}(s)$, with the anode on the left.
• Faraday: $Q = It$ and one mole of electrons carries $96500\ \text{C}$.

Electrode potentials

• The standard hydrogen electrode is $0.00\ \text{V}$: platinum, $\text{H}^+$ at $1\ \text{mol/L}$, $\text{H}_2$ at $1\ \text{bar}$.
• $E^\circ$ is a reduction potential at $1\ \text{mol/L}$, $1\ \text{bar}$ and $298\ \text{K}$.
• $E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}$.
• Reverse a half-reaction: change the sign. Multiply it: no change.

Predicting reactions

• Lower $E^\circ$: stronger reducing agent. Higher $E^\circ$: stronger oxidising agent.
• A reaction can happen if $E^\circ(\text{reduced}) - E^\circ(\text{oxidised})$ is positive.
• Metals with $E^\circ < 0$ release hydrogen from dilute acid.
• $\Delta G^\circ = -nFE^\circ$.
• $E^\circ$ says whether a reaction can happen, not how fast.

A worked example

A cell has a zinc anode and a silver cathode. Find its potential and $\Delta G^\circ$, for the reaction $\text{Zn} + 2\text{Ag}^+ \rightarrow \text{Zn}^{2+} + 2\text{Ag}$.

1. Potential. $E^\circ_{cell} = 0.80 - (-0.76) = 1.56\ \text{V}$.
2. Electrons. The equation moves $n = 2$.
3. Energy. $\Delta G^\circ = -2 \times 96500 \times 1.56 = -301\,080\ \text{J/mol}$, or $-301.1\ \text{kJ/mol}$.

Practice with new numbers

A program to try

The program finds the cell potential and the free energy change of three cells.

Mistakes to avoid

• Anode minus cathode. Always cathode minus anode.
• Doubling $E^\circ$ with the equation. $E^\circ$ does not change.
• Calling the anode positive. In a cell that makes electricity it is negative.
• Electrons in the salt bridge. Only ions move there.
• Losing the minus sign in $\Delta G^\circ = -nFE^\circ$.

Check yourself

1. Which electrode is the cathode in a Daniell cell?

2. Does silver dissolve in a solution of copper sulphate?

3. A cell has $E^\circ = 0.50\ \text{V}$ and $n = 2$. Is $\Delta G^\circ$ positive or negative?

4. Why does zinc protect iron from rusting?