Electrolysis, Batteries and Corrosion: Quick Revision

A brief note on Electrolysis, Batteries and Corrosion. Read it before a test. Try each question at the end before you open its answer.

Products at the electrodes

• Electrolytic cell: anode positive, oxidation. Cathode negative, reduction. A battery forces the reaction.
• Cathode: the higher reduction potential is reduced. Water ($-0.83\ \text{V}$) beats $\text{Na}^+$, $\text{K}^+$, $\text{Mg}^{2+}$ and $\text{Al}^{3+}$, so hydrogen forms. $\text{Cu}^{2+}$ and $\text{Ag}^+$ give the metal.
• Anode: the lower potential is oxidised. Water (about $1.60\ \text{V}$ with overpotential) loses to chloride, bromide and iodide, so the halogen forms. Sulphate and nitrate are not oxidised, so oxygen forms.
• An active electrode can dissolve, as copper does in copper sulphate. Plating and refining use this.

Faraday's laws

• $Q = It$, with the time in seconds. $F = 96500\ \text{C/mol}$. One ampere-hour is $3600\ \text{C}$.
• First law: $m = ZQ$. In moles, $m = \dfrac{Q}{zF}M$.
• Second law: the same charge gives masses in the ratio $M/z$.
• Per mole of electrons: $\tfrac{1}{2}\ \text{mol}$ of $\text{H}_2$ and $\tfrac{1}{4}\ \text{mol}$ of $\text{O}_2$.

Batteries, fuel cells and corrosion

• Primary cells are used once. Secondary cells are recharged.
• Dry cell: zinc anode, $\text{MnO}_2$ cathode, about $1.5\ \text{V}$. Mercury cell: steady $1.35\ \text{V}$.
• Lead storage battery: Pb anode, $\text{PbO}_2$ cathode. Discharge makes $\text{PbSO}_4$ and water. About $2\ \text{V}$ a cell.
• Fuel cell: $\text{H}_2$ and $\text{O}_2$ fed in, water out, $E^\circ = 1.23\ \text{V}$.
• Rusting needs oxygen and water. Iron is the anode. Zinc and magnesium protect iron by sacrifice. Tin does not once it is scratched.

A worked example

A silver spoon is plated with a current of $1.15\ \text{A}$ for $17.5$ minutes. What mass of silver deposits?

1. Charge. $Q = 1.15 \times 17.5 \times 60 = 1207.5\ \text{C}$.
2. Electrons. $\dfrac{1207.5}{96500} = 0.0125\ \text{mol}$. Silver needs 1 for each ion.
3. Mass. $0.0125 \times 107.9 = 1.35\ \text{g}$.

Practice with new numbers

A program to try

The program has two functions. One finds the mass for a given current and time. The other finds the time for a given mass.

Mistakes to avoid

• Using minutes in $Q = It$. The time must be in seconds.
• Leaving out $z$. Copper needs 2 electrons and aluminium needs 3.
• Forgetting water at the electrodes. In a solution it competes, and sodium ions are never reduced.
• Calling the anode negative in an electrolytic cell. It is positive.

Check yourself

1. What forms at the cathode when aqueous sodium sulphate is electrolysed with platinum electrodes?

2. The same charge passes through a silver cell and a copper cell. Which gives more moles of metal?

3. What happens to the acid in a lead storage battery as it discharges?

4. Why does zinc protect a scratched iron sheet?