Electrolysis and Fuel Cells: Revision Notes

Condensed for the final weeks. For the full explanation, use the Electrolysis and Fuel Cells study guide.

The basics

Electrolysis — the breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity. It only works because ions are free to move and carry charge once the solid is melted or dissolved — a solid ionic lattice does not conduct, since its ions are locked in a fixed position.

Term	Meaning
Electrolyte	The molten or aqueous ionic compound
Cathode	Negative electrode → attracts cations (positive ions) → reduction
Anode	Positive electrode → attracts anions (negative ions) → oxidation

Memory hooks: PANIC — Positive is Anode, Negative Is Cathode. AN OIL RIG CAT — Anode Oxidation, Cathode Reduction. Note this is the reverse of a battery cell, where the positive terminal is conventionally treated differently — electrolysis and cell notation are a common source of confusion precisely because the electrode polarity convention flips.

Ions must be free to move, which is why solids do not conduct.

Molten compounds — simple

Only two ions present.

Molten PbBr2:
  Cathode:  Pb2+  +  2e-  ->  Pb        (silvery metal)
  Anode:    2Br-  ->  Br2  +  2e-       (red-brown vapour)

Aqueous solutions — water competes

Water supplies H⁺ and OH⁻, so four ions are present. Use these rules:

At the cathode: the less reactive of the metal and hydrogen is discharged. → Metal below hydrogen in the reactivity series (Cu, Ag) → metal deposited. → Metal above hydrogen (Na, K, Ca, Mg, Al, Zn, Fe) → hydrogen gas.

At the anode: halides are discharged in preference to OH⁻ if concentrated; otherwise oxygen from OH⁻.

Concentrated NaCl(aq):   cathode H2      anode Cl2
Dilute NaCl(aq):         cathode H2      anode O2
CuSO4(aq), inert:        cathode Cu      anode O2
Dilute H2SO4, inert:     cathode H2      anode O2   (ratio H2:O2 = 2:1 by volume)

Dilute sulfuric acid contains no halide, so it behaves like dilute NaCl at the anode: hydrogen at the cathode (squeaky pop with a lit splint), oxygen at the anode (relights a glowing splint), collected in a 2 : 1 volume ratio of hydrogen to oxygen — the same ratio as in water, H₂O.

Copper sulfate is a special case worth knowing separately. With inert (carbon/graphite) electrodes, copper deposits at the cathode and oxygen forms at the anode, as the table above shows. But with copper electrodes, copper still deposits at the cathode — the copper anode dissolves instead of oxygen being released. This is the basis of both electroplating (below) and purifying copper.

Electroplating

Object to be plated = cathode. Pure plating metal = anode. Electrolyte contains ions of the plating metal. Purpose: appearance and corrosion resistance. Metal ions from the dissolving anode are deposited onto the object at the cathode, so the plating solution’s concentration stays roughly constant throughout.

Hydrogen–oxygen fuel cell

A hydrogen-oxygen fuel cell produces electricity directly from a chemical reaction, rather than by burning the fuel first.

Overall:  2H2  +  O2  ->  2H2O

Advantages: only product is water, no CO₂ at point of use, more efficient than combustion, no recharging needed.

Disadvantages: hydrogen is hard to store and transport, highly flammable, expensive, and producing the hydrogen may itself release CO₂.

Fuel cell vs petrol/gasoline engine:

	Fuel cell	Petrol engine
Product	Water only	CO₂, water, and pollutants (CO, NOₓ)
Efficiency	Generally higher	Loses more energy as heat
Fuel storage	Hydrogen is difficult and costly to store	Petrol is easy to store and transport
Infrastructure	Refuelling not widely available	Petrol stations are everywhere

Exam traps

• Cathode is negative in electrolysis — the reverse of a battery cell.
• In aqueous solutions, remember water contributes ions; don’t ignore it.
• Half equations must balance charge with electrons: electrons on the right for oxidation, left for reduction.
• Silvery metal at the cathode, gas at the anode — describe observations when asked.
• A fuel cell is not “pollution-free” overall if the hydrogen came from fossil fuels.
• Forgetting that copper sulfate electrolysed with copper electrodes behaves differently from inert electrodes — the anode dissolves rather than releasing oxygen.
• Assuming a fuel cell is automatically more efficient than every other power source without qualifying the comparison against a specific alternative, such as a petrol engine.

Self-test

1. Name the products of electrolysing molten aluminium oxide.
2. Concentrated potassium chloride solution — give the product at each electrode.
3. Write the cathode half equation for copper(II) sulfate solution.
4. Which electrode is the object during electroplating?
5. Give one advantage and one disadvantage of a hydrogen fuel cell.
6. How does electrolysing copper sulfate with copper electrodes differ from using inert electrodes?
7. Give one difference between a hydrogen fuel cell and a petrol engine, other than the product formed.

Answers: 1. Aluminium at the cathode, oxygen at the anode. 2. Cathode: hydrogen (potassium is above hydrogen in reactivity). Anode: chlorine (halide, concentrated). 3. Cu²⁺ + 2e⁻ → Cu. 4. The cathode (negative electrode). 5. Advantage: the only product is water, with no CO₂ at point of use. Disadvantage: hydrogen is difficult and dangerous to store and transport, and may be made using fossil fuels. 6. With copper electrodes, the anode dissolves into solution instead of oxygen being released, while copper still deposits at the cathode either way. 7. Any one: a fuel cell is generally more efficient (loses less energy as heat), or hydrogen is harder to store and transport than petrol, or hydrogen refuelling infrastructure is far less widely available.

For the full worked explanation with additional detail, see the Electrolysis and Fuel Cells study guide; for exam-style questions with full mark schemes, see the Electrolysis practice questions.