Electrolysis of Aqueous Solutions and Representation of Reactions at Electrodes as Half Equations [HT for half equations]

SPEC 4.4.3.4 – 4.4.3.5 • PAPER 1 • RP3

Aqueous Electrolysis

Four ions in solution. Decision rules at each electrode. Gas tests. Half equations on Higher Tier. The most-tested electrolysis topic.

Why Aqueous Is Different from Molten

In molten electrolysis (lesson 4.07), only the compound's two ions are present. In aqueous electrolysis, water is part of the system — and water itself partially ionises.

The Key Idea — Four Ions

Any aqueous ionic solution contains four ions: the two ions from the dissolved compound, PLUS H⁺ and OH⁻ from water (water partially ionises: H₂O ⇌ H⁺ + OH⁻).

Aqueous NaCl contains: Na⁺ + Cl⁻ + H⁺ + OH⁻

Examiner Alert (P1 Jun 2022 Q06.4)

"Most students recognised that hydrogen ions would be present in aqueous sodium chloride. However, only just over a half of students also recognised that hydroxide ions were present. Some stated that the fourth ion would be oxygen, showing a lack of understanding of the ionisation of water." The fourth ion is OH⁻ — not "oxygen", not "oxide".

Cathode (–) Decision Rule

At the cathode, two positive ions compete for discharge: the metal cation (e.g. Na⁺, Cu²⁺) and H⁺ (from water). One rule decides which wins.

The Rule

The less reactive ion is discharged at the cathode.

Metal LESS reactive than H

Metal is discharged → metal deposited on cathode.

Cu²⁺ + 2e⁻ → Cu

Examples: Cu, Ag.

Metal MORE reactive than H

H⁺ is discharged → hydrogen gas released.

2H⁺ + 2e⁻ → H₂

Examples: K, Na, Li, Ca, Mg, Al, Zn, Fe.

Reactivity Series Anchor

In the AQA reactivity series, hydrogen sits between iron and copper. So: K, Na, Li, Ca, Mg, Al, Zn, Fe are all more reactive than H → produce H₂ at cathode. Cu, Ag are less reactive than H → metal deposits at cathode.

Anode (+) Decision Rule

At the anode, two negative ions compete: the compound's anion (e.g. Cl⁻, SO₄²⁻) and OH⁻ (from water). One rule decides.

The Rule

If a halide ion (Cl⁻, Br⁻, I⁻) is present → the halogen is produced. Otherwise → OH⁻ is discharged → oxygen produced.

Halide present

Halogen is produced.

2Cl⁻ → Cl₂ + 2e⁻

2Br⁻ → Br₂ + 2e⁻

2I⁻ → I₂ + 2e⁻

No halide (e.g. SO₄²⁻, NO₃⁻)

Oxygen is produced from OH⁻.

4OH⁻ → O₂ + 2H₂O + 4e⁻

Examiner Alert (P1 Jun 2023 Q07.3)

ER: "Few students described the initial step outlined in the specification; that water molecules break down to form H⁺ ions and OH⁻ ions. Many seem to think that oxide or 'oxygen' ions are responsible, rather than hydroxide ions." The correct chain: H₂O ionises → OH⁻ → discharged at anode → O₂ produced. Never write "O²⁻" — water ionises to H⁺ + OH⁻, not O²⁻.

The Word "Discharged"

One of the most-penalised vocabulary mistakes in electrolysis. The examiner reports flag this every series.

"The term 'discharged' is widely misunderstood — the word means 'loses its charge', so to say that oxygen is discharged is not correct." — AQA Examiner Report, P1 Jun 2023 Q07.3

The Rule

"Discharged" applies to ions — they lose their charge to become neutral atoms or molecules. The resulting element is "produced" or "released" — never "discharged".

✗ Wrong✓ Right

"Oxygen is discharged""OH⁻ is discharged; oxygen is produced"

"Hydrogen is discharged""H⁺ is discharged; hydrogen is produced"

"Copper is discharged""Cu²⁺ is discharged; copper is produced"

"Aluminium is discharged""Al³⁺ is discharged; aluminium is released" (Jun23 Q07.2 ER)

The Four Worked Examples

Every aqueous electrolysis question follows the same two decision rules. These four examples cover all combinations.

SolutionCathode (–)Anode (+)Why

Aqueous NaCl (Na⁺, Cl⁻, H⁺, OH⁻)H₂Cl₂Na more reactive than H → H₂; halide present → Cl₂

Aqueous CuSO₄ (Cu²⁺, SO₄²⁻, H⁺, OH⁻)CuO₂Cu less reactive than H → Cu; no halide → O₂ from OH⁻

Aqueous Na₂SO₄ (Na⁺, SO₄²⁻, H⁺, OH⁻)H₂O₂Na more reactive than H → H₂; no halide → O₂

Aqueous CuCl₂ (Cu²⁺, Cl⁻, H⁺, OH⁻)CuCl₂Cu less reactive than H → Cu; halide present → Cl₂

Pattern

The cathode answer depends only on the metal; the anode answer depends only on whether a halide is present. Two independent decisions for any aqueous solution.

The Four Half Equations HT Only

These four cover every aqueous electrolysis case at GCSE. Memorise them — each must balance in both atoms and charges.

Cathode Options

2H⁺ + 2e⁻ → H₂

(metal more reactive than H)

Cu²⁺ + 2e⁻ → Cu

(metal less reactive than H)

Anode Options

2Cl⁻ → Cl₂ + 2e⁻

(halide present — also Br⁻, I⁻)

4OH⁻ → O₂ + 2H₂O + 4e⁻

(no halide)

Watch the OH⁻ Equation

The 4OH⁻ → O₂ + 2H₂O + 4e⁻ equation has FOUR products to track on the right side. Forget the H₂O and the equation won't balance: 4 O atoms on left = 2 (in H₂O) + 2 (in O₂) on right; 4 H atoms on left = 4 (in H₂O) on right; charge −4 on left = −4 from electrons on right. ✓

RP3 — Investigating Aqueous Electrolysis

Required Practical 3: Investigate what happens when aqueous solutions are electrolysed using inert electrodes.

Variables

Independent variable: the solution used (e.g. NaCl, CuSO₄, Na₂SO₄, CuCl₂)

Dependent variable: the products at each electrode (identified by tests)

Controls: same voltage / current; same concentration; same volume of solution; same inert electrodes (graphite or platinum)

The Method

Pour the aqueous solution into an electrolysis cell.

Insert two inert electrodes (graphite) connected to a power supply.

Switch on the power supply and electrolyse for a fixed time.

Collect any gas products in test tubes / measuring cylinders inverted over each electrode.

Test the gases (see next slide) to identify them.

Observe the cathode for any solid metal deposited.

Repeat with each of the four salt solutions; record results in a table.

Why "Inert" Electrodes

Inert means the electrode does NOT react with the products. Graphite (carbon) and platinum are the standard choices. If a reactive electrode were used, it could itself be oxidised at the anode, contaminating the result.

Identifying the Three Gas Products

Three gases can be produced at the electrodes. Each has a distinctive test that examiners expect verbatim.

GasTestPositive result

Hydrogen (H₂)Hold a lit splint at the mouth of the test tubeSqueaky pop

Oxygen (O₂)Insert a glowing splint into the test tubeSplint relights

Chlorine (Cl₂)Hold damp blue litmus paper at the mouthBleaches white (turns red briefly, then bleaches)

Vocabulary Precision

"Squeaky pop" not "popping sound" — exact phrase required.

Glowing splint for O₂ (not "lit splint" — that's for H₂).

"Damp blue litmus" — must be damp, not dry.

"Bleaches" — preferred to "turns white". Some mark schemes describe both stages: turns red briefly, then bleaches white.

The Apparatus — Aqueous Electrolysis

What to Note

Two inverted test tubes sit over each electrode to collect any gas produced. Both electrodes are graphite (inert). The aqueous solution is the electrolyte. After electrolysis, gases in the test tubes are tested with lit/glowing splints or damp litmus paper.

Worked Case — Aqueous CuSO₄

An aqueous solution of copper sulfate is blue. As electrolysis proceeds with inert electrodes, the colour fades. Why?

The Four Ions

CuSO₄(aq) contains: Cu²⁺, SO₄²⁻, H⁺, OH⁻.

Cathode (−)

Cu is less reactive than H → Cu²⁺ discharged.

Cu²⁺ + 2e⁻ → Cu

Pinkish-brown copper deposits on the cathode.

Anode (+)

No halide → OH⁻ discharged.

4OH⁻ → O₂ + 2H₂O + 4e⁻

Oxygen bubbles released at anode.

Examiner Alert (P1 Jun 2024 Q05.5)

ER: "Some students seem to believe that copper is blue (rather than copper ions)." Mark scheme: "Cu²⁺ / copper ions are blue". Copper metal is pinkish-red and insoluble; the blue colour comes from Cu²⁺ ions in solution. As Cu²⁺ is removed (discharged) the solution fades.

Mark-Scheme Phrase (P1 Nov 2021 Q07.7)

"Copper ions are discharged (from the solution)" — also accept "copper ions are removed / used up". Always specify ions, not "copper".

Worked Case — Aqueous NaCl + Industry

The chlor-alkali process: brine (concentrated aqueous NaCl) is electrolysed industrially to produce chlorine, hydrogen, and sodium hydroxide solution.

The Four Ions and the Decisions

NaCl(aq) contains Na⁺, Cl⁻, H⁺, OH⁻. Cathode: Na is more reactive than H → H⁺ discharged → H₂ gas. Anode: Cl⁻ is a halide → Cl⁻ discharged → Cl₂ gas.

Why an Alkaline Solution Forms (P1 Jun 2022 Q06.5–06.6)

After H⁺ and Cl⁻ are both discharged, the ions left in solution are Na⁺ and OH⁻ — that is sodium hydroxide solution (NaOH), which is alkaline.

"Sodium ions and hydroxide ions are left (in solution) (because) hydrogen ions are discharged / reduced (at the negative electrode to form hydrogen) (and because) chloride ions are discharged / oxidised (at the positive electrode to form chlorine)." — Mark scheme verbatim, P1 Jun 2022 Q06.6

Examiner Alert (P1 Jun 2022 Q06.6)

ER: "Many students did not refer to ions… Some otherwise creditworthy responses were let down by referring to chlorine ions instead of chloride ions, or by confusing the polarities of anode and cathode, or by confusing oxidation and reduction." Use chloride ion, not "chlorine ion".

Common Errors → Mark-Scheme Phrases

✗ Common error✓ Correct phrase

"2 ions in aqueous NaCl: Na⁺, Cl⁻""4 ions: Na⁺, Cl⁻, H⁺, OH⁻ "

"O²⁻ ions discharged at anode"" OH⁻ ions discharged ; oxygen produced"

"Oxide ions discharged"" Hydroxide ions (OH⁻) discharged"

"Oxygen is discharged""OH⁻ is discharged; oxygen is produced / released "

"Copper is blue"" Copper ions / Cu²⁺ are blue "

"Chlorine ions"" Chloride ions (Cl⁻)"

"Aluminium is discharged""Al³⁺ is discharged; aluminium is released "

"Popping sound"" Squeaky pop "

Why So Many Vocabulary Traps

Aqueous electrolysis pulls in chemistry from three places at once: ionic equations, water chemistry, and the reactivity series. The mark schemes test whether students can keep these vocabularies separate. Discharge → ions; colour → ions in solution, not metal; halide ion → "chloride" not "chlorine".

Practice 1 — The Four Ions

Aqueous sodium chloride solution is electrolysed in an industrial process. Two gases and an alkaline solution are produced. Which two ions are present in aqueous sodium chloride solution in addition to sodium ions and chloride ions?2 MARKS

Citation: P1 Jun 2022 Q06.4 — verbatim AQA stem.

MP1

hydrogen / H⁺ (ions)

MP2

hydroxide / OH⁻ (ions)

★

These are the two ions from water's partial ionisation (H₂O ⇌ H⁺ + OH⁻). Together with Na⁺ and Cl⁻, that's all four ions present in aqueous NaCl.

⚠️

Common wrong answers: "oxygen" or "oxide" — both wrong. Water ionises into H⁺ and OH⁻, NOT into H⁺ and O²⁻.

Practice 2 — Decision Rules

3 MARKS

Citation: P1 Jun 2021 (Nov21) Q07.3 — verbatim AQA stem.

Salt solutionPositive electrodeNegative electrode

Copper nitrate?copper

Potassium iodide??

MP1

Copper nitrate, positive electrode: oxygen. No halide present (NO₃⁻ is not a halide) → OH⁻ discharged → O₂ produced.

MP2

Potassium iodide, positive electrode: iodine. Halide (I⁻) present → I⁻ discharged → I₂ produced.

MP3

Potassium iodide, negative electrode: hydrogen. K is more reactive than H → H⁺ discharged → H₂ produced.

★

Pattern: for each electrode, apply ONE decision rule. Cathode: metal vs H reactivity. Anode: halide present or not.

Practice 3 — Half Equation HT Only

What is the half equation for the reaction at the positive electrode when aqueous calcium chloride solution is electrolysed?1 MARK

A 2Cl⁻ → Cl₂ + 2e⁻

B Cl₂ + 2e⁻ → 2Cl⁻

C 4OH⁻ → O₂ + 2H₂O + 4e⁻

D O₂ + 2H₂O + 4e⁻ → 4OH⁻Citation: P1 Jun 2024 Q05.4 — verbatim AQA stem.

1

Identify the anode product: Cl⁻ is a halide and is present (CaCl₂) → halogen is produced → Cl₂.

2

Identify direction: Cl⁻ is the reactant (ion is discharged); Cl₂ is the product. Eliminates B.

3

Eliminate C and D: these are the OH⁻/O₂ equations — the no-halide case. CaCl₂ has a halide, so anode produces Cl₂, not O₂.

★

Answer: A — 2Cl⁻ → Cl₂ + 2e⁻. Mark scheme verbatim.

⚠️

Examiner caution (Jun24 Q05.4 ER): "Two thirds of students identified the correct half equation, with the most popular distractor being the reverse half equation, with chlorine as a reactant rather than the product." Always: ion = reactant, element = product.

Lesson Checkpoint

Foundation Outcomes

State that aqueous solutions contain 4 ions : compound's ions + H⁺ + OH⁻ from water

Apply the cathode rule : metal more reactive than H → H₂; less reactive → metal

Apply the anode rule : halide present → halogen; no halide → O₂ from OH⁻

Predict products for the four standard cases (NaCl, CuSO₄, Na₂SO₄, CuCl₂)

Identify gases by tests: squeaky pop (H₂) , relights glowing splint (O₂) , bleaches damp blue litmus (Cl₂)

Use precise vocabulary: "discharged" applies to ions ; elements are produced

Higher Tier Outcomes

Write the four half equations: 2H⁺ + 2e⁻ → H₂; Cu²⁺ + 2e⁻ → Cu; 2Cl⁻ → Cl₂ + 2e⁻; 4OH⁻ → O₂ + 2H₂O + 4e⁻

Balance both atoms and charges in each half equation

Explain why oxygen is produced from water at the anode (water ionises → OH⁻ moves to anode → discharged)

Next Lesson

5.01: Energy Transfer During Exo/Endothermic Reactions (RP4). A new module — energy changes in chemistry. RP4 investigates how temperature changes during reactions.

Exam-Ready Summary

The Six Essentials

Aqueous solutions = 4 ions (compound's two + H⁺ + OH⁻).

Cathode: metal more reactive than H → H₂; less reactive → metal.

Anode: halide → halogen; no halide → O₂ from OH⁻.

Gas tests: squeaky pop / glowing splint relights / bleaches damp litmus.

"Discharged" = ions lose charge; elements are produced.

Coloured solutions: it's the ion that's coloured, not the metal.

Top Lost Marks

"2 ions in aqueous NaCl" — should be 4 (Jun22 Q06.4).

"O²⁻ / oxide / oxygen ions discharged" — should be OH⁻ (Jun23 Q07.3).

"Oxygen is discharged" — wrong; ions discharged, gases produced (Jun23 Q07.3).

"Copper is blue" — wrong; Cu²⁺ ions are blue (Jun24 Q05.5).

"Chlorine ions" — should be chloride ions (Jun22 Q06.6).

Reversed half equation with element as reactant (Jun24 Q05.4).

Exam Technique

Two questions to ask for any aqueous electrolysis: (1) is the metal more or less reactive than H? → cathode product. (2) is a halide present? → anode product. That's the entire decision tree. For half equations: ion = reactant, element = product, balance atoms AND charges.