GCSE Chemistry revision

Electrolysis of aqueous solutions

Electrolysis

AQA 4.4.3.4 Foundation & Higher

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is Electrolysis of Aqueous Solutions?

Electrolysis splits compounds into their elements by passing an electric current through an electrolyte, where ions must be free to move.
Soluble compounds like copper sulfate and sodium chloride can simply be dissolved in water to form an aqueous (aq) electrolyte, rather than being melted.
In aqueous solutions, water partially splits into hydrogen ions (H⁺) and hydroxide ions (OH), adding extra ions alongside those from the dissolved compound.

1. What is Electrolysis of Aqueous Solutions? AQA

Notes

Electrolysis splits compounds into their elements by passing an electric current through an electrolyte, where ions must be free to move.
Soluble compounds like copper sulfate and sodium chloride can simply be dissolved in water to form an aqueous (aq) electrolyte, rather than being melted.
In aqueous solutions, water partially splits into hydrogen ions (H⁺) and hydroxide ions (OH), adding extra ions alongside those from the dissolved compound.

Setting Up an Electrolysis Experiment

A typical electrolysis setup includes a beaker of electrolyte, two electrodes, a connecting wire, and a power supply.
The positive electrode is called the anode and the negative electrode is called the cathode.
Positive ions (cations) are attracted to the negative cathode, while negative ions (anions) are attracted to the positive anode.

2. Setting Up an Electrolysis Experiment AQA

Notes

A typical electrolysis setup includes a beaker of electrolyte, two electrodes, a connecting wire, and a power supply.
The positive electrode is called the anode and the negative electrode is called the cathode.
Positive ions (cations) are attracted to the negative cathode, while negative ions (anions) are attracted to the positive anode.

Which Ion is Discharged at the Cathode?

At the cathode, both the metal ion from the compound and the hydrogen ion (H⁺) from water are attracted, but only one is discharged.
The rule is that the ion of the least reactive element is discharged at the cathode.
Using the reactivity series, if the metal is more reactive than hydrogen (e.g. sodium), then hydrogen ions are discharged instead of the metal ions.
If the metal is less reactive than hydrogen (e.g. copper), then the metal ions are discharged and the metal is deposited at the cathode.

3. Which Ion is Discharged at the Cathode? AQA

Notes

At the cathode, both the metal ion from the compound and the hydrogen ion (H⁺) from water are attracted, but only one is discharged.
The rule is that the ion of the least reactive element is discharged at the cathode.
Using the reactivity series, if the metal is more reactive than hydrogen (e.g. sodium), then hydrogen ions are discharged instead of the metal ions.
If the metal is less reactive than hydrogen (e.g. copper), then the metal ions are discharged and the metal is deposited at the cathode.

Which Ion is Discharged at the Anode?

At the anode, the negative ion from the compound and the hydroxide ion (OH⁻) from water compete to be discharged.
If a halide ion is present (e.g. Cl⁻, Br⁻, F⁻), the halide ion is always discharged in preference to the hydroxide ion.
If no halide ion is present, the hydroxide ion (OH⁻) is always the one that gets discharged at the anode.

4. Which Ion is Discharged at the Anode? AQA

Notes

At the anode, the negative ion from the compound and the hydroxide ion (OH⁻) from water compete to be discharged.
If a halide ion is present (e.g. Cl⁻, Br⁻, F⁻), the halide ion is always discharged in preference to the hydroxide ion.
If no halide ion is present, the hydroxide ion (OH⁻) is always the one that gets discharged at the anode.

Electrolysis of Aqueous Copper Sulfate

In aqueous copper sulfate, the four ions present are: Cu²⁺, SO₄²⁻, H⁺, and OH⁻.
At the cathode, copper ions are discharged (not hydrogen) because copper is less reactive than hydrogen in the reactivity series, forming solid copper that accumulates on the cathode.
At the anode, hydroxide ions are discharged (not sulfate, as sulfate is not a halide), producing oxygen gas and water according to: 4OH⁻ → 2H₂O + O₂ + 4e⁻

5. Electrolysis of Aqueous Copper Sulfate AQA

Notes

In aqueous copper sulfate, the four ions present are: Cu²⁺, SO₄²⁻, H⁺, and OH⁻.
At the cathode, copper ions are discharged (not hydrogen) because copper is less reactive than hydrogen in the reactivity series, forming solid copper that accumulates on the cathode.
At the anode, hydroxide ions are discharged (not sulfate, as sulfate is not a halide), producing oxygen gas and water according to: 4OH⁻ → 2H₂O + O₂ + 4e⁻

Electrolysis of Aqueous Sodium Chloride

In aqueous sodium chloride, the four ions present are: Na⁺, Cl⁻, H⁺, and OH⁻.
At the cathode, hydrogen ions are discharged (not sodium) because sodium is more reactive than hydrogen, producing hydrogen gas (H₂).
At the anode, chloride ions are discharged (not hydroxide) because chloride is a halide, producing chlorine gas (Cl₂).

6. Electrolysis of Aqueous Sodium Chloride AQA

Notes

In aqueous sodium chloride, the four ions present are: Na⁺, Cl⁻, H⁺, and OH⁻.
At the cathode, hydrogen ions are discharged (not sodium) because sodium is more reactive than hydrogen, producing hydrogen gas (H₂).
At the anode, chloride ions are discharged (not hydroxide) because chloride is a halide, producing chlorine gas (Cl₂).

Half Equations at the Electrodes

At the cathode, positive ions gain electrons (reduction) — for example, copper ions: Cu²⁺ + 2e⁻ → Cu
At the cathode, if hydrogen is discharged: 2H⁺ + 2e⁻ → H₂
At the anode, negative ions lose electrons (oxidation) — for example, chloride ions: 2Cl⁻→ Cl₂ + 2e⁻
At the anode, if hydroxide is discharged: 4OH⁻ → 2H₂O + O₂ + 4e⁻

7. Half Equations at the Electrodes AQA

Notes

At the cathode, positive ions gain electrons (reduction) — for example, copper ions: Cu²⁺ + 2e⁻ → Cu
At the cathode, if hydrogen is discharged: 2H⁺ + 2e⁻ → H₂
At the anode, negative ions lose electrons (oxidation) — for example, chloride ions: 2Cl⁻→ Cl₂ + 2e⁻
At the anode, if hydroxide is discharged: 4OH⁻ → 2H₂O + O₂ + 4e⁻

Key Rules Summary for Aqueous Electrolysis

At the cathode: compare the metal ion and H⁺ using the reactivity series — the less reactive element's ion is discharged.
At the anode: if a halide ion is present, it is discharged; if not, the hydroxide ion (OH⁻) is always discharged.
These rules allow you to predict the products at both electrodes for any aqueous electrolyte in your GCSE exam.

8. Key Rules Summary for Aqueous Electrolysis AQA

Notes

At the cathode: compare the metal ion and H⁺ using the reactivity series — the less reactive element's ion is discharged.
At the anode: if a halide ion is present, it is discharged; if not, the hydroxide ion (OH⁻) is always discharged.
These rules allow you to predict the products at both electrodes for any aqueous electrolyte in your GCSE exam.

Practical Applications and Importance

The electrolysis of aqueous copper sulfate is used industrially for electroplating and purifying copper.
The electrolysis of brine (aqueous sodium chloride) is a major industrial process that produces chlorine gas, hydrogen gas, and sodium hydroxide solution — all very useful chemicals.
Understanding which ions are discharged helps chemists and engineers control and design electrolysis processes for specific products.

9. Practical Applications and Importance AQA

Notes

The electrolysis of aqueous copper sulfate is used industrially for electroplating and purifying copper.
The electrolysis of brine (aqueous sodium chloride) is a major industrial process that produces chlorine gas, hydrogen gas, and sodium hydroxide solution — all very useful chemicals.
Understanding which ions are discharged helps chemists and engineers control and design electrolysis processes for specific products.