Electrolysis Principles and Applications

Chemistry· Unit 6: Redox Reactions and Electrolysis, Topic 2· 25 min read

1. Core Electrolytic Cell Concepts★☆☆☆☆⏱ 8 min

📘 Definition

Electrolytic Cell

An electrochemical cell that uses external electrical energy from a battery to drive a non-spontaneous redox reaction. This is the opposite of a spontaneous galvanic (voltaic) cell.

Example:

A cell for electrolysis of molten sodium chloride

In all electrochemical cells, oxidation always occurs at the anode, and reduction always occurs at the cathode. In electrolytic cells, the anode is connected to the positive terminal of the battery, so it is positively charged, and the cathode is connected to the negative terminal, so it is negatively charged.

📐 Worked Example

Write the half-equations for electrolysis of molten sodium chloride with inert platinum electrodes, and identify which reaction occurs at which electrode.

1
List the ions present in molten NaCl:
2
$Na^+(l), Cl^-(l)$
3
Reduction occurs at the negative cathode: Na⁺ gains electrons:
4
$Na^+(l) + e^- \rightarrow Na(l)$
5
Oxidation occurs at the positive anode: Cl⁻ loses electrons:
6
$2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$

2. Predicting Electrolysis Products★★★☆☆⏱ 10 min

To predict products, you must first identify if the electrolyte is molten or aqueous, and check if the electrodes are inert (do not react) or reactive. For molten electrolytes, only the ions provided can react. For aqueous electrolytes, water can also be oxidized or reduced, so you need to compare the reactivity of all possible species, accounting for overpotential in concentrated solutions.

📘 Definition

Overpotential

An extra voltage required to drive a kinetically slow reaction at the electrode. This can change the expected product in concentrated solutions.

Example:

Concentrated aqueous sodium chloride produces chlorine instead of oxygen at the anode due to overpotential.

📐 Worked Example

Predict the products of electrolysis of dilute sulfuric acid with inert graphite electrodes.

1
List all species present: H⁺ (from acid and water), SO₄²⁻, OH⁻ (from water)
2
At the cathode, possible reductions both produce hydrogen gas:
3
$2H^+(aq) + 2e^- \rightarrow H_2(g)$
4
At the anode, compare possible oxidations: sulfate oxidation has a higher electrode potential than water oxidation, so water is oxidized preferentially to oxygen:
5
$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$
6
Final products: hydrogen at cathode, oxygen at anode. Overall reaction is electrolysis of water.

Exam tip:

Always check the concentration of halide solutions before predicting the anode product. Concentrated chlorides give chlorine, dilute give oxygen.

3. Quantitative Electrolysis: Faraday's Laws★★★☆☆⏱ 7 min

Faraday's laws relate the amount of product formed to the electric charge passed through the electrolyte. Charge is calculated as $Q = I \times t$ , where $Q$ is in coulombs, $I$ is current in amps, and $t$ is time in seconds. The moles of electrons passed is $n (e^{-}) = \frac{Q}{F}$ , where $F = 96500$ C mol⁻¹.

📐 Worked Example

Calculate the mass of copper deposited at the cathode when a current of 2.0 A is passed through copper(II) sulfate solution for 30 minutes. ( $A_{r}$ Cu = 63.5, F = 96500 C mol⁻¹)

1
Convert time from minutes to seconds: $30 \times 60 = 1800$ s
2
Calculate total charge:
3
$Q = I \times t = 2.0 \times 1800 = 3600 \ C$
4
Calculate moles of electrons passed:
5
$n(e^-) = \frac{3600}{96500} \approx 0.0373 \ mol$
6
Copper(II) requires 2 electrons per atom: $C u^{2 +} + 2 e^{-} \to C u$ , so:
7
$n(Cu) = \frac{0.0373}{2} = 0.01865 \ mol$
8
Calculate mass of copper:
9
$mass = n \times A_r = 0.01865 \times 63.5 \approx 1.2 \ g$

4. Industrial Applications of Electrolysis★★☆☆☆⏱ 5 min

Electrolysis is used widely in industry for three main purposes: extraction of reactive metals that cannot be reduced by carbon, purification of impure metals, and electroplating for corrosion resistance or decoration. Three key examples for CIE are detailed below:

Extraction of aluminium: Purified aluminium oxide (bauxite) is dissolved in molten cryolite to lower the melting point from ~2000°C to ~900°C, reducing energy costs.
Purification of copper: Impure copper acts as the anode, pure copper as the cathode, with copper sulfate electrolyte. Copper dissolves from the anode and deposits on the cathode, impurities fall as anode mud.
Electroplating: A thin layer of a more expensive/less reactive metal is deposited onto an object. The object to be plated is the cathode, the plating metal is the anode.

📐 Worked Example

Explain why cryolite is used in aluminium extraction.

1
Pure aluminium oxide has an extremely high melting point of ~2000°C, which requires very high energy input and expensive infrastructure to maintain.
2
Cryolite lowers the melting point of aluminium oxide to ~900°C, which drastically reduces the energy required and production costs.
3
Cryolite also acts as a solvent, allowing the molten mixture to conduct electricity required for electrolysis.

5. Common Pitfalls

Wrong move:

Claiming the cathode is positive in an electrolytic cell

Why:

Students mix up charge assignments from galvanic cells

Correct move:

In electrolytic cells: anode = positive, cathode = negative; oxidation always at anode for all cells

Wrong move:

Predicting sodium metal as a product of aqueous sodium chloride electrolysis

Why:

Students forget water is present and is reduced preferentially to sodium ions

Correct move:

Hydrogen gas is produced at the cathode for aqueous sodium chloride, not sodium

Wrong move:

Using time in minutes directly to calculate charge Q = I × t

Why:

Current is measured in coulombs per second, so time must be in seconds

Correct move:

Always multiply time in minutes by 60 to convert to seconds before calculation

Wrong move:

Using 1 mole of electrons per mole of copper for copper(II) electrolysis

Why:

Students forget copper(II) ions have a +2 charge, so require 2 electrons per atom

Correct move:

Always check the charge of the ion to find the number of electrons transferred per mole of product

Wrong move:

Claiming oxygen is always the anode product for aqueous chloride solutions

Why:

Students ignore the effect of concentration and overpotential

Correct move:

For concentrated aqueous chloride solutions, chlorine is produced at the anode instead of oxygen

6. Quick Reference Cheatsheet

Concept	Key Fact	Formula
Anode/Cathode	Oxidation at anode, reduction at cathode \| Electrolysis: Anode +, Cathode -
Charge calculation	Charge = current × time	Q = I \times t
Moles of electrons	Moles of electrons = total charge ÷ Faraday constant	n(e^-) = \frac{Q}{F}, F = 96500 \ C \ mol^{-1}
Molten electrolyte	Only the ions present are oxidized/reduced
Dilute aqueous NaCl	Cathode: H₂, Anode: O₂
Concentrated aqueous NaCl	Cathode: H₂, Anode: Cl₂
Aluminium extraction	Cryolite lowers melting point of Al₂O₃ to ~900°C
Copper purification	Impure anode, pure cathode, CuSO₄ electrolyte

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 1
Product prediction electrolysis
2022 · 2
Faraday's law calculation
2021 · 4
Aluminium extraction process

What's Next

Electrolysis is a core part of electrochemistry in CIE A-Level Chemistry, and connects to broader topics including redox equilibria, standard electrode potentials, and industrial chemistry. The skills you learned here for predicting products and completing quantitative calculations are frequently tested in both multiple choice and structured questions, and are required for more advanced physical chemistry topics. Understanding industrial applications also links to inorganic chemistry topics covering production of key chemicals and materials. Mastery of this sub-topic will give you a strong foundation for combined questions that link electrolysis to other redox concepts that appear often in A-Level papers.