Electrolysis of aqueous solutions
Lajoy Tucker
Teacher
What Is Aqueous Electrolysis?
Definition:
Electrolysis of an aqueous solution is the decomposition of an ionic compound dissolved in water into their elements using electricity.
An aqueous solution contains ions from both the ionic compound and ions from water, so more than one possible ion can be discharged at each electrode.
The Electrolyte Mixture
When an ionic compound dissolves in water, four ions are usually present:
Source | Ions Produced |
From the ionic compound (e.g. NaCl) | Na⁺ and Cl⁻ |
From water | H⁺ and OH⁻ |
The ions compete to be discharged at the electrodes depending on reactivity.
Water’s Role
Water partially ionises:
H⁺ may be reduced to hydrogen gas at the cathode.
OH⁻ may be oxidised to oxygen gas at the anode.
Rules for Predicting Products
Electrode | Ions Attracted | Discharge Rule | Product Formed |
Cathode (–) | Positive ions (cations) | The less reactive ion is discharged. If the metal is more reactive than hydrogen, hydrogen gas is formed. | Metal (if less reactive) or hydrogen gas |
Anode (+) | Negative ions (anions) | Halide ions (Cl⁻, Br⁻, I⁻) are discharged first. If no halides, hydroxide ions (OH⁻) from water discharge to form oxygen gas. | Halogen gas or oxygen gas |
Half Equations (HT Only)
At the Cathode (Reduction):
H+ ions gain electrons to form hydrogen gas
HT Only:
At the Anode (Oxidation):
OH- ions lose electrons to form oxygen gas and water
HT Only:
Water is decomposed into hydrogen and oxygen.
Example 1 - Electrolysis of Aqueous Sodium Chloride (NaCl(aq))
Ions present: Na⁺, Cl⁻, H⁺, OH⁻
At Cathode (–):
H+ ions gain electrons to form hydrogen gas
HT Only:
(Hydrogen discharged instead of sodium because sodium is more reactive.)
At Anode (+):
Cl- ions lose electrons to form oxygen gas and water
HT Only:
(Chloride ions discharged because they are halide ions.)
Products:
Hydrogen gas (at cathode)
Chlorine gas (at anode)
Remaining solution: Sodium hydroxide (NaOH)
No answer provided.
Example 2 - Electrolysis of Aqueous Copper(II) Sulfate (CuSO₄(aq))
Ions present: Cu²⁺, SO₄²⁻, H⁺, OH⁻
At Cathode (–):
Cu2+ ions gain electrons to form hydrogen gas
HT Only:
(Copper is less reactive than hydrogen.)
At Anode (+):
OH- ions lose electrons to form oxygen gas and water
HT Only:
Products:
Copper (solid) at cathode
Oxygen (gas) at anode
Observations:
Blue colour fades as Cu²⁺ ions are removed.
Copper coats the cathode.
Bubbles of gas appear at the anode.
No answer provided.
Required Practical 3 - Electrolysis of Aqueous Solutions
Aim:
Investigate what happens when aqueous solutions are electrolysed using inert (graphite) electrodes.
Method:
1. Pour electrolyte (e.g. CuSO₄ solution) into a beaker.
2. Insert graphite electrodes and connect to a power supply.
3. Pass an electric current through the solution.
4. Observe what happens at both electrodes.
5. Test any gases produced.
The products depend on which ions are present and their relative reactivity.
Common Examples Summary
Solution | Cathode Product | Anode Product | Notes/Observations |
NaCl (aq) | H₂ | Cl₂ | Na⁺ and OH⁻ stay in solution as NaOH |
CuSO₄ (aq) | Cu | O₂ | Blue colour fades |
H₂SO₄ (aq) | H₂ | O₂ | From H⁺ and OH⁻ only |
Na₂SO₄ (aq) | H₂ | O₂ | Na⁺ and SO₄²⁻ remain in solution |
Gas Tests and Observations
Gas | Test | Observation |
Hydrogen (H₂) | Insert a lit splint into the gas. | A squeaky pop sound is heard. |
Oxygen (O₂) | Insert a glowing splint into the gas. | The splint relights. |
Chlorine (Cl₂) | Hold damp blue litmus paper in the gas. | The paper turns red, then bleaches white. |
Each test confirms the identity of the gas produced during electrolysis.
Important Terms
Term | Definition |
Discharged | Ion loses or gains electrons to become neutral. |
Inert electrode | Does not react during electrolysis (e.g. graphite). |
Reduction | Gain of electrons (at cathode). |
Oxidation | Loss of electrons (at anode). |
No answer provided.
Summary - Foundation vs Higher Tier
Concept | Foundation Tier | Higher Tier (HT Only) |
Definition of electrolysis | Splitting of ionic compound with electricity | Use and balance half equations |
Predicting products | Apply basic reactivity rules | Use competition between ions |
Discharge rules | Halide before hydroxide | Link oxidation/reduction with half equations |
Gas testing | Know correct tests | Explain observations using ionic reactions |
Required practical | Identify products and gases | Write half equations and balance charges |
No answer provided.