Electrolysis
Higher Tier
The majority of this topic (spec points 1.55C to 1.60C) is Higher Tier chemistry-only content.
Conductivity of Compounds
Electric current is the flow of charged particles. This can be electrons (in metals and graphite) or ions (in solutions and molten compounds).
Covalent Compounds
Most covalent compounds do not conduct electricity in any state because:
- They have no freely moving ions or electrons
- Their molecules carry no overall electric charge
Covalent substances are used as electrical insulators (e.g. plastic cable coatings, rubber).
Exceptions: graphite and graphene conduct due to their delocalised electrons.
Ionic Compounds
Ionic compounds:
- Cannot conduct when solid: ions are held in fixed positions in the lattice and cannot move.
- Can conduct when molten or dissolved in water (aqueous solution): the ions are free to move and carry charge.
Cations and Anions
- Cation: positively charged ion (e.g. Na+, Cu2+). During electrolysis, cations migrate towards the cathode (negative electrode).
- Anion: negatively charged ion (e.g. Cl-, O2-). During electrolysis, anions migrate towards the anode (positive electrode).
Electrolysis
Electrolysis is the decomposition of an ionic compound by passing an electric current through it when the compound is molten or in solution.
Key components:
- Electrolyte: the ionic compound that is molten or dissolved and conducts electricity
- Electrodes: conductors dipped into the electrolyte; usually graphite (inert, does not react)
- Cathode: negative electrode; where reduction occurs (positive ions gain electrons)
- Anode: positive electrode; where oxidation occurs (negative ions lose electrons)
At the cathode: positive ions (cations) gain electrons and are reduced to form neutral atoms or molecules.
At the anode: negative ions (anions) lose electrons and are oxidised to form neutral atoms or molecules.
Electrolysis of Molten Compounds
Binary ionic compounds (two elements only, e.g. lead(II) bromide) produce their constituent elements on electrolysis when molten.
Predicting products:
- Positive ion migrates to cathode; the element formed is always the metal (or hydrogen).
- Negative ion migrates to anode; the element formed is always the non-metal.
Electrolysis of Molten Lead(II) Bromide
Method: Heat lead(II) bromide until molten. Insert graphite electrodes. Pass a current.
- Cathode: Pb2+ ions gain 2 electrons each and grey lead metal forms.
- Anode: Br- ions each lose 1 electron; bromine gas (brown) is produced.
Half-equations:
- Cathode (reduction): Pb2+ + 2e- → Pb
- Anode (oxidation): 2Br- - 2e- → Br2, or equivalently: 2Br- → Br2 + 2e-
Electrolysis of Aqueous Solutions
Aqueous solutions always contain water molecules, which dissociate to a small extent:
H2O ⇌ H+ + OH-
So the electrolyte contains ions from the dissolved compound AND H+ and OH- from water.
Which ion is discharged depends on the relative reactivity:
At the Cathode (reduction)
The less reactive ion (or element) is preferentially discharged:
- If the metal ion comes from an element below hydrogen in the reactivity series, the metal is deposited.
- If the metal ion comes from an element above hydrogen in the reactivity series, hydrogen gas is produced (the more reactive metal ion stays in solution).
At the Anode (oxidation)
- If halide ions (Cl-, Br-, I-) are present, the halogen is produced.
- If no halide ions are present, oxygen is produced (from OH- ions or water molecules).
Summary of Common Aqueous Electrolysis Results
| Solution | Cathode product | Anode product |
|---|---|---|
| Sodium chloride (NaCl) | Hydrogen gas (H2) | Chlorine gas (Cl2) |
| Dilute sulfuric acid (H2SO4) | Hydrogen gas (H2) | Oxygen gas (O2) |
| Copper(II) sulfate (CuSO4) | Copper metal (Cu) | Oxygen gas (O2) |
Writing Half-Equations
Half-equations show what happens at each electrode. Charges must balance on both sides.
Electrolysis of Aqueous Sodium Chloride
Cathode: 2H+(aq) + 2e- → H2(g) [reduction]
Anode: 2Cl-(aq) → Cl2(g) + 2e- [oxidation]
Electrolysis of Dilute Sulfuric Acid
Cathode: 2H+(aq) + 2e- → H2(g) [reduction]
Anode: 2H2O(l) → O2(g) + 4H+(aq) + 4e- [oxidation]
Electrolysis of Aqueous Copper(II) Sulfate
Cathode: Cu2+(aq) + 2e- → Cu(s) [reduction]
Anode: 2H2O(l) → O2(g) + 4H+(aq) + 4e- [oxidation]
Electrolysis of Molten Lead(II) Bromide
Cathode: Pb2+(l) + 2e- → Pb(l) [reduction]
Anode: 2Br-(l) → Br2(g) + 2e- [oxidation]
Oxidation and Reduction in Electrolysis
- Oxidation is loss of electrons: anions lose electrons at the anode (OIL: Oxidation Is Loss).
- Reduction is gain of electrons: cations gain electrons at the cathode (RIG: Reduction Is Gain).
Memory aid: OIL RIG (Oxidation Is Loss, Reduction Is Gain). Or remember: the ANode is where ANions go and OXidation occurs.
Practical: Electrolysis of Aqueous Solutions
Aim: To electrolyse solutions of sodium chloride, sulfuric acid and copper(II) sulfate and to collect and identify the products at each electrode.
Method:
- Add the aqueous solution to a beaker and cover the electrodes.
- Invert two small test tubes over each electrode to collect gaseous products.
- Connect electrodes to a power pack and switch on.
- Observe each electrode and test any gases collected.
Testing the products:
- Colourless gas at the cathode that burns with a “squeaky pop” with a burning splint: hydrogen
- Gas at the anode that relights a glowing splint: oxygen
- Gas at the anode that bleaches damp blue litmus paper: chlorine
- Pink-brown solid deposit at the cathode: copper
Exam Questions
Q: Explain why sodium chloride cannot conduct electricity when solid but can when dissolved in water.
When solid, the Na+ and Cl- ions are held in fixed positions in the ionic lattice and cannot move to carry charge. When dissolved in water, the ions are free to move through the solution and can carry charge.
Q: In the electrolysis of molten lead(II) bromide, explain what is formed at the cathode.
Lead. The positively charged Pb2+ ions are attracted to the negatively charged cathode. They gain 2 electrons each (are reduced) to form lead metal, which deposits at the cathode.
Q: Write the half-equation for the reaction at the anode during electrolysis of aqueous sodium chloride.
2Cl-(aq) → Cl2(g) + 2e- [oxidation]
Q: In the electrolysis of aqueous copper(II) sulfate, hydrogen is not produced at the cathode even though H+ ions are present. Explain why.
Copper is below hydrogen in the reactivity series, so Cu2+ ions are less reactive than H+ ions and are preferentially discharged at the cathode. The Cu2+ ions gain electrons and are reduced to copper metal rather than H+ ions being reduced to hydrogen gas.
Exam Tip
A common mistake is to say electrons flow through the electrolyte. They do not: electrons flow through the external circuit (wires and electrodes). It is the IONS in the electrolyte that move and carry charge inside the solution.