Electrolysis
Introduction
Electrolysis is the process of decomposing a chemical compound in a molten or aqueous state by passing an electric current through it. In ICSE Class 10 Chemistry, you study the mechanism of electrolysis and its practical applications.
Electrolytes and Non-Electrolytes
| Property | Electrolyte | Non-Electrolyte |
|---|---|---|
| Conducts electricity in molten/aqueous state | Yes | No |
| Dissociates into ions | Yes | No |
| Examples | NaCl, CuSO₄, H₂SO₄ | Sugar, alcohol, urea, benzene |
Strong vs Weak Electrolytes
| Strong electrolyte | Weak electrolyte |
|---|---|
| Fully ionised in solution | Partially ionised |
| High conductivity | Low conductivity |
| Examples: NaCl, HCl, NaOH, KOH | Examples: CH₃COOH, NH₄OH, H₂CO₃ |
Key Terms
| Term | Definition |
|---|---|
| Electrolysis | Decomposition using electric current |
| Electrolyte | Compound that conducts electricity in molten/aqueous state |
| Electrodes | Conductors that carry current into the electrolyte |
| Anode | Positive electrode (anions go here, oxidation occurs) |
| Cathode | Negative electrode (cations go here, reduction occurs) |
| Anions | Negatively charged ions (move to anode) |
| Cations | Positively charged ions (move to cathode) |
Electrolysis of Molten PbBr₂
Electrolyte: Molten lead(II) bromide (PbBr₂)
Ionisation: PbBr₂ → Pb²⁺ + 2Br⁻
| Electrode | Reaction | Product |
|---|---|---|
| Cathode (−) | Pb²⁺ + 2e⁻ → Pb (Reduction) | Lead metal |
| Anode (+) | 2Br⁻ → Br₂ + 2e⁻ (Oxidation) | Bromine gas |
Observation: Greyish lead metal deposits at the cathode; reddish-brown bromine fumes at the anode.
Electrolysis of Acidified Water
Electrolyte: Water with a few drops of H₂SO₄ (to provide ions).
Ionisation: H₂SO₄ → 2H⁺ + SO₄²⁻; H₂O ⇌ H⁺ + OH⁻
| Electrode | Reaction | Product |
|---|---|---|
| Cathode (−) | 2H⁺ + 2e⁻ → H₂ (Reduction) | Hydrogen gas |
| Anode (+) | 4OH⁻ → 2H₂O + O₂ + 4e⁻ (Oxidation) | Oxygen gas |
Volume ratio: H₂ : O₂ = 2 : 1 (at cathode: 2 volumes H₂; at anode: 1 volume O₂)
Electrolysis of Aqueous CuSO₄
Electrolyte: Aqueous copper(II) sulphate
Using INERT electrodes (platinum/graphite):
| Electrode | Reaction | Product |
|---|---|---|
| Cathode (−) | Cu²⁺ + 2e⁻ → Cu (Reduction) | Copper metal deposited |
| Anode (+) | 4OH⁻ → 2H₂O + O₂ + 4e⁻ (Oxidation) | Oxygen gas |
Using ACTIVE electrode (copper anode):
| Electrode | Reaction | Change |
|---|---|---|
| Cathode (−) | Cu²⁺ + 2e⁻ → Cu | Copper deposited (cathode gains mass) |
| Anode (+) | Cu → Cu²⁺ + 2e⁻ | Copper dissolves (anode loses mass) |
This is the basis of electrorefining of copper.
Applications of Electrolysis
Electroplating
Coating a cheap metal with a thin layer of a more expensive metal.
| Application | Metal coated | Object (cathode) | Electrolyte |
|---|---|---|---|
| Silver plating | Silver | Object to be plated | AgNO₃ + KCN |
| Chrome plating | Chromium | Object to be plated | Cr₂(SO₄)₃ |
| Gold plating | Gold | Object to be plated | K[Au(CN)₂] |
Electrorefining of Copper
- Impure copper is made the anode.
- Pure copper is made the cathode.
- Electrolyte: CuSO₄ + H₂SO₄.
- Pure copper deposits on the cathode; impurities settle below the anode.
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing anode and cathode reactions | Anode: oxidation (loss of e⁻); Cathode: reduction (gain of e⁻) |
| Thinking H₂O ionises fully | Water is a weak electrolyte (partial ionisation) |
| Writing wrong product at anode for CuSO₄ with inert electrodes | O₂ is produced from OH⁻, not SO₄²⁻ |
| Forgetting that active anode dissolves | With copper anode, Cu dissolves instead of O₂ being produced |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: electrolysis of molten PbBr₂, acidified water, CuSO₄; electroplating; electrorefining; identifying products at each electrode.
Marks Blueprint: Electrolysis mechanisms — 4 marks, Electroplating/Electrorefining — 2 marks, Terminology — 2 marks.
Self-Test Questions
-
Define electrolysis. Differentiate between electrolytes and non-electrolytes.
-
Describe the electrolysis of molten PbBr₂ with electrode reactions.
-
During electrolysis of acidified water, which gas is collected at the cathode and which at the anode? What is the volume ratio?
-
Explain the electrorefining of copper with a labelled diagram.
-
What is electroplating? Why is the object to be plated made the cathode?
-
Distinguish between strong and weak electrolytes with examples.
