SSC CGL Chemistry · Chemical Reactions
Types of Reactions, Redox and Rates
Combination and decomposition reactions with the textbook examples, physical and chemical changes, rancidity, oxidation and reduction, and the order of a reaction.
Why this matters
Sixteen questions in the bank: one from the 2023 shifts and 15 from the 2024 shifts. Most describe an experiment from the school textbook and ask its type or its product, so the examples table is the core.
Concept 1 of 3: Types of reaction and their examples
Definition
Combination: quicklime + water: CaO + H2O gives Ca(OH)2 (slaked lime), with heat.
Decomposition:
- By heat: lead nitrate gives brown nitrogen dioxide fumes.
- By light: white silver chloride turns grey as it splits into silver and chlorine.
- By electricity: water splits into hydrogen and oxygen.
Changes:
- Curdling milk is a chemical change. Heating iron red hot, evaporating diesel and subliming NH4Cl are physical.
- Rancidity (rancidification): fats and oils are oxidised and smell bad. Antioxidants and nitrogen in chip packets prevent it.
Named reaction: Sandmeyer: a diazonium salt to an aryl halide with a copper(I) halide.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Chemical Reactions · Types of Reactions, Redox and Rates
Rancid fats are oxidised
Concept 2 of 3: Oxidation and reduction
Definition
The rules:
- A redox reaction is the transfer of electrons between reactants.
- Oxidation: oxidation number rises (electrons lost). Reduction: it falls (electrons gained).
- In 4NH3 + 5O2 giving 4NO + 6H2O, nitrogen goes from minus 3 to plus 2 (oxidised) and oxygen from 0 to minus 2 (reduced).
- CaCO3 splitting into CaO and CO2 is not redox: no oxidation number changes.
History: Lavoisier showed that combustion is reaction with oxygen, ending the phlogiston theory.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Chemical Reactions · Types of Reactions, Redox and Rates
Not every decomposition is redox
Concept 3 of 3: Order of a reaction and temperature
Definition
Orders:
- Zero order: rate does not depend on concentration. It happens when a surface or catalyst is saturated, as with ammonia on hot platinum at high pressure.
- First order: rate proportional to the first power of concentration.
- Second and third order: the second and third powers.
Temperature: a 10 degree rise roughly doubles the rate constant.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 3 · Chemical Reactions · Types of Reactions, Redox and Rates
Saturated surface means zero order
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Watch out for (3)
- Rancid fats are oxidised→ Types of reaction and their examples
- Not every decomposition is redox→ Oxidation and reduction
- Saturated surface means zero order→ Order of a reaction and temperature
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