Types of Chemical Reactions
~8 min read
- Five types: Combination, Decomposition, Displacement, Double Displacement, Redox.
- Energy: Exothermic (heat released) vs Endothermic (heat absorbed).
- Reversibility: Reversible (⇌) vs Irreversible (→).
Chemical reactions can be sorted into a few patterns. NDA tests recognition with examples.
The Five Reaction Types
| Type | Form | Example |
|---|---|---|
| Combination | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ (on heating) |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu |
| Double displacement | AB + CD → AD + CB | NaCl + AgNO₃ → AgCl↓ + NaNO₃ |
| Redox | Combined oxidation + reduction | 2Mg + O₂ → 2MgO (Mg oxidised, O reduced) |
Exothermic and Endothermic
- Exothermic: Releases heat. Surroundings get warm. ΔH negative.
- Combustion (burning).
- Neutralisation.
- Respiration (slow biological oxidation).
- Quick lime + water (slaking).
- Endothermic: Absorbs heat. Surroundings get cool. ΔH positive.
- Decomposition of CaCO₃ on heating.
- Photosynthesis (absorbs sunlight energy).
- Melting of ice; dissolution of NH₄Cl in water.
Precipitation Reactions
- Type of double displacement.
- Two soluble salts in solution react to form one insoluble product (precipitate).
- Example: Lead nitrate + Potassium iodide → Lead iodide (yellow precipitate) + Potassium nitrate.
- Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃.
- Used in qualitative analysis to detect ions in solution.
Balancing Chemical Equations
- Law of conservation of mass: atoms cannot be created or destroyed in a chemical reaction.
- Equations must be balanced — same number of each type of atom on both sides.
- Unbalanced: H₂ + O₂ → H₂O.
- Balanced: 2H₂ + O₂ → 2H₂O.
NDA PYQ Examples
Q: 2H₂ + O₂ → 2H₂O is an example of:
(a) Decomposition (b) Combination (c) Displacement (d) Double displacement
Answer: (b) Combination.
Q: Burning of magnesium ribbon is:
(a) Endothermic (b) Exothermic (c) Reversible (d) Photolysis
Answer: (b) Exothermic — releases heat and light.
Q: Reaction of iron with copper sulphate solution is:
(a) Combination (b) Decomposition (c) Displacement (d) Double displacement
Answer: (c) Displacement — more reactive iron displaces copper from solution.
Q: Photosynthesis is:
(a) Exothermic (b) Endothermic (c) Neither (d) Both
Answer: (b) Endothermic — absorbs solar energy.
Drill Types of Chemical Reactions for NDA
NDA-pattern items on Types of Chemical Reactions with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
How do you know if a reaction is exothermic?
Observe heat release: temperature of surroundings rises. Examples: burning, neutralisation, respiration. Endothermic reactions absorb heat (surroundings get cooler) — photosynthesis, decomposition of CaCO₃.
What is a displacement reaction?
A more reactive element displaces a less reactive one from its compound. Iron displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. The reactivity series (K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Hg, Ag, Au) tells which displaces which.
Why must equations be balanced?
Law of conservation of mass — atoms can't be created or destroyed. Balanced equation reflects the actual ratio of atoms. Also tells you molar ratios for calculations.
What is a redox reaction in simple terms?
A reaction where one substance loses electrons (oxidation) and another gains them (reduction). The electron donor is the reducing agent; the electron acceptor is the oxidising agent. Most combustion, corrosion, and energy-producing reactions are redox.
Why does adding water to quick lime get hot?
CaO + H₂O → Ca(OH)₂. The reaction is strongly exothermic — releases ~63 kJ/mol heat. Same principle behind ready-to-eat meal heaters and self-warming cans.