Chemical Reactions
1. What is a Chemical Reaction?
A process in which ONE or MORE substances (reactants) are CONVERTED into different substances (products).
Reactants → Products
Signs of a chemical reaction:
- Change in COLOUR
- Evolution of GAS
- Formation of PRECIPITATE
- Change in TEMPERATURE
- Change in ODOUR
- Evolution of LIGHT or SOUND
2. Types of Chemical Reactions
Combination Reaction
TWO or more substances combine to form a SINGLE new substance.
General form: A + B → AB
Examples: 2Mg + O₂ → 2MgO (burning magnesium — dazzling white light) 2H₂ + O₂ → 2H₂O (formation of water) C + O₂ → CO₂ (burning carbon) CaO + H₂O → Ca(OH)₂ (slaking of lime — exothermic)
Decomposition Reaction
A SINGLE compound BREAKS DOWN into two or more simpler substances.
General form: AB → A + B
Examples: 2H₂O → 2H₂ + O₂ (electrolysis of water) CaCO₃ → CaO + CO₂ (heating limestone — thermal decomposition) 2H₂O₂ → 2H₂O + O₂ (catalysed by manganese dioxide) 2KClO₃ → 2KCl + 3O₂ (heating potassium chlorate)
Displacement Reaction
A MORE reactive element DISPLACES a LESS reactive element from its compound.
General form: A + BC → AC + B
Examples: Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper — blue solution turns GREEN) Zn + H₂SO₄ → ZnSO₄ + H₂ (zinc displaces hydrogen) Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag (copper displaces silver)
Double Displacement Reaction
TWO compounds EXCHANGE their ions to form NEW compounds.
General form: AB + CD → AD + CB
Examples: AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white precipitate of AgCl) Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (white precipitate of BaSO₄) HCl + NaOH → NaCl + H₂O (neutralisation — acid + base → salt + water)
3. Exothermic and Endothermic Reactions
Exothermic Reactions
Reactions that RELEASE heat energy (the temperature of the surroundings INCREASES).
Examples:
- Burning of fuels (coal, wood, petrol)
- Respiration (glucose + O₂ → CO₂ + H₂O + energy)
- Neutralisation (acid + base → salt + water)
- Slaking of lime (CaO + H₂O → Ca(OH)₂ + heat)
Endothermic Reactions
Reactions that ABSORB heat energy (the temperature of the surroundings DECREASES).
Examples:
- Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — needs sunlight)
- Electrolysis of water (needs electrical energy)
- Thermal decomposition (CaCO₃ → CaO + CO₂ — needs heat)
- Dissolving ammonium chloride in water (solution becomes COLD)
4. Oxidation and Reduction
Oxidation: GAIN of oxygen or LOSS of hydrogen. Reduction: LOSS of oxygen or GAIN of hydrogen.
Redox reaction: Oxidation and reduction occur SIMULTANEOUSLY.
Worked Example: CuO + H₂ → Cu + H₂O
CuO → Cu: LOSS of oxygen = REDUCTION H₂ → H₂O: GAIN of oxygen = OXIDATION H₂ is the REDUCING AGENT. CuO is the OXIDISING AGENT.
5. Energy Changes in Reactions
| Type | Energy Change | Surroundings | Examples |
|---|---|---|---|
| Exothermic | Heat RELEASED | Temperature RISES | Burning, neutralisation |
| Endothermic | Heat ABSORBED | Temperature FALLS | Photosynthesis, electrolysis |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'All reactions that give off light are exothermic' | Most are, but some (like glow sticks) involve CHEMILUMINESCENCE — light without significant heat |
| 'Decomposition always needs heat' | Decomposition can be triggered by heat (thermal), electricity (electrolytic), or light (photolytic) |
| 'Combination and displacement are the same' | Combination: A + B → AB. Displacement: A + BC → AC + B. DIFFERENT patterns |
| 'Neutralisation is not a double displacement' | Neutralisation IS a double displacement: HX + MOH → MX + H₂O. The water forms from H⁺ and OH⁻ |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define the type of reaction with one example
- 3-mark questions: Distinguish between exothermic and endothermic reactions
- 4-mark questions: Classify reactions with balanced equations
- 6-mark questions: Identify type, balance, and explain energy change
Self-Test
Q1. What type of reaction is 2Mg + O₂ → 2MgO? A1. COMBINATION reaction (two substances combine to form one product) and OXIDATION (Mg gains oxygen).
Q2. Give one example each of exothermic and endothermic reactions. A2. Exothermic: Burning of methane (CH₄ + 2O₂ → CO₂ + 2H₂O + heat). Endothermic: Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — needs sunlight energy).
Q3. Balance: Zn + HCl → ZnCl₂ + H₂. Identify the reaction type. A3. Zn + 2HCl → ZnCl₂ + H₂. DISPLACEMENT reaction (Zn displaces H from HCl).
Q4. Distinguish between displacement and double displacement reactions. A4. Displacement: One element displaces another (A + BC → AC + B). Double displacement: Two compounds exchange ions (AB + CD → AD + CB).
Q5. What happens when iron is added to copper sulphate solution? A5. Fe + CuSO₄ → FeSO₄ + Cu. The blue colour of CuSO₄ FADES and a REDDISH-BROWN deposit of copper forms. This is a DISPLACEMENT reaction.
Q6. Why is photosynthesis considered endothermic? A6. Photosynthesis ABSORBS sunlight energy to convert CO₂ and H₂O into glucose and oxygen. The energy is stored in the chemical bonds of glucose.
