Chapter 10 — Types of Chemical Reactions: Long Answer Questions
1. Combination (Synthesis) Reaction
Two or more reactants combine to form a single product.
Examples:
- 2Mg + O₂ → 2MgO (magnesium burning — gives white light)
- CaO + H₂O → Ca(OH)₂ (quicklime + water → slaked lime; exothermic)
- N₂ + 3H₂ → 2NH₃ (Haber process — manufacture of ammonia)
- SO₃ + H₂O → H₂SO₄ (industrial sulphuric acid production)
2. Decomposition Reaction
A single compound breaks down into two or more simpler substances when heat, light, or electricity is applied.
Types:
- Thermal decomposition (heat):
- 2KClO₃ → 2KCl + 3O₂ (MnO₂ catalyst)
- CaCO₃ → CaO + CO₂ (limestone → quicklime; industrial process)
- Photolytic decomposition (light):
- 2AgBr → 2Ag + Br₂ (used in black-and-white photography)
- 2AgCl → 2Ag + Cl₂
- Electrolytic decomposition (electricity):
- 2H₂O → 2H₂ + O₂ (electrolysis of water)
- 2NaCl → 2Na + Cl₂ (electrolysis of molten NaCl)
3. Displacement (Single Displacement) Reaction
A more reactive element displaces a less reactive element from its compound.
Examples:
- Fe + CuSO₄ → FeSO₄ + Cu↓ (iron displaces copper — iron is more reactive)
- Zn + H₂SO₄ → ZnSO₄ + H₂↑ (zinc displaces hydrogen from acid)
- 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂↑
- 2Na + 2H₂O → 2NaOH + H₂↑ (sodium displaces hydrogen from water)
Reactivity series (Activity series): K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Au A metal displaces metals below it in the series.
4. Double Displacement Reaction
Two compounds exchange their ions to form two new compounds. Usually involves formation of a precipitate, gas, or water.
Examples:
- Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (white precipitate)
- AgNO₃ + NaCl → AgCl↓ + NaNO₃ (white precipitate — test for Cl⁻)
- Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Neutralisation is a special double displacement reaction:
- NaOH + HCl → NaCl + H₂O (acid-base reaction)
5. Combustion Reaction
Rapid reaction with oxygen producing heat and light.
- Complete combustion: CH₄ + 2O₂ → CO₂ + 2H₂O
- Incomplete combustion: 2CH₄ + 3O₂ → 2CO + 4H₂O (toxic CO produced)
Definitions of Oxidation
| In terms of | Oxidation |
|---|---|
| Oxygen | Gain of oxygen (or addition of oxygen) |
| Hydrogen | Loss of hydrogen |
| Electrons | Loss of electrons |
| Oxidation state | Increase in oxidation number |
Definitions of Reduction
| In terms of | Reduction |
|---|---|
| Oxygen | Loss of oxygen (removal of oxygen) |
| Hydrogen | Gain of hydrogen |
| Electrons | Gain of electrons |
| Oxidation state | Decrease in oxidation number |
Mnemonic: OIL RIG
- Oxidation Is Loss (of electrons)
- Reduction Is Gain (of electrons)
Oxidising Agent and Reducing Agent
- Oxidising agent: The substance that oxidises others by accepting electrons (itself gets reduced)
- Reducing agent: The substance that reduces others by donating electrons (itself gets oxidised)
Example 1: CuO + H₂ → Cu + H₂O
- H₂ is oxidised (H: 0 → +1 in H₂O) → H₂ is the reducing agent
- Cu is reduced (Cu: +2 in CuO → 0) → CuO is the oxidising agent
Example 2: 2Fe + 3Cl₂ → 2FeCl₃
- Fe is oxidised (0 → +3) → Fe is the reducing agent
- Cl₂ is reduced (0 → −1) → Cl₂ is the oxidising agent
Common oxidising agents: O₂, Cl₂, F₂, H₂O₂, KMnO₄, K₂Cr₂O₇, HNO₃ Common reducing agents: H₂, C, CO, Na, Mg, Al, Fe, SO₂
Corrosion
Corrosion is the gradual deterioration of a metal due to its reaction with the surrounding environment (oxygen, moisture, acids, salts).
Rusting of iron (most common corrosion):
Rusting requires both oxygen and water.
Reaction:
Type of reaction: Oxidation — iron loses electrons to become Fe³⁺.
Factors that accelerate rusting:
- Presence of electrolytes (salt water rusts iron faster — explains faster corrosion near sea)
- Acid rain (SO₂, NOₓ dissolve in water)
- Scratches exposing fresh metal
Prevention of rusting: 1. Painting — physical barrier against oxygen and moisture 2. Galvanisation — coating with zinc (Zn is more reactive than Fe; zinc corrodes sacrificially, protecting iron) 3. Electroplating — coating with chromium or nickel (for appearance and protection) 4. Alloying — making stainless steel (Fe + 10–18% Cr + Ni; Cr forms passive Cr₂O₃ layer) 5. Oiling and greasing — for machinery 6. Cathodic protection — connecting iron to more reactive metal (magnesium or zinc) as anode — iron becomes cathode; doesn't corrode
Rancidity
Rancidity is the oxidation of fats and oils in food, producing unpleasant taste and smell, making food unsafe to eat.
Chemical process:
- Unsaturated fatty acids in fats react with atmospheric oxygen
- Peroxides and aldehydes form → rancid smell and taste
Relation to oxidation: Rancidity is an oxidation reaction — fats are oxidised by oxygen in air.
Prevention of rancidity: 1. Antioxidants — added to food (Vitamin C, Vitamin E, BHA, BHT) — they react with O₂ preferentially, protecting fats 2. Vacuum packaging — removes oxygen from packaging 3. Nitrogen flushing — packs filled with nitrogen (inert) instead of air (potato chips, biscuits) 4. Refrigeration — slows oxidation reaction rate 5. Use of airtight containers — limits exposure to oxygen 6. Adding salt — inhibits bacterial growth (secondary prevention)
Both corrosion and rancidity are oxidation reactions — preventing oxidation is the key to preventing both.
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