Chemical Reactions and Equations — Practice

Question 1

Balance: H₂ + N₂ → NH₃.

Accepted Answer

Step 1 — Count atoms. N: 2 → 1; H: 2 → 3.

Step 2 — Balance N with a coefficient 2 on NH₃: H₂ + N₂ → 2NH₃. Now H: 2 → 6, N: 2 → 2.

Step 3 — Balance H by putting 3 before H₂: 3H₂ + N₂ → 2NH₃.

Step 4 — Check: H 6↔6, N 2↔2. ✓

✦ Answer: 3H₂ + N₂ → 2NH₃.

Question 2

Name the type of reaction: CaO + H₂O → Ca(OH)₂.

Accepted Answer

Two reactants form a single product, with heat released.

✦ Answer: Combination reaction (also exothermic).

Question 3

Define a decomposition reaction and give one example.

Accepted Answer

A reaction in which a single reactant breaks down into two or more simpler products, usually on supplying heat, light or electricity.

Example: 2FeSO₄ →(heat) Fe₂O₃ + SO₂ + SO₃.

✦ Answer: definition + any valid example (CaCO₃ → CaO + CO₂ also accepted).

Question 4

Write the balanced equation with physical states for the reaction of zinc with dilute sulphuric acid.

Accepted Answer

Step 1 — Skeletal: Zn + H₂SO₄ → ZnSO₄ + H₂.

Step 2 — Count: Zn 1↔1, H 2↔2, S 1↔1, O 4↔4 — already balanced.

Step 3 — Add states.

✦ Answer: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).

Question 5

When an iron nail is dipped in copper sulphate solution, the blue colour fades and a brown deposit forms on the nail. Write the equation and name the reaction.

Accepted Answer

Step 1 — Iron is more reactive than copper, so it displaces copper.

Step 2 — Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).

Step 3 — Blue CuSO₄ → pale green FeSO₄ (colour fades); brown copper deposits on the nail.

✦ Answer: Displacement reaction; Fe + CuSO₄ → FeSO₄ + Cu.

Question 6

Identify the precipitate and write the balanced equation: sodium sulphate solution + barium chloride solution.

Accepted Answer

Step 1 — Ions exchange partners.

Step 2 — Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq).

Step 3 — BaSO₄ is insoluble — a white precipitate.

✦ Answer: white BaSO₄ precipitate; double displacement (precipitation) reaction.

Question 7

In the reaction CuO + H₂ → Cu + H₂O, identify the substance oxidised and the substance reduced.

Accepted Answer

Step 1 — CuO loses oxygen → it is reduced (to Cu).

Step 2 — H₂ gains oxygen → it is oxidised (to H₂O).

✦ Answer: CuO is reduced; H₂ is oxidised. (Hence it is a redox reaction.)

Question 8

Why must a chemical equation be balanced? State the law involved and illustrate with the formation of water.

Accepted Answer

Step 1 — Atoms are neither created nor destroyed in a chemical reaction, only rearranged — the law of conservation of mass.

Step 2 — So the number of atoms of each element must be equal on both sides; only then is mass conserved.

Step 3 — 2H₂ + O₂ → 2H₂O: H 4↔4, O 2↔2; mass of reactants (4 + 32 = 36 g) = mass of products (36 g).

✦ Answer: To obey the law of conservation of mass — equal atoms (and hence equal mass) on both sides.

Question 9

What is corrosion? Why does an iron tawa rust faster during the monsoon? List two methods to prevent rusting.

Accepted Answer

Step 1 — Corrosion is the slow eating away of a metal by the attack of moist air (oxygen + water), forming compounds like rust (hydrated iron(III) oxide).

Step 2 — Rusting needs both oxygen and water. The monsoon air carries high moisture, so rusting speeds up.

Step 3 — Prevention (any two): painting/oiling/greasing, galvanisation (zinc coating), alloying (stainless steel), chrome plating.

✦ Answer: definition + 'high humidity supplies the water needed' + any two prevention methods.

Question 10

Why do oily snacks like namkeen develop an unpleasant smell over time? Name this phenomenon and give two ways to slow it down.

Accepted Answer

Step 1 — The fats and oils in the food are oxidised by oxygen in the air, producing compounds with an unpleasant taste and smell.

Step 2 — This oxidation of fats/oils is called rancidity.

Step 3 — Slowing it down (any two): add antioxidants, flush the pack with nitrogen gas, refrigerate, store in airtight containers away from light.

✦ Answer: oxidation of fats/oils = rancidity; any two valid prevention methods.

Question 11

A green coloured ferrous sulphate crystal is heated strongly. (a) Name the type of reaction. (b) Write the balanced equation. (c) State the colour change observed. (d) Name the gases evolved.

Accepted Answer

Step 1 (a) — A single reactant breaks down on heating → thermal decomposition reaction.

Step 2 (b) — 2FeSO₄(s) →(heat) Fe₂O₃(s) + SO₂(g) + SO₃(g).

Step 3 (c) — Green ferrous sulphate crystals lose water and turn brown (Fe₂O₃).

Step 4 (d) — Sulphur dioxide (SO₂) and sulphur trioxide (SO₃), with a smell of burning sulphur.

✦ Answer: (a) decomposition, (b) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃, (c) green → brown, (d) SO₂ and SO₃.

Question 12

Assertion (A): Silver chloride is stored in dark coloured bottles. Reason (R): Silver chloride decomposes in sunlight. Choose: both true and R explains A / both true but R does not explain A / A true R false / A false R true.

Accepted Answer

AgCl undergoes photodecomposition: 2AgCl →(sunlight) 2Ag + Cl₂, turning grey. Storing it in dark bottles prevents this.

✦ Answer: Both A and R are true, and R is the correct explanation of A.

Practice — Chemical Reactions and Equations

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