Chemical Reactions and Equations

Exemplar Problems

Short Answer Type

Part 1

Question 19: Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.

Answer: N2 + 3H2 + Catalyst/773 K ⇨ 2NH3

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

Answer: NaOH + CH3COOH ⇨ CH3COONa + H2O

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H2SO4 .

Answer: C2H5OH + CH3COOH ⇨ CH3COOC2H5 + H2O

(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer: C2H4 + 3O2 ⇨ 2CO2 + 2H2O + Heat + Light

(a) and (c) are combination reactions, (b) is double displacement reaction and (d) is redox reaction.

Chapter List

Short Answer Questions from Exemplar Problems

Question 20: Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

Answer: Fe2O3 + 2Al ⇨ 2Fe + Al2O3 + Heat + Light

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

Answer: 3MgS + N2 ⇨ 2Mg3N2

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

Answer: 2KI + Cl2 ⇨ 2KCl + I2

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer: C2H5OH + 3O2 ⇨ 2CO2 + 3H2O + Heat

Question 21: Complete the missing components/variables given as '?' in the following reactions

(a) Pb(NO3 )2 (aq) + 2KI (aq) ⇨ PbI2 (?) + 2KNO3 (?)

Answer: Pb(NO3 )2 (aq) + 2KI (aq) ⇨ PbI2 (s) + 2KNO3 (a)

(b) Cu (s) + 2AgNO3 (aq) ⇨ Cu(NO3 )2 + ? (?)

Answer: Cu (s) + 2AgNO3 (aq) ⇨ Cu(NO3 )2 + Ag (s)

(c) Zn (s) + H2SO4 (aq) ⇨ ZnSO4 (?) + H2 (?)

Answer: Zn (s) + H2SO4 (aq) ⇨ ZnSO4 (aq) + H2 (g)

(d) CaCO3 + ? ⇨ CaO (s) + CO2 (g)

Answer: CaCO3 + Heat ⇨ CaO (s) + CO2 (g)

Question 22: Which among the following changes are exothermic or endothermic in nature?

  1. Decomposition of ferrous sulphate
  2. Dilution of sulphuric acid
  3. Dissolution of sodium hydroxide in water
  4. Dissolution of ammonium chloride in water

Answer: (a) and (d) are endothermic, while (b) and (c) are exothermic

Question 23: Identify the reducing agent in the following reactions

  1. 4NH3 + 5O2 ⇨ 4NO + 6H2O
    Answer: NH3
  2. H2O + F2 ⇨ HF + HOF
    Answer: F2
  3. Fe2O3 + 3CO ⇨ 2Fe + 3CO2
    Answer: CO
  4. 2H2 + O2 ⇨ 2H2O
    Answer: H2

Question 24: Identify the oxidising agent (oxidant) in the following reactions

  1. Pb3O4 + 8HCl ⇨ 3PbCl2 + Cl2 + 4H2O
    Answer: Pb3O4
  2. 2Mg + O2 ⇨ 2MgO
    Answer: O2
  3. CuSO4 + Zn ⇨ Cu + ZnSO4
    Answer: CuSO4
  4. V2O5 + 5Ca ⇨ 2V + 5CaO
    Answer: V2O5
  5. 3Fe + 4H2O ⇨ Fe3O4 + 4H2O
    Answer: 4H2O
  6. CuO + H2 ⇨ Cu + H2O
    Answer: CuO

Question 25: Write the balanced chemical equations for the following reactions

  1. Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogencarbonate.
    Answer: Na2CO3 + HCl ⇨ NaCl + NaHCO3
  2. Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
    Answer: NaHCO3 + HCl ⇨ NaCl + H2O + CO2
  3. Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu2I2), liberates iodine gas and also forms potassium sulphate.
    Answer: 2CuSO4 + 4KI ⇨ Cu2I2 + I2 + 2K2SO4

Question 26: A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

Answer: KCl + AgNO3 ⇨ AgCl + KNO3

This is a Precipitation, Double Displacement Reaction.

Question 27: Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer: 2FeSO4 + Heat ⇨ Fe2O3 + SO2 + SO3

Sulphur dioxide evolves in this smells like burning of sulphur. This reaction is a Decomposition Reaction.

Question 28: Why do fire flies glow at night?

Answer: There is a luminous bag found at the lower abdomen of fire flies. The light emits from this because of secretion of luciferase enzyme. Luciferase acts on the substrate luciferin which results in emission of light. In this complex process, oxidation of magnesium by oxygen molecule is involved. You must have read how magnesium ribbon emits dazzling white light when it is burnt in presence of oxygen.

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