CBSE Class 10 Chemistry Chemical Reactions and Equations MCQs Set 37

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Chapter-wise Objective Questions: Chemical Reactions and Equations

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Multiple Choice Questions

Question 1: A reaction in which both reactants and products are present at equilibrium is called
(a) reversible
(b) irreversible
(c) endothermic
(d) exothermic
Show Answer & Explanation

Answer: (a) reversible

Explanation:
1. In a reversible reaction the products can change back into the reactants.
2. At equilibrium both are therefore present.

Teacher's Note:
1. In an irreversible reaction the reactants are used up completely.
2. Heat change does not decide reversibility.

Question 2: In the reaction 2Al + Fe2O3 → Al2O3 + 2Fe, which substance is oxidised?
(a) Al
(b) Fe
(c) Fe2O3
(d) none of these
Show Answer & Explanation

Answer: (a) Al

Explanation:
1. Aluminium gains oxygen and its oxidation number rises from 0 to +3.
2. So aluminium is oxidised, and it acts as the reducing agent.

Teacher's Note:
1. Iron(III) oxide loses oxygen and is reduced.
2. The oxidised species is the reducing agent.

Question 3: A change is said to be a chemical change when
(a) energy change occurs
(b) new substances are formed
(c) the change cannot be easily reversed
(d) all the statements are correct
Show Answer & Explanation

Answer: (b) new substances are formed

Explanation:
1. A chemical change is identified by the formation of new substances with new properties.
2. Energy changes also accompany physical changes, and some chemical changes can be reversed.

Teacher's Note:
1. Colour change, gas, precipitate, temperature change help identify chemical change.
2. Only the formation of a new substance is decisive.

Question 4:
Assertion (A): The oxidation state of carbon in its compounds is sometimes +4.
Reason (R): An element has a fixed oxidation state.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Carbon is +4 in CO2, so A is true.
2. Carbon is –4 in CH4, 0 in the element and +2 in CO, so an element does not have a fixed oxidation state and R is false.

Teacher's Note:
1. Most elements show more than one oxidation state.
2. Transition elements show many.

Question 5: Which one of the following reactions is an example of electrical decomposition?
(a) CaCO3(s) → CaO(s) + CO2(g)
(b) 2HOCl(aq) → O2(g) + 2HCl(g)
(c) 2H2O(l) → 2H2(g) + O2(g)
(d) 2AgCl(s) → 2Ag(s) + Cl2(g)
Show Answer & Explanation

Answer: (c) 2H2O(l) → 2H2(g) + O2(g)

Explanation:
1. The splitting of water into hydrogen and oxygen is done by passing an electric current through acidified water.
2. So it is an electrical decomposition (electrolysis).

Teacher's Note:
1. Hydrogen and oxygen are collected in the volume ratio 2 : 1.
2. The current is passed through acidified water.

Question 6: Chemically, rust is
(a) hydrated ferrous oxide
(b) hydrated ferric oxide
(c) only ferric oxide
(d) none of these
Show Answer & Explanation

Answer: (b) hydrated ferric oxide

Explanation:
1. Rust is the reddish-brown coating formed when iron reacts with oxygen and moisture.
2. Its formula is Fe2O3·xH2O, hydrated iron(III) (ferric) oxide.

Teacher's Note:
1. Rusting needs both air and water.
2. Iron(III) means ferric.

Question 7: For the reaction CuO + H2 → Cu + H2O, which of the following is correct?
(a) CuO is reduced and H2 is oxidised
(b) CuO is oxidised and H2 is reduced
(c) both are oxidised
(d) both are reduced
Show Answer & Explanation

Answer: (a) CuO is reduced and H2 is oxidised

Explanation:
1. CuO loses oxygen and copper goes from +2 to 0, so CuO is reduced.
2. H2 gains oxygen and hydrogen goes from 0 to +1, so H2 is oxidised.

Teacher's Note:
1. CuO is the oxidising agent and H2 is the reducing agent.
2. Hot copper(II) oxide changes from black to brown.

Question 8: Which of the following is the correct statement about the oxidation numbers? (i) oxygen can be 0, +2, –1 or –2 (ii) fluorine can be 0 or –1 (iii) fluorine is always –1 (iv) sodium in sodium amalgam is 0
(a) (i), (ii) and (iv)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (iii) and (iv)
Show Answer & Explanation

Answer: (a) (i), (ii) and (iv)

Explanation:
1. Oxygen is 0 in O2, +2 in OF2, –1 in peroxides and –2 in oxides.
2. Fluorine is 0 in F2 and –1 in compounds, so (ii) is true but (iii) is false because F2 is zero.
3. In an amalgam sodium is present as an element, so its oxidation number is 0.

Teacher's Note:
1. Oxidation number of an element in its free state is zero.
2. Fluorine never has a positive oxidation number.

Question 9: Which of the following is the correct balanced equation for H2O2 + Fe2+ → Fe3+ + H2O in acidic medium?
(a) H2O2 + Fe2+ + H+ → Fe3+ + H2O
(b) H2O2 + 2Fe2+ + 2H+ → 2Fe3+ + 2H2O
(c) H2O2 + 2Fe2+ → 2Fe3+ + 2H2O
(d) 2H2O2 + Fe2+ + 2H+ → Fe3+ + 4H2O
Show Answer & Explanation

Answer: (b) H2O2 + 2Fe2+ + 2H+ → 2Fe3+ + 2H2O

Explanation:
1. Oxidation: Fe2+ → Fe3+ + e–, taken twice gives 2 electrons.
2. Reduction: H2O2 + 2H+ + 2e– → 2H2O.
3. Adding gives H2O2 + 2Fe2+ + 2H+ → 2Fe3+ + 2H2O, balanced in atoms and charge (+6 on both sides).

Teacher's Note:
1. Balance atoms, then charge, using H+ in acidic medium.
2. The electrons lost must equal the electrons gained.

Question 10: For the reaction 2CuI → Cu + CuI2, the reaction is a
(a) neutralisation
(b) redox reaction (disproportionation)
(c) pure oxidation
(d) pure reduction
Show Answer & Explanation

Answer: (b) redox reaction (disproportionation)

Explanation:
1. Copper goes from +1 in CuI to 0 in Cu (reduction) and to +2 in CuI2 (oxidation).
2. Both happen together, so it is a redox reaction (disproportionation).

Teacher's Note:
1. If a species is both oxidised and reduced, it disproportionates.
2. Neutralisation has no oxidation number change.

Question 11: When caesium metal reacts with water, it forms
(a) Cs and O2
(b) CsOH and H2
(c) CsO
(d) Cs2O only
Show Answer & Explanation

Answer: (b) CsOH and H2

Explanation:
1. Alkali metals react with water to give the hydroxide and hydrogen.
2. 2Cs + 2H2O → 2CsOH + H2.

Teacher's Note:
1. The reaction of Cs is extremely violent.
2. Reactivity increases down group 1.

Question 12: When 1.84 g of a mixture of CaCO3 and MgCO3 is heated to constant weight, the residue weighs 0.96 g. The percentage of CaCO3 in the mixture is
(a) 45.65%
(b) 60%
(c) 54.35%
(d) 50%
Show Answer & Explanation

Answer: (c) 54.35%

Explanation:
1. Let the mass of CaCO3 be x g: CaO from it = 56x/100 and MgO from the rest = 40(1.84 – x)/84.
2. 0.56x + 0.4762(1.84 – x) = 0.96 gives x = 1.00 g.
3. % CaCO3 = (1.00 / 1.84) × 100 = 54.35%, and MgCO3 is 45.65%.

Teacher's Note:
1. Write the mass of each oxide formed using the molar mass ratio.
2. Solve the single linear equation.

Question 13:
Assertion (A): The reaction of ammonia solution with calomel (Hg2Cl2) is a disproportionation reaction giving Hg(II) amido chloride and mercury.
Reason (R): In a disproportionation reaction the species is neither oxidised nor reduced.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Show Answer & Explanation

Answer: (c) A is true, but R is false.

Explanation:
1. Hg(I) goes to Hg(II) (oxidation) and to Hg (reduction), so A is true.
2. In disproportionation the same species is both oxidised and reduced, so R is false.

Teacher's Note:
1. Hg2Cl2 + 2NH3 → Hg(NH2)Cl + Hg + NH4Cl.
2. The black residue of mercury gives the reaction its name (calomel = beautiful black).

Question 14: The decomposition reaction which is useful in black and white photography is that of
(a) calcium carbonate
(b) lead nitrate
(c) silver bromide
(d) water
Show Answer & Explanation

Answer: (c) silver bromide

Explanation:
1. Silver bromide decomposes in light to silver and bromine: 2AgBr → 2Ag + Br2.
2. The silver formed makes the dark parts of the photograph.

Teacher's Note:
1. This is photodecomposition.
2. Silver chloride behaves in the same way.

Question 15: Food items containing oils and fats are flushed with nitrogen or kept in a refrigerator because
(a) this adds moisture
(b) this makes them tastier
(c) this converts them to proteins
(d) this slows down or stops oxidation and so rancidity
Show Answer & Explanation

Answer: (d) this slows down or stops oxidation and so rancidity

Explanation:
1. Nitrogen keeps oxygen away and low temperature slows chemical reactions.
2. Both reduce the oxidation of fats, which prevents rancidity.

Teacher's Note:
1. Antioxidants such as BHA are also added.
2. Air-tight packing is used for the same reason.

Question 16: Phosphorus and chlorine react to form two compounds. Their formulae are
(a) PCl2 and PCl4
(b) P2Cl3 and P2Cl5
(c) PCl and PCl2
(d) PCl3 and PCl5
Show Answer & Explanation

Answer: (d) PCl3 and PCl5

Explanation:
1. Phosphorus shows valencies 3 and 5.
2. So with chlorine it forms PCl3 (2P + 3Cl2 → 2PCl3) and PCl5 (2P + 5Cl2 → 2PCl5).

Teacher's Note:
1. Phosphorus shows +3 and +5 oxidation states.
2. Excess chlorine gives PCl5.

Question 17: Which of the following are chemical changes? (i) digestion of food (ii) liquefaction of air (iii) ripening of fruit (iv) dissolution of sulphur in carbon disulphide (v) freezing of water
(a) (ii) and (v) only
(b) (iv) and (v) only
(c) (i), (ii) and (iii)
(d) (i) and (iii) only
Show Answer & Explanation

Answer: (d) (i) and (iii) only

Explanation:
1. Digestion and ripening form new substances, so they are chemical changes.
2. Liquefaction, dissolution and freezing change only the state or mixture, so they are physical changes.

Teacher's Note:
1. A chemical change forms a new substance and is hard to reverse.
2. Physical changes affect only physical properties.

Question 18: The reaction CuO(s) + H2(g) → Cu(s) + H2O(l) shows that
(a) hydrogen is oxidised and copper(II) oxide is reduced
(b) hydrogen is reduced and copper(II) oxide is oxidised
(c) both are reduced
(d) neither undergoes a change in oxidation state
Show Answer & Explanation

Answer: (a) hydrogen is oxidised and copper(II) oxide is reduced

Explanation:
1. Hydrogen gains oxygen to form water, so it is oxidised.
2. CuO loses oxygen to give copper, so it is reduced.

Teacher's Note:
1. Both processes occur at the same time.
2. Copper changes from black to brown.

Question 19: A solution of a substance X is used for white washing. X and its reaction with water are
(a) limestone, CaCO3 + H2O → CaO
(b) quick lime, CaO + H2O → Ca(OH)2 + heat
(c) common salt, NaCl + H2O → NaOH
(d) slaked lime, Ca(OH)2 + H2O → CaO
Show Answer & Explanation

Answer: (b) quick lime, CaO + H2O → Ca(OH)2 + heat

Explanation:
1. The substance used is quick lime, CaO.
2. It reacts vigorously with water to give slaked lime with the release of heat: CaO + H2O → Ca(OH)2.

Teacher's Note:
1. Slaked lime is the actual whitewash.
2. On the wall it forms a thin shiny layer of CaCO3 after 2 to 3 days.

Question 20: Respiration is considered an exothermic reaction because
(a) it produces only oxygen
(b) it absorbs heat from the body
(c) it decomposes water
(d) glucose is oxidised in the cells with release of energy: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Show Answer & Explanation

Answer: (d) glucose is oxidised in the cells with release of energy: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Explanation:
1. In respiration the glucose from food combines with oxygen in cells and heat energy is released.
2. A reaction that releases heat is exothermic.

Teacher's Note:
1. This energy keeps our body warm and active.
2. It is a slow oxidation.

Question 21: What is the sum of all the coefficients when the equation MnO2 + Al → Mn + Al2O3 is balanced using the smallest whole numbers?
(a) 12
(b) 10
(c) 11
(d) 15
Show Answer & Explanation

Answer: (a) 12

Explanation:
1. Balance the atoms of each element in turn.
2. The balanced equation is 3MnO2 + 4Al → 3Mn + 2Al2O3.
3. The coefficients add up to 3+4+3+2 = 12.

Teacher's Note:
1. Never change a subscript while balancing, only the coefficients.
2. Check every element on both sides at the end.

Question 22: What is the sum of all the coefficients when the equation As2O3 + H2S → As2S3 + H2O is balanced using the smallest whole numbers?
(a) 9
(b) 11
(c) 8
(d) 6
Show Answer & Explanation

Answer: (c) 8

Explanation:
1. Balance the atoms of each element in turn.
2. The balanced equation is As2O3 + 3H2S → As2S3 + 3H2O.
3. The coefficients add up to 1+3+1+3 = 8.

Teacher's Note:
1. Never change a subscript while balancing, only the coefficients.
2. Check every element on both sides at the end.

Question 23: What is the sum of all the coefficients when the equation BaCl2 + Al2(SO4)3 → BaSO4 + AlCl3 is balanced using the smallest whole numbers?
(a) 10
(b) 7
(c) 9
(d) 8
Show Answer & Explanation

Answer: (c) 9

Explanation:
1. Balance the atoms of each element in turn.
2. The balanced equation is 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3.
3. The coefficients add up to 3+1+3+2 = 9.

Teacher's Note:
1. Never change a subscript while balancing, only the coefficients.
2. Check every element on both sides at the end.

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