CBSE Class 11 Chemistry Redox Reactions MCQs Set 04

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. In acidic medium, MnO₄⁻ is reduced to Mn²⁺ (3d⁵, half-filled, very stable) while Cr₂O₇²⁻ is reduced to Cr³⁺.
2. The greater the stability of the reduced species, the greater the tendency of the oxidising agent to be reduced; E° (MnO₄⁻/Mn²⁺) = +1.51 V is higher than E° (Cr₂O₇²⁻/Cr³⁺) = +1.33 V.
3. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

Teacher's Note:
1. A higher standard reduction potential means a stronger oxidising agent.
2. Mn²⁺ (d⁵) is extra stable because of its half-filled d subshell.

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

Explanation:
1. The reaction of potassium permanganate and potassium iodide (acidic medium) is:
10KI + 2KMnO₄ + 8H₂SO₄ → 2MnSO₄ + 6K₂SO₄ + 8H₂O + 5I₂
2. The oxidation state of Mn decreases from +7 (in KMnO₄) to +2 (in MnSO₄), so permanganate is reduced and acts as the oxidising agent, while I⁻ is oxidised to I₂.
3. Thus, Assertion is correct but Reason is incorrect (the change is from +7 to +2, not +2 to +7).

Teacher's Note:
1. An oxidising agent is reduced, so its oxidation number must decrease.
2. Watch the direction of change in such statements – it is a common trap.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. KI₃ can be regarded as KI + I₂ (K⁺ and the I₃⁻ ion, formed from I⁻ and I₂).
2. Average oxidation number of I = \( \frac{-1 \times 1 + 0 \times 2}{1 + 2} = -\frac{1}{3} \)
3. The iodine atoms actually have −1 (in KI) and 0 (in I₂) oxidation numbers, and their average is −1/3.
4. Thus, either the two values are reported separately or one average value is reported. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. From the formula alone: +1 + 3x = 0 gives x = −1/3.
2. Fractional oxidation numbers always indicate atoms of the same element in different states.

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

Explanation:
1. PbCl₂ is more stable than PbCl₄, i.e. Pb²⁺ is more stable than Pb⁴⁺ due to the inert pair effect.
2. So, Pb⁴⁺ readily gains 2 electrons: Pb⁴⁺ + 2e⁻ → Pb²⁺ (reduction). Hence, PbCl₄ acts as a powerful oxidising agent.
3. Assertion is correct but Reason is incorrect.

Teacher's Note:
1. Down group 14, the +2 state becomes more stable than +4 because of the inert pair effect.
2. The less stable higher oxidation state tends to be reduced, so it behaves as an oxidising agent.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Oxygen is the most electronegative element after fluorine.
2. Therefore, in the compounds of oxygen with fluorine (OF₂: +2, O₂F₂: +1), oxygen shows a positive oxidation state.
3. Both Assertion and Reason are correct and Reason is the correct explanation of the Assertion.

Teacher's Note:
1. Shared electrons are assigned to the more electronegative atom – with F, that is fluorine.
2. Fluorine never shows a positive oxidation number.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The maximum oxidation state of S is +6 (it has 6 valence electrons), and in H₂SO₄ sulphur is already +6.
2. It cannot be oxidised further, so H₂SO₄ cannot act as a reducing agent (it can act only as an oxidising agent).
3. Both Assertion and Reason are correct and Reason is the correct explanation of the Assertion.

Teacher's Note:
1. An element in its highest oxidation state can only be reduced.
2. Concentrated H₂SO₄ is a good oxidising agent, e.g. it oxidises Cu to CuSO₄ and is reduced to SO₂.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. F₂ is the best oxidant among the halogens because it has the highest standard reduction potential (E° = +2.87 V).
2. This is mainly due to its low F–F bond dissociation enthalpy and the very high hydration enthalpy of the small F⁻ ion, and not merely its electronegativity.
3. Both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Oxidising power in solution is decided by E°, which includes bond, electron-gain and hydration enthalpies.
2. Electronegativity is a property of a bonded atom, not of the free molecule.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Fe₃O₄ = FeO·Fe₂O₃; the oxidation states of Fe in FeO and Fe₂O₃ are +2 and +3 respectively.
2. Fe²⁺ ions decolourise acidified KMnO₄: Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺ (pink MnO₄⁻ changes to colourless Mn²⁺).
3. Both Assertion and Reason are correct statements, but Reason is not the correct explanation of the Assertion.

Teacher's Note:
1. Fe₃O₄ actually contains one Fe²⁺ and two Fe³⁺ per formula unit.
2. KMnO₄ acts as its own indicator in titrations because MnO₄⁻ is pink and Mn²⁺ is nearly colourless.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. The structure of Br₃O₈ (tribromooctaoxide) is:
2. Thus, the oxidation state of the two corner Br atoms is +6 and of the middle one is +4. The difference in oxidation states is due to the difference in bonding situations.
3. Average oxidation state = \( \frac{6 + 4 + 6}{3} = \frac{16}{3} \)
4. Thus, both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. The average value (16/3) comes from the formula; the actual values come from the structure.
2. Whenever an average oxidation number is fractional, the atoms are in different oxidation states.

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

Explanation:
1. Due to its high electronegativity, high reduction potential and the high hydration/lattice energies of the small fluoride ion, fluorine oxidises elements to their highest oxidation states, e.g. SF₆, IF₇, OsF₆.
2. So, the Assertion is incorrect.
3. The fluoride ion is very difficult to oxidise back to F₂ (it can be done only electrolytically), so the reverse reaction (F₂ → F⁻) occurs easily. The Reason is correct.
4. Hence, Assertion is incorrect but Reason is correct.

Teacher's Note:
1. Fluorine brings out the highest oxidation states of elements, e.g. S in SF₆ (+6) and I in IF₇ (+7).
2. Since F⁻ cannot be oxidised chemically, F₂ is prepared only by electrolysis.

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

Explanation:
1. The oxidation number of O in O₃ is zero (elementary state) and that of S in SO₂ is +4. So, Assertion is correct.
2. O in O₃ can only decrease from 0 to −1 or −2 (it cannot increase to +2 under normal conditions), so O₃ acts only as an oxidising agent.
3. In SO₂, S is +4; it can go down to −2 or up to +6. Its oxidation number can either decrease or increase, so SO₂ acts both as an oxidising and a reducing agent.
4. Assertion is correct, but Reason is incorrect (it is reversed).

Teacher's Note:
1. A species in an intermediate oxidation state (like S +4 in SO₂) can act both ways.
2. SO₂ usually acts as a reducing agent, e.g. it decolourises acidified KMnO₄.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Structure of the tetrathionate ion (S₄O₆²⁻):
2. Formal (average) oxidation number of sulphur: 2(+1) + 4x + 6(−2) = 0 ⇒ x = +2.5 [= (2 × 5 + 2 × 0)/4].
3. The two middle S-atoms are bonded only to S-atoms (oxidation number 0), while the two terminal S-atoms are bonded to O-atoms (oxidation number +5). So, the Reason is also correct.
4. The Reason explains why the actual oxidation numbers of the S-atoms differ (0 and +5), but the formal value 2.5 simply comes from the formula. Hence, both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Formal oxidation number = average from the formula; actual values come from the structure.
2. Tetrathionate is formed when thiosulphate is oxidised by iodine in iodometric titrations.

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

Explanation:
1. In acidic medium, MnO₄⁻ is reduced to Mn²⁺: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. So, Assertion is correct.
2. MnO₄⁻ is reduced to MnO₂ only in neutral or mildly basic medium, and in strongly basic medium it is reduced to MnO₄²⁻. So, the Reason is incorrect.
3. Assertion is correct but Reason is incorrect.

Teacher's Note:
1. Remember: acidic → Mn²⁺ (5e⁻), neutral/weakly basic → MnO₂ (3e⁻), strongly basic → MnO₄²⁻ (1e⁻).
2. The equivalent weight of KMnO₄ therefore depends on the medium.

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

Explanation:
1. Cl₂ is an oxidising agent. In the presence of moisture it gives nascent oxygen (Cl₂ + H₂O → 2HCl + [O]), which bleaches articles permanently by oxidation.
2. Assertion is correct, but Reason is incorrect.

Teacher's Note:
1. Bleaching by chlorine is permanent (oxidation), while bleaching by SO₂ is temporary (reduction).
2. Chlorine bleaches only in the presence of moisture.

Answer: (c) 3

Explanation:
1. Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 3I₂ + 7H₂O
2. Each Cr changes from +6 to +3, accepting 3e⁻; one Cr₂O₇²⁻ (2 Cr) accepts 6 electrons.
3. 6 electrons oxidise 6 I⁻ to 3 I₂. So, 1 mole of K₂Cr₂O₇ liberates 3 moles of I₂, i.e. n = 3.

Teacher's Note:
1. Count electrons per formula unit: 2 Cr × 3 e⁻ = 6 e⁻.
2. Each I₂ molecule formed needs 2 electrons to be removed from 2 I⁻.

Answer: (b) evolved

Explanation:
1. Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is a spontaneous redox reaction.
2. If a zinc rod is dipped in copper sulphate solution, due to the transfer of electrons from zinc to copper ions heat is evolved (the solution warms up).

Teacher's Note:
1. In a Daniell cell the same reaction is used to produce electrical energy instead of heat.
2. The blue colour of the solution fades as Cu²⁺ ions are removed.

Answer: (b) stronger reducing agent than H⁺/H2 couple

Explanation:
1. The standard electrode potential of the H⁺/H2 couple is taken as zero.
2. A negative E° means that the redox couple is a stronger reducing agent than the H⁺/H2 couple.
3. Negative E° = stronger reducing agent; positive E° = weaker reducing agent (stronger oxidising agent).

Teacher's Note:
1. The more negative the E°, the stronger the reduced form is as a reducing agent (e.g. Li, K).
2. Equivalently, the oxidised form of such a couple is a weaker oxidising agent than H⁺.

Answer: (b) In redox reaction, the oxidation number of oxidant increases, while that of reductant decreases

Explanation:
1. Oxidant (oxidising agent) is itself reduced, so its oxidation number decreases.
2. Reductant (reducing agent) is itself oxidised, so its oxidation number increases.
3. So, statement (b) is incorrect. Statements (a), (c) (N is +3 in HNO₂, an intermediate state) and (d) are correct.

Teacher's Note:
1. Oxidant → gets reduced → ON decreases; reductant → gets oxidised → ON increases.
2. HNO₂ (N = +3) can go up to +5 or down to lower states, so it acts both ways.

Answer: (c) 2.5

Explanation:
1. Cl: +4 (in ClO₂) → −1 (in Cl⁻), gain of 5e⁻ per ClO₂. O: −1 (in H₂O₂) → 0 (in O₂), loss of 2e⁻ per H₂O₂.
2. In alkaline medium, the balanced equation is: 2ClO₂ + 5H₂O₂ + 2OH⁻ → 2Cl⁻ + 5O₂ + 6H₂O
3. 2 moles of ClO₂ oxidise 5 moles of H₂O₂.
4. ∴ 1 mole of ClO₂ will oxidise \( \frac{5}{2} \times 1 = 2.5 \) moles of H₂O₂.

Teacher's Note:
1. Moles oxidised = electrons gained per ClO₂ ÷ electrons lost per H₂O₂ = 5/2.
2. Here H₂O₂ acts as a reducing agent.

Answer: (a) KMnO₄

Explanation:
1. Let the oxidation state of Mn be x.
2. (a) KMnO₄: +1 + x + 4(−2) = 0 ⇒ x = +7
3. (b) K₂MnO₄: 2(+1) + x + 4(−2) = 0 ⇒ x = +6
4. (c) Mn₂O₃: 2x + 3(−2) = 0 ⇒ x = +3
5. (d) MnO₂: x + 2(−2) = 0 ⇒ x = +4
6. Thus, the oxidation state of Mn is highest in KMnO₄.

Teacher's Note:
1. +7 is the maximum oxidation state of Mn (3d⁵ 4s²: all 7 valence electrons used).
2. KMnO₄ is purple, while K₂MnO₄ (manganate) is green.

Answer: (c) Fluorine

Explanation:
1. Fluorine is the most electronegative element.
2. Hence, it does not possess a positive oxidation number in any of its compounds (it is always −1).
3. Oxygen is positive in OF₂ and O₂F₂, and Cl and Br are positive in their oxoacids and in compounds with O and F.

Teacher's Note:
1. The most electronegative atom always gets the shared electrons, so it can never be positive.
2. Fluorine also has no d-orbitals in its valence shell, so it shows no higher oxidation states.

Answer: (c) +6

Explanation:
1. On fusion with an oxidising agent (KNO₃) in strongly alkaline conditions (NaOH), the manganous salt (Mn²⁺) is oxidised to green manganate, MnO₄²⁻.
2. In MnO₄²⁻: x + 4(−2) = −2 ⇒ x = +6
3. So, the oxidation number of Mn changes from +2 to +6.

Teacher's Note:
1. In strongly alkaline fusion, manganese ends up as manganate (MnO₄²⁻, Mn +6), not permanganate.
2. This green fusion mass is a test for manganese in qualitative analysis.

Answer: (d) +1

Explanation:
1. The complex ion is [Fe(H₂O)₅NO]²⁺ (balanced by SO₄²⁻). H₂O is a neutral ligand and NO is present as nitrosonium, NO⁺.
2. Let the oxidation state of Fe be x: x + 5(0) + (+1) = +2 ⇒ x = +1
3. So, iron is in +1 oxidation state in the brown ring complex.

Teacher's Note:
1. In the brown ring test, NO transfers one electron to Fe²⁺, giving Fe⁺ and NO⁺.
2. If NO were treated as a neutral ligand the answer would be +2, so read the options carefully.

Answer: (c) Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

Explanation:
1. (a) S: +4 (SO₂) and −2 (H₂S) → 0 (S), a redox reaction.
2. (b) Na: 0 → +1 and O: 0 → −1, a redox reaction.
3. (c) Na (+1), O (−1 in Na₂O₂ and in H₂O₂), S (+6) and H (+1) do not change – no change in valency.
4. (d) Cl: +5 → +7 (KClO₄) and −1 (KCl), a disproportionation reaction.

Teacher's Note:
1. Na₂O₂ and H₂O₂ are both peroxides, so O stays −1 in this acid–base reaction.
2. Making H₂O₂ from a peroxide and an acid is not a redox reaction.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Cr: +6 (in K₂Cr₂O₇) → +3 (in Cr₂(SO₄)₃) – reduction.
2. S: +4 (in SO₂) → +6 (in Cr₂(SO₄)₃) – oxidation.
3. ∴ Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. SO₂ acts as the reducing agent and decolourises orange dichromate to green Cr³⁺.
2. A reaction is redox precisely because oxidation and reduction occur together.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. 2H₂O₂ → 2H₂O + O₂
2. O: −1 (in H₂O₂) → −2 (in H₂O), reduction; O: −1 → 0 (in O₂), oxidation.
3. The same element in the same species is both oxidised and reduced, so this is a disproportionation reaction.
4. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. For disproportionation, the element must be in an intermediate oxidation state.
2. This is why H₂O₂ is stored in dark bottles with stabilisers – it slowly decomposes.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. The structure of Br₃O₈ (tribromooctaoxide) is:
2. Thus, the oxidation state of the two corner Br atoms is +6 and of the middle one is +4. The difference in oxidation states is due to the difference in bonding situations.
3. Average oxidation state = \( \frac{6 + 4 + 6}{3} = \frac{16}{3} \)
4. Thus, both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Average oxidation number is calculated from the formula; actual values need the structure.
2. Each corner Br forms three Br=O bonds, while the middle Br forms two.

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

Explanation:
1. The reaction of potassium permanganate and potassium iodide (acidic medium) is:
10KI + 2KMnO₄ + 8H₂SO₄ → 2MnSO₄ + 6K₂SO₄ + 8H₂O + 5I₂
2. The oxidation state of Mn decreases from +7 to +2, so permanganate is the oxidising agent.
3. Thus, Assertion is correct but Reason is incorrect.

Teacher's Note:
1. An increase from +2 to +7 would be oxidation, which an oxidising agent cannot undergo.
2. I⁻ is oxidised to I₂, so KI is the reducing agent.

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

Explanation:
1. The H⁺/H2 couple has zero standard reduction potential.
2. Couples having positive E° values are weaker reducing agents, while couples having negative E° values are stronger reducing agents than the H⁺/H2 couple.
3. Thus, Assertion is incorrect but Reason is correct.

Teacher's Note:
1. Negative E° → stronger reducing agent; positive E° → stronger oxidising agent.
2. Metals with negative E° displace hydrogen from dilute acids.

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

Explanation:
1. Let the oxidation state of N in N₃H be x: 3x + 1 = 0 ⇒ x = −1/3. So, Assertion is correct.
2. Nitrogen (3.0) is more electronegative than hydrogen (2.1), which is why H is given +1 here.
3. ∴ Assertion is correct but Reason is incorrect.

Teacher's Note:
1. H is +1 with non-metals more electronegative than itself and −1 with metals (hydrides).
2. N₃H is hydrazoic acid; its azide ion N₃⁻ also has an average N oxidation number of −1/3.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. 2H₂O₂ → 2H₂O + O₂
2. O: −1 → −2 (in H₂O), reduction; O: −1 → 0 (in O₂), oxidation.
3. Thus, the above reaction is an example of disproportionation reaction.
4. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. In disproportionation, one species acts as both the oxidising and the reducing agent.
2. MnO₂ or catalase speed up this decomposition.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. In a Daniell cell, electrons are transferred from Zn to Cu²⁺ through the metallic wire which connects the two rods.
2. Inside the cell, electricity flows through the salt bridge by migration of ions from one beaker to the other, completing the circuit.
3. Both Assertion and Reason are correct statements but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Electrons flow only through the external wire; ions carry the current through the salt bridge.
2. The salt bridge also keeps the two solutions electrically neutral.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A redox couple is the combination of the oxidised and reduced forms of a substance taking part in a half-cell reaction.
2. In the representations E°(Fe³⁺/Fe²⁺) and E°(Cu²⁺/Cu), Fe³⁺/Fe²⁺ and Cu²⁺/Cu are redox couples (the oxidised form is written first).
3. Both Assertion and Reason are correct statements but Reason is not the correct explanation of Assertion (it only gives examples).

Teacher's Note:
1. In a redox couple, the oxidised form is written on the left of the slash.
2. Examples are not explanations – look for a cause in the Reason.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Oxidation numbers of elements in their compounds or ions are obtained using some rules, e.g. hydrogen is assigned an oxidation number of +1 in general and −1 in metal hydrides.
2. CaH₂ is an ionic metal hydride (Ca²⁺ and 2H⁻), so H is −1: +2 + 2x = 0 ⇒ x = −1.
3. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. In metal hydrides, hydrogen is the more electronegative partner and exists as H⁻.
2. Such hydrides react with water to give H₂ gas, e.g. CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂.