Chemistry · Redox reactions · chapter 7 of 9

Class 11 Chemistry Chapter 7: Redox Reactions MCQs with answers

10 multiple-choice questions on Redox Reactions, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
In the classical sense (addition or removal of oxygen, hydrogen or an electronegative element), which of the following conversions is a reduction?
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Answer: (c) ZnO → Zn, on heating zinc oxide with carbon

Removal of oxygen from ZnO is reduction. Adding chlorine to FeCl₂, removing hydrogen from ethanol and adding oxygen to copper are all oxidations in the classical sense.

Question 2
On the electron-transfer view, an oxidising agent is a species that
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Answer: (a) accepts electrons and is itself reduced

An oxidising agent is an acceptor of electrons; gain of electrons is reduction, so the oxidant is itself reduced. A donor of electrons is a reducing agent.

Question 3
In which pair does oxygen have the same oxidation number in both compounds?
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Answer: (b) H₂O₂ and Na₂O₂

Both H₂O₂ and Na₂O₂ are peroxides, with each oxygen at −1. In H₂O oxygen is −2 but in OF₂ it is +2; in KO₂ it is −½ against −1 in Na₂O₂; in O₃ it is 0 against −2 in H₂O.

Question 4
Hydrogen has an oxidation number of −1 in
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Answer: (d) MgH₂

Hydrogen is −1 only when bonded to a metal in a binary compound, as in the hydride MgH₂. In H₂S, NH₄⁺ and HF it is bonded to non-metals and is +1.

Question 5
The oxidation number of nitrogen in hydroxylamine, NH₂OH, is
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Answer: (a) −1

With H = +1 (three H atoms) and O = −2, the sum is zero: x + 3(+1) + (−2) = 0, so x = −1. −3 is nitrogen in NH₃; +1 comes from taking O as −2 but dropping the sign of the hydrogens.

Question 6
The average oxidation number of carbon in propane, CH₃–CH₂–CH₃, is −8/3. Assigning the electrons of each C–H bond to carbon and splitting each C–C bond equally, the oxidation numbers of the three carbon atoms, in order, are
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Answer: (c) −3, −2, −3

A C–C bond between like atoms shifts no electrons. Each terminal carbon carries three H (+1 each), so it is −3; the middle carbon carries two H, so it is −2. Their average is (−3 − 2 − 3)/3 = −8/3: the fraction is only an average, as for C₃O₂.

Question 7
Vanadium pentoxide, V₂O₅, written in Stock notation is
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Answer: (b) V₂(V)O₅

Each V is +5 (2x − 10 = 0), and Stock notation puts that number as a Roman numeral after the metal's symbol: V₂(V)O₅. X would be the total for both atoms; II is the subscript, not an oxidation number.

Question 8
Tungsten is obtained by heating its oxide in hydrogen: WO₃ + 3H₂ → W + 3H₂O. The element whose oxidation number decreases, and the reducing agent, are respectively
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Answer: (d) tungsten in WO₃; H₂

W falls from +6 to 0, so tungsten is reduced. H rises from 0 to +1, so H₂ lowers the oxidation number of W and is the reductant. Oxygen stays at −2 throughout.

Question 9
Which of the following decomposition reactions is NOT a redox reaction?
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Answer: (c) NH₄Cl(s) → NH₃(g) + HCl(g)

In NH₄Cl → NH₃ + HCl, N stays −3, H +1 and Cl −1, and no element is formed. Ag₂O gives Ag (+1 → 0) and O₂ (−2 → 0); NaNO₃ gives N +5 → +3; ammonium dichromate gives N −3 → 0 and Cr +6 → +3.

Question 10
Which of the following combination reactions is NOT a redox reaction?
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Answer: (a) CaO(s) + CO₂(g) → CaCO₃(s)

A combination is redox only when A or B (or both) is an element. CaO and CO₂ are both compounds, and Ca (+2), C (+4) and O (−2) keep their oxidation numbers. The other three start from elements.

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