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Redox Reactions — NEET & JEE MCQs with answers
Redox Reactions (Unit 7) covers the evolution of oxidation and reduction from classical oxygen/hydrogen definitions to the modern electron-transfer framework. Oxidation numbers are assigned using established rules and used to identify oxidants and reductants, balance equations, and classify reactions (combination, decomposition, displacement, disproportionation). Electrode processes connect redox chemistry to electrochemical cells.
Topics in this chapter
- 7.1 Classical Idea of Redox Reactions — Oxidation and Reduction
- 7.2 Redox Reactions in Terms of Electron Transfer Reactions
- 7.3 Oxidation Number
- 7.4 Redox Reactions and Electrode Processes
Free practice questions
In the combination reaction CH4 + 2O2 -> CO2 + 2H2O, the change in oxidation number of carbon is
- A. +4 to -4
- B. -2 to +2
- C. 0 to +4
- D. -4 to +4
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Answer: D — In CH4 carbon is -4 (H is +1) and in CO2 carbon is +4; carbon is oxidised from -4 to +4, while oxygen is reduced 0 to -2, making this combustion a redox combination reaction.
NCERT Class 11 Chemistry · Ch 7 · 7.3 Oxidation Number
In the reaction 2 Na (s) + H2 (g) → 2 NaH (s), hydrogen is:
- A. Both oxidised and reduced
- B. Oxidised
- C. Neither oxidised nor reduced
- D. Reduced
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Answer: D — Using the concept of electronegativity, hydrogen is reduced (it is the more electronegative species here, gaining electron character).
NCERT Class 11 Chemistry · Ch 7 · 7.1 Classical Idea of Redox Reactions
Using the E° values of Table 7.1 (Mg²⁺/Mg = −2.36 V, Al³⁺/Al = −1.66 V, Zn²⁺/Zn = −0.76 V, Fe²⁺/Fe = −0.44 V), the correct DECREASING order of reducing strength is:
- A. Zn > Fe > Mg > Al
- B. Fe > Zn > Al > Mg
- C. Al > Mg > Fe > Zn
- D. Mg > Al > Zn > Fe
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Answer: D — Reducing strength increases as E° becomes more negative. The order of E° from most to least negative is Mg (−2.36) > Al (−1.66) > Zn (−0.76) > Fe (−0.44), which is the order of decreasing reducing strength.
NCERT Class 11 Chemistry · Ch 7 · 7.4 Redox Reactions and Electrode Processes (Table 7.1)
Which statement best explains why the zinc–copper electron-transfer reaction can be harnessed in a Galvanic cell while a copper strip in zinc sulphate cannot?
- A. Both reactions are equally spontaneous
- B. The Zn–Cu²⁺ reaction is spontaneous (favours products) and supplies electrical energy; the reverse is not
- C. Zinc sulphate is coloured while copper nitrate is colourless
- D. Copper sulphate is more soluble than zinc sulphate
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Answer: B — Only a spontaneous reaction that greatly favours products (Zn → Zn²⁺, Cu²⁺ → Cu) provides usable electrical energy; the reverse (Cu in ZnSO₄) is non-spontaneous, giving no detectable reaction and no useful cell.
NCERT Class 11 Chemistry · Ch 7 · 7.2.1 Competitive Electron Transfer Reactions
The term that describes a reaction where oxidation and reduction occur together is derived from combining the words:
- A. Reduction and oxidation
- B. Reaction and oxygen
- C. Removal and oxidation
- D. Reduction and oxygen
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Answer: A — 'Redox' is coined by combining REDuction and OXidation.
NCERT Class 11 Chemistry · Ch 7 · 7.1 Classical Idea of Redox Reactions
Under which complete set of conditions is the potential of an electrode called its Standard Electrode Potential?
- A. Zero concentration, no gas, 298 K
- B. Unit concentration, gas at 10 atm, 373 K
- C. Unit concentration, gas at 1 atm, 298 K
- D. Any concentration, gas at 2 atm, 273 K
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Answer: C — Standard electrode potential requires unit concentration of each species, gases at 1 atm, and a temperature of 298 K.
NCERT Class 11 Chemistry · Ch 7 · 7.4 Redox Reactions and Electrode Processes
Which of these is NOT a correct description of a half reaction?
- A. A reduction half reaction shows gain of electrons
- B. A half reaction never shows electrons
- C. An oxidation half reaction shows loss of electrons
- D. The sum of the two half reactions gives the overall reaction
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Answer: B — A half reaction explicitly shows electrons (either lost or gained). The option claiming a half reaction never involves electrons is false; the others are correct.
NCERT Class 11 Chemistry · Ch 7 · 7.2 Redox Reactions in Terms of Electron Transfer Reactions
What is the change in oxidation number of carbon in the following reaction ?
$CH_4(g) + 4Cl_2(g) \rightarrow CCl_4(g) + 4HCl(g)$
- A. $+4$ to $+4$
- B. $0$ to $+4$
- C. $-4$ to $+4$
- D. $0$ to $-4$
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Answer: C — In $CH_4$, C is $-4$ (H is +1). In $CCl_4$, C is $+4$ (Cl is −1). So the oxidation number of carbon changes from $-4$ to $+4$. Option C.
Class 11 · Chemistry · Ch 7 Redox Reactions · Sec 7.3 Oxidation Number
Using E°(Fe³⁺/Fe²⁺) = +0.77 V and E°(I₂/I⁻) = +0.54 V, the reaction 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ is:
- A. feasible, since E°cell = +0.23 V
- B. feasible, since E°cell = +1.31 V
- C. feasible only at high temperature
- D. not feasible, since E°cell = −0.23 V
Show answer
Answer: A — Fe³⁺ (higher E°) is reduced and I⁻ is oxidised. E°cell = E°(reduction) − E°(oxidation) = 0.77 − 0.54 = +0.23 V (> 0), so the reaction is feasible — Fe³⁺ oxidises iodide to iodine.
NCERT Class 11 Chemistry · Ch 7 · 7.4 Redox Reactions and Electrode Processes (Table 7.1; cf. Ex. 7.26a)
The electrochemical (metal activity) series is built by listing metals and their ions on the basis of their tendency to:
- A. Form coloured oxides
- B. Release electrons
- C. Release protons
- D. Absorb water
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Answer: B — NCERT: metals and their ions are listed on the basis of their tendency to release electrons, just as acids are listed by their tendency to release protons.
NCERT Class 11 Chemistry · Ch 7 · 7.2.1 Competitive Electron Transfer Reactions
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