Redox Reactions - MCQs with Explanations

1. Select the correct coefficients in the given reaction, when it is balanced:
\( xMnO_{4}^{-} + yH_{2}O_{2} + zH^{+} \rightarrow pMn^{2+} + qO_{2} + rH_{2}O \)

• (a) \( x=1, y=5, z=8 \)
• (b) \( x=5, y=1, z=8 \)
• (c) \( x=1, y=5, z=6 \)
• (d) \( x=2, y=5, z=6 \)

Correct Answer: (d)

Explanation: The balanced redox reaction in acidic medium is \( 2MnO_{4}^{-} + 5H_{2}O_{2} + 6H^{+} \rightarrow 2Mn^{2+} + 5O_{2} + 8H_{2}O \).

2. The oxidation number of S in \( Na_{2}S_{4}O_{6} \) is:

• (a) +2.5 for each S atom
• (b) +2 and +3 (two S have + 2 and other two have + 3)
• (c) +2 and + 3 (three S have +2 and one S has +3)
• (d) +5 and 0 (two S have + 5 and other two have 0).

Correct Answer: (d)

Explanation: In the structure of the tetrathionate ion (\( S_{4}O_{6}^{2-} \)), the two middle sulphur atoms linked to each other have an oxidation number of zero, while the two terminal sulphur atoms have an oxidation number of +5.

3. A compound of Xe and F is found to have 53.3% Xe. Oxidation number of Xe in this compound is:

• (a) -4
• (b) zero
• (c) +4
• (d) +6

Correct Answer: (d)

Explanation: Based on the mass percentages (\( Xe = 53.3/131 \approx 0.4 \); \( F = 46.7/19 \approx 2.45 \)), the simplest ratio is 1:6, giving the formula \( XeF_{6} \), where Xenon is in the +6 state.

4. The decomposition of \( KClO_{3} \) to \( KCl \) and \( O_{2} \) on heating is an example of:

• (a) intermolecular redox change
• (b) intramolecular redox change
• (c) disproportionation or auto redox change
• (d) none of the above.

Correct Answer: (b)

Explanation: It is an intramolecular redox change because two different elements within the same compound undergo oxidation (\( O^{2-} \) to \( O_{2} \)) and reduction (\( Cl^{5+} \) to \( Cl^{-} \)).

5. The reaction is balanced if:
\( 5H_{2}O_{2} + XClO_{2} + 2OH^{-} \rightarrow XCl^{-} + YO_{2} + 6H_{2}O \)

• (a) \( X=5, Y=2 \)
• (b) \( X=2, Y=5 \)
• (c) \( X=4, Y=10 \)
• (d) \( X=5, Y=5 \)

Correct Answer: (b)

Explanation: The balanced equation for the reaction is \( 5H_{2}O_{2} + 2ClO_{2} + 2OH^{-} \rightarrow 2Cl^{-} + 5O_{2} + 6H_{2}O \).

6. The oxidation number of carbon in \( CH_{2}O \) is:

• (a) -2
• (b) +2
• (c) 0
• (d) +4

Correct Answer: (c)

Explanation: Letting the oxidation state of carbon be \( x \): \( x + 2(+1) + (-2) = 0 \), which gives \( x = 0 \).

7. When \( KMnO_{4} \) acts as an oxidising agent and ultimately forms \( MnO_{4}^{2-} \), \( MnO_{2} \), \( Mn_{2}O_{3} \), and \( Mn^{2+} \), then the number of electrons transferred in each case respectively is:

• (a) 4, 3, 1, 5
• (b) 1, 5, 3, 7
• (c) 1, 3, 4, 5
• (d) 3, 5, 7, 1

Correct Answer: (c)

Explanation: Starting from Mn(+7): to \( MnO_{4}^{2-} \) (+6) is 1e; to \( MnO_{2} \) (+4) is 3e; to \( Mn_{2}O_{3} \) (+3) is 4e; to \( Mn^{2+} \) (+2) is 5e.

8. In the conversion of \( Br_{2} \) to \( BrO_{3}^{-} \), the oxidation state of bromine changes from:

• (a) 0 to +5
• (b) -1 to +5
• (c) 0 to -3
• (d) +2 to +5

Correct Answer: (a)

Explanation: Bromine in its elemental form (\( Br_{2} \)) has an oxidation state of 0, and in the bromate ion (\( BrO_{3}^{-} \)), its oxidation state is +5.

9. In \( NH_{4}NO_{2} \rightarrow N_{2} + 2H_{2}O \), Nitrogen (N) is:

• (a) oxidised
• (b) reduced
• (c) both (a) and (b)
• (d) none of these.

Correct Answer: (c)

Explanation: The nitrogen in the ammonium ion is in the -3 state and the nitrogen in the nitrite ion is in the +3 state. Both are converted to \( N_{2} \) (0 state), meaning one is oxidised and the other is reduced.

10. In which of the following compounds, the oxidation number of iodine is fractional?

• (a) \( IF_{7} \)
• (b) \( I_{3}^{-} \)
• (c) \( IF_{5} \)
• (d) \( IF_{3} \)

Correct Answer: (b)

Explanation: In the triiodide ion (\( I_{3}^{-} \)), the average oxidation state is calculated as \( 3x = -1 \), hence \( x = -1/3 \).

11. One mole of \( N_{2}H_{4} \) loses ten moles of electrons to form a new compound Y. Assuming that all the nitrogen appears in the new compound, what is the oxidation state of nitrogen in Y?

• (a) -1
• (b) -3
• (c) +3
• (d) +5

Correct Answer: (c)

Explanation: In hydrazine (\( N_{2}H_{4} \)), Nitrogen is in the -2 state. Losing 10 electrons from 2 nitrogen atoms means each atom loses 5 electrons, shifting the state from -2 to +3.

12. Excess of KI reacts with \( CuSO_{4} \) solution and then \( Na_{2}S_{2}O_{3} \) solution is added to it. Which of the statements is incorrect for this reaction?

• (a) Evolved \( I_{2} \) is reduced.
• (b) \( CuI_{2} \) is formed.
• (c) \( Na_{2}S_{2}O_{3} \) is oxidised.
• (d) \( Cu_{2}I_{2} \) is formed.

Correct Answer: (b)

Explanation: The reaction with excess KI forms cuprous iodide (\( Cu_{2}I_{2} \)) and liberates iodine. \( CuI_{2} \) is unstable and is not the final product.

13. Which one of the following reactions demonstrates that \( H_{2}O_{2} \) acts as an oxidising agent in the basic medium?

• (a) \( Na_{2}CO_{3} + H_{2}O_{2} \rightarrow Na_{2}O_{2} + H_{2}O + CO_{2} \)
• (b) \( 2[Fe(CN)_{6}]^{3-} + H_{2}O_{2} + 2OH^{-} \rightarrow 2[Fe(CN)_{6}]^{4-} + 2H_{2}O + O_{2} \)
• (c) \( Mn^{2+} + H_{2}O_{2} + 2OH^{-} \rightarrow MnO_{2} + 2H_{2}O \)
• (d) \( MnO_{2} + H_{2}O_{2} + 2H^{+} \rightarrow Mn^{2+} + 2H_{2}O + O_{2} \)

Correct Answer: (c)

Explanation: In basic medium, \( H_{2}O_{2} \) oxidises \( Mn^{2+} \) to \( MnO_{2} \) (where Mn moves from +2 to +4).

14. Carbon has an oxidation number of _______ and _______ respectively in diamond and graphite.

• (a) +2, +2
• (b) +1, +1
• (c) +4, -4
• (d) zero, zero

Correct Answer: (d)

Explanation: In their elemental allotropic forms (diamond and graphite), the oxidation state of carbon is zero.

15. The oxidation state of chromium in the final product formed by the reaction between KI and acidified potassium dichromate solution is:

• (a) +2
• (b) +3
• (c) +4
• (d) +5

Correct Answer: (b)

Explanation: Acidified potassium dichromate oxidises KI to \( I_{2} \), while the chromium itself is reduced to the chromium(III) state, forming \( Cr_{2}(SO_{4})_{3} \).

16. Oxidation number of sulphur in Caro's acid is:

• (a) +6
• (b) +4
• (c) +8
• (d) +7

Correct Answer: (a)

Explanation: Caro's acid (\( H_{2}SO_{5} \)) contains a peroxide linkage (\( -O-O- \)). The oxidation state of sulphur is +6.

17. The oxide which cannot act as a reducing agent is:

• (a) \( CO_{2} \)
• (b) \( ClO_{2} \)
• (c) \( NO_{2} \)
• (d) \( SO_{2} \)

Correct Answer: (a)

Explanation: In \( CO_{2} \), carbon is already in its maximum oxidation state (+4), so it cannot be further oxidised and thus cannot act as a reducing agent.

18. Oxidation number of S in \( H_{2}S_{2}O_{8} \) is:

• (a) +7
• (b) +6
• (c) +8
• (d) -2

Correct Answer: (b)

Explanation: In Marshall's acid (\( H_{2}S_{2}O_{8} \)), there is one peroxide linkage. The oxidation state of each sulphur atom is +6.

19. In the balanced chemical reaction, \( IO_{3}^{-} + aI^{-} + bH^{+} \rightarrow cH_{2}O + dI_{2} \), a, b, c and d respectively correspond to:

• (a) 5, 6, 3, 3
• (b) 5, 3, 6, 3
• (c) 3, 5, 3, 6
• (d) 5, 6, 5, 5

Correct Answer: (a)

Explanation: The balanced equation is \( IO_{3}^{-} + 5I^{-} + 6H^{+} \rightarrow 3H_{2}O + 3I_{2} \).

20. The oxidation states of sulphur in the anions \( SO_{3}^{2-} \), \( S_{2}O_{4}^{2-} \) and \( S_{2}O_{6}^{2-} \) follow the order:

• (a) \( S_{2}O_{4}^{2-} < SO_{3}^{2-} < S_{2}O_{6}^{2-} \)
• (b) \( SO_{3}^{2-} < S_{2}O_{4}^{2-} < S_{2}O_{6}^{2-} \)
• (c) \( S_{2}O_{4}^{2-} < S_{2}O_{6}^{2-} < SO_{3}^{2-} \)
• (d) \( S_{2}O_{6}^{2-} < S_{2}O_{4}^{2-} < SO_{3}^{2-} \)

Correct Answer: (a)

Explanation: The oxidation states are +3 for \( S_{2}O_{4}^{2-} \), +4 for \( SO_{3}^{2-} \), and +5 for \( S_{2}O_{6}^{2-} \).

21. For the redox reaction, \( MnO_{4}^{-} + C_{2}O_{4}^{2-} + H^{+} \rightarrow Mn^{2+} + CO_{2} + H_{2}O \), the correct coefficients of the reactants for the balanced reaction are:

	\( MnO_{4}^{-} \)	\( C_{2}O_{4}^{2-} \)	\( H^{+} \)
(a)	2	5	16

Correct Answer: (a)

Explanation: The balanced equation is \( 2MnO_{4}^{-} + 5C_{2}O_{4}^{2-} + 16H^{+} \rightarrow 2Mn^{2+} + 10CO_{2} + 8H_{2}O \).

22. Which of the following statements is not correct?

• (a) Potassium permanganate is a powerful oxidising substance.
• (b) Potassium permanganate is a weaker oxidising agent than potassium dichromate.
• (c) Potassium permanganate is a stronger oxidising agent than potassium dichromate.
• (d) Potassium dichromate oxidises a secondary alcohol into a ketone.

Correct Answer: (b)

Explanation: Potassium permanganate is actually a stronger oxidising agent than potassium dichromate because it gains more electrons per mole in acidic solution.

23. In the disproportionation reaction, \( 3HClO_{3} \rightarrow HClO_{4} + Cl_{2} + 2O_{2} + H_{2}O \), the equivalent mass of the oxidising agent is: (molar mass = 84.45)

• (a) 16.89
• (b) 32.22
• (c) 84.45
• (d) 28.15

Correct Answer: (a)

Explanation: Chlorine moves from +5 in \( ClO_{3}^{-} \) to 0 in \( Cl_{2} \), a change of 5. Equivalent mass = \( 84.45 / 5 = 16.89 \).

24. In which of the following reactions, \( H_{2}O_{2} \) acts as a reducing agent?

• (a) \( 2KI + H_{2}O_{2} \rightarrow 2KOH + I_{2} \)
• (b) \( Na_{2}SO_{3} + H_{2}O_{2} \rightarrow Na_{2}SO_{4} + H_{2}O \)
• (c) \( PbO_{2} + H_{2}O_{2} \rightarrow PbO + H_{2}O + O_{2} \)
• (d) \( KNO_{2} + H_{2}O_{2} \rightarrow KNO_{3} + H_{2}O \)

Correct Answer: (c)

Explanation: In this reaction, \( PbO_{2} \) is reduced to \( PbO \), meaning \( H_{2}O_{2} \) acts as the reducing agent.

25. The bleaching properties of \( H_{2}O_{2} \) are due to its:

• (a) acidic nature
• (b) basic nature
• (c) oxidising nature
• (d) reducing nature.

Correct Answer: (c)

Explanation: The bleaching action is due to the nascent oxygen liberated during its decomposition, which oxidises coloured material to colourless.

26. In acidic medium, \( MnO_{4}^{2-} \):

• (a) disproportionates to \( MnO_{2} \) and \( MnO_{4}^{-} \)
• (b) is oxidised to \( MnO_{4}^{-} \)
• (c) is reduced to \( MnO_{2} \)
• (d) is reduced to \( Mn^{2+} \).

Correct Answer: (a)

Explanation: The manganate ion (\( MnO_{4}^{2-} \)) is unstable in acidic medium and disproportionates into the permanganate ion and manganese dioxide.

27. Oxidation number of Cl in \( NOClO_{4} \) is:

• (a) +7
• (b) -7
• (c) +5
• (d) -5

Correct Answer: (a)

Explanation: Considering the compound as \( [NO]^{+}[ClO_{4}]^{-} \), the oxidation number of Cl is found by \( x + 4(-2) = -1 \), giving \( x = +7 \).

28. The oxidation state of phosphorus in cyclotrimetaphosphoric acid is:

• (a) +3
• (b) +5
• (c) -3
• (d) +2

Correct Answer: (b)

Explanation: In cyclotrimetaphosphoric acid \( (HPO_{3})_{3} \), the calculation \( 3[1 + x + 3(-2)] = 0 \) yields \( x = +5 \).

29. The product of oxidation of \( I^{-} \) with \( MnO_{4} \) in alkaline medium is:

• (a) \( IO_{3}^{-} \)
• (b) \( IO_{4}^{-} \)
• (c) \( IO^{-} \)
• (d) \( I_{2} \)

Correct Answer: (a)

Explanation: In alkaline solution, permanganate oxidises iodide ions to iodate (\( IO_{3}^{-} \)).

30. A compound contains atoms X, Y and Z. Oxidation numbers are \( X=+3, Y=+5, Z=-2 \). The possible formula is:

• (a) \( XYZ_{2} \)
• (b) \( Y_{2}(XZ_{3})_{2} \)
• (c) \( X_{3}(YZ_{4})_{3} \)
• (d) \( X_{3}(Y_{4}Z)_{2} \)

Correct Answer: (c)

Explanation: In \( X_{3}(YZ_{4})_{3} \), the sum of oxidation numbers is \( 3(3) + 3(5) + 12(-2) = 9 + 15 - 24 = 0 \).

31. Equivalent weight of \( FeC_{2}O_{4} \) in the change, \( FeC_{2}O_{4} \rightarrow Fe^{3+} + CO_{2} \) is:

• (a) \( M/3 \)
• (b) \( M/6 \)
• (c) \( M/2 \)
• (d) \( M/1 \)

Correct Answer: (a)

Explanation: There is a 3-electron change overall (1 from \( Fe^{2+} \rightarrow Fe^{3+} \) and 2 from \( C_{2}O_{4}^{2-} \rightarrow 2CO_{2} \)). Thus, Eq wt = \( M/3 \).

32. \( KMnO_{4} \) gets reduced to:

• (a) \( K_{2}MnO_{4} \) in neutral medium
• (b) \( MnO_{2} \) in acidic medium
• (c) \( Mn^{2+} \) in alkaline medium
• (d) \( MnO_{2} \) in neutral medium.

Correct Answer: (d)

Explanation: In neutral or weakly alkaline medium, \( KMnO_{4} \) is reduced to \( MnO_{2} \).

33. For the redox reaction \( Zn + NO_{3}^{-} \rightarrow Zn^{2+} + NH_{4}^{+} \) (in basic medium), the coefficients of Zn, \( NO_{3}^{-} \) and \( OH^{-} \) are:

• (a) 4, 1, 7
• (b) 7, 4, 1
• (c) 4, 1, 10
• (d) 1, 4, 10

Correct Answer: (c)

Explanation: Balancing the reaction yields \( 4Zn + NO_{3}^{-} + 7H_{2}O \rightarrow 4Zn^{2+} + NH_{4}^{+} + 10OH^{-} \).

34. Balanced equation coefficients for \( K_{2}Cr_{2}O_{7} + 4KCl + 6H_{2}SO_{4} \rightarrow 2CrO_{2}Cl_{2} + 6KHSO_{4} + 3H_{2}O \) is:

• (a) a=2, b=4, c=6 etc.
• (d) a=1, b=4, c=6 and x=2, y=6, z=3

Correct Answer: (d)

Explanation: The stoichiometric coefficients are 1, 4, 6, 2, 6, and 3 respectively for the reactants and products in the chromyl chloride test.

35. In \( PCl_{3} + Cl_{2} \rightarrow PCl_{5} \):

• (a) \( Cl_{2} \) is oxidant, \( PCl_{3} \) is reductant.
• (b) \( Cl_{2} \) is reductant, \( PCl_{3} \) is oxidant.

Correct Answer: (a)

Explanation: Phosphorus is oxidised from +3 to +5, while Chlorine is reduced from 0 to -1.

36. In the equation \( BiO_{3}^{-} + 6H^{+} + xe^{-} \rightarrow Bi^{3+} + 3H_{2}O \), the value of x is:

• (a) 2
• (b) 3
• (c) 4
• (d) 6

Correct Answer: (a)

Explanation: Balancing by charge: \( -1 + 6 - x = +3 \), which gives \( x = 2 \).

37. Number of oxidants out of \( Fe^{3+}, SO_{3}, NO_{3}^{-}, I^{-}, Na \) are:

• (a) 1
• (b) 2
• (c) 3
• (d) 4

Correct Answer: (c)

Explanation: \( Fe^{3+} \), \( SO_{3} \), and \( NO_{3}^{-} \) are oxidants as they can be reduced. \( Na \) and \( I^{-} \) are reductants.

38. Oxidation number of Mn in the product of alkaline oxidative fusion of \( MnO_{2} \) is:

• (a) 5
• (b) 7
• (c) 6
• (d) 2

Correct Answer: (b)

Explanation: Alkaline fusion of \( MnO_{2} \) with an oxidising agent produces \( KMnO_{4} \), where Mn is in the +7 state.

39. Identify the substance reduced in the Tollens' reagent reaction:

• (a) \( HCHO \)
• (b) \( [Ag(NH_{3})_{2}]^{+} \)
• (c) \( OH^{-} \)
• (d) \( NH_{3} \)

Correct Answer: (b)

Explanation: Silver is reduced from the +1 state in the complex ion to metallic silver (0 state).

40. Oxidation number of O in \( H_{2}O_{2} \) is:

• (a) -2
• (b) -1
• (c) +1
• (d) +2

Correct Answer: (b)

Explanation: In hydrogen peroxide, each oxygen atom has an oxidation number of -1.

41. What is the oxidation number of Fe in \( Fe(CO)_{5} \)?

• (a) Zero
• (b) 5
• (c) -5
• (d) +3

Correct Answer: (a)

Explanation: In metal carbonyls, the oxidation state of the metal is always zero.

42. In \( Ag_{2}O + H_{2}O + 2e^{-} \rightarrow 2Ag + 2OH^{-} \):

• (a) water is oxidised
• (b) silver is oxidised
• (c) silver is reduced
• (d) hydrogen is reduced.

Correct Answer: (c)

Explanation: Silver moves from the +1 state in \( Ag_{2}O \) to the zero state in Ag, meaning it is reduced.

43. Gas X bubbled through Y and Z-. If reduction potentials are \( Z > Y > X \), then:

• (a) Y will oxidize X but not Z
• (b) Y will oxidize Z but not X

Correct Answer: (a)

Explanation: A substance with higher reduction potential is a stronger oxidising agent. Therefore, Y can oxidise X but is not strong enough to oxidise Z.

44. Which reaction depicts the oxidising behaviour of \( H_{2}SO_{4} \)?

• (a) \( 2HI + H_{2}SO_{4} \rightarrow I_{2} + SO_{2} + 2H_{2}O \)
• (b) \( Ca(OH)_{2} + H_{2}SO_{4} \rightarrow CaSO_{4} + 2H_{2}O \)

Correct Answer: (a)

Explanation: Sulphuric acid oxidises HI to \( I_{2} \) and itself is reduced to \( SO_{2} \).

45. The oxidation number of phosphorus in \( Ba(H_{2}PO_{2})_{2} \) is:

• (a) +3
• (b) +2
• (c) +1
• (d) -1

Correct Answer: (c)

Explanation: Calculation: \( +2 + 2[2(+1) + x + 2(-2)] = 0 \), resulting in \( x = +1 \).