MCQs of Redox Reactions for NEET

Redox Reactions

Question 1. Which of the following is a redox reaction?

1. Evaporation of H₂O
2. Both oxidation and reduction
3. H₂SO₄ with NaOH
4. In the atmosphere O₃ from O₂ by lightning

Answer: 2. Both oxidation and reduction

Redox reactions are those chemical reactions which involve both oxidation and reduction simultaneously.

Question 2. Without losing its concentration, ZnCl₂ solution cannot be kept in contact with

1. Pb
2. Al
3. Au
4. Ag

Answer: 2. Al

Only ‘AI’ lies above ‘Zn’ in the electrochemical series, which can displace Zn from the ZnCl₂ solution. Therefore, conc. of ZnCl₂ will decrease when kept in an ‘Al’ container.

⇒ \(2 \mathrm{Al}+3 \mathrm{ZnCl}_2 \rightarrow 2 \mathrm{AlCl}_3+3 \mathrm{Zn}\)

Question 3. On balancing the given redox reaction, \(a \mathrm{Cr}_2 \mathrm{O}_7^{2-}+b \mathrm{SO}_{3(\mathrm{mq})}^{2-}+c \mathrm{H}_{(\mathrm{mq})}^{+} \rightarrow 2 a \mathrm{Cr}_{[(a q)}^{3+}+b \mathrm{SO}_{4(\mathrm{aq})}^{2-}+\frac{c}{2} \mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}\) the coefficient a, b and c are found to be respectively.

1. 1,8,3
2. 8,1,3
3. 1,3,8
4. 3,8, 1

Answer: 3. 1,3,8

⇒ \(\mathrm{Cr}_2 \mathrm{O}_{7(a q)}^{2-}+3 \mathrm{SO}_{3(a q)}^{2-}+8 \mathrm{H}_{(a q)}^{+} \longrightarrow 2 \mathrm{Cr}_{(a q)}^{3+}+3 \mathrm{SO}_{4(a q)}^{2-}+4 \mathrm{H}_2 \mathrm{O}_{(l)}\)

Question 4. Which of the following reactions is the metal displacement reaction? Choose the right option.

1. \(2 \mathrm{~Pb}\left(\mathrm{NO}_3\right)_2 \rightarrow 2 \mathrm{PbO}+4 \mathrm{NO}_2+\mathrm{O}_2 \uparrow\)
2. \(2 \mathrm{KClO}_3\)\(\longrightarrow{\Delta}\)\(2 \mathrm{KCl}+3 \mathrm{O}_2\)
3. \(\mathrm{Cr}_2 \mathrm{O}_3+2 \mathrm{Al}\)\(\longrightarrow{\Delta}\)\(\mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{Cr}\)
4. \(\mathrm{Fe}+2 \mathrm{HCl} \longrightarrow \mathrm{FeCl}_2+\mathrm{H}_2 \uparrow\)

Answer: 3. \(\mathrm{Cr}_2 \mathrm{O}_3+2 \mathrm{Al}\)\(\longrightarrow{\Delta}\)\(\mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{Cr}\)

When a metal from the electrochemical series is mixed with the ions of a metal lower down in the electrochemical series, then more active metal displaces the less active one, this is known as metal displacement.

Hence, the correct reaction is \(\mathrm{Cr}_2 \mathrm{O}_3+2 \mathrm{Al}\)\(\longrightarrow{\Delta}\)\(\mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{Cr}\)

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Question 5. What is the change in the oxidation number of carbon in the following reaction? \(\mathrm{CH}_{4(\mathrm{~g})}+4 \mathrm{Cl}_{2(\mathrm{~g})} \rightarrow \mathrm{CCl}_{4(l)}+4 \mathrm{HCl}_{(\mathrm{g})}\)

1. +4 to+4
2. 0 to+4
3. -4 to+4
4. 0 to-4

Answer: 3. -4 to+4

In CH₄ the oxidation number of carbon is -4 while in CCl₄, the oxidation number of carbon is +4. Thus, the change in oxidation number of carbon in the given reaction is from -4 to +4.

Question 6. The correct structure of tribromooctaoxide is Which of the following reactions are disproportionation reactions?

Answer: 2

Question 7. Which of the following reactions are disproportionation reactions?

1. \(2 \mathrm{Cu}^{+} \longrightarrow \mathrm{Cu}^{2+}+\mathrm{Cu}^0\)
2. \(3 \mathrm{MnO}_4^{2-}+4 \mathrm{H}^{+} \longrightarrow 2 \mathrm{MnO}_4^{-}+\mathrm{MnO}_2+2 \mathrm{H}_2 \mathrm{O}\)
3. \(2 \mathrm{KMnO}_4\) \(\longrightarrow{\Delta}\)\(\mathrm{K}_2 \mathrm{MnO}_4+\mathrm{MnO}_2+\mathrm{O}_2\)
4. \(2 \mathrm{MnO}_4^{-}+3 \mathrm{Mn}^{2+}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow 5 \mathrm{MnO}_2+4 \mathrm{H}^{+}\)

Select the correct option from the following.

1. (1) and (4) only
2. (1) and (2) only
3. (1), (2) and (3)
4. (1), (3) and (4)

Answer: 2. (1) and (2) only

Disproportionation reactions are those in which the same element/compound gets oxidised and reduced simultaneously.

⇒ \(2 \mathrm{Cu}^{+}\)\(\longrightarrow\)\(\mathrm{Cu}^{2+}+\mathrm{Cu}^0\)

⇒ \(3 \mathrm{MnO}_4^{2-}+4 \mathrm{H}^{+} \longrightarrow \stackrel{+7}{\mathrm{MnO}_4^{-}}+\stackrel{+4}{\mathrm{MnO}_2}+2 \mathrm{H}_2 \mathrm{O}\)

Question 8. The oxidation state of Cr in CrO₅ is

1. -6
2. +12
3. +6
4. +4

Answer: 3. +6

CrO₅ has a butterfly structure having two peroxo bonds.

Peroxo oxygen has -1 oxidation state.

Let the oxidation state of Cr be ‘x’

CrO₅: x+4(-1)+1(-2) =0 + x=+6

Question 9. The correct order of N-compounds in its decreasing order of oxidation states is

1. \(\mathrm{HNO}_3, \mathrm{NO}, \mathrm{N}_2, \mathrm{NH}_4 \mathrm{Cl}\)
2. \(\mathrm{HNO}_3, \mathrm{NO}, \mathrm{NH}_4 \mathrm{Cl}, \mathrm{N}_2\)
3. \(\mathrm{HNO}_3, \mathrm{NH}_4 \mathrm{Cl}, \mathrm{NO}, \mathrm{N}_2\)
4. \(\mathrm{NH}_4 \mathrm{Cl}, \mathrm{N}_2, \mathrm{NO}, \mathrm{HNO}_3\)

Answer: 1. \(\mathrm{HNO}_3, \mathrm{NO}, \mathrm{N}_2, \mathrm{NH}_4 \mathrm{Cl}\)

⇒ \(\stackrel{+5}{\mathrm{HNO}_3,} \stackrel{+2}{\mathrm{NO}}, \stackrel{0}{\mathrm{~N}}, \stackrel{-3}{\mathrm{NH}_4 \mathrm{Cl}}\)

Question 10. For the redox reaction, \(\mathrm{MnO}_4^{-}+\mathrm{C}_2 \mathrm{O}_4^{2-}+\mathrm{H}^{+} \rightarrow \mathrm{Mn}^{2+}+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\). The correct coefficients of the reactants for the balanced equation are

Answer: 2

The correct balanced equation is \(2 \mathrm{MnO}_4^{-}+5 \mathrm{C}_2 \mathrm{O}_4^{2-}+16 \mathrm{H}^{+} \rightarrow 2 \mathrm{Mn}^{2+}+10 \mathrm{CO}_2+8 \mathrm{H}_2 \mathrm{O}\)

Question 11. Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour?

1. \(\mathrm{Cu}+2 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CuSO}_4+\mathrm{SO}_2+2 \mathrm{H}_2 \mathrm{O}\)
2. \(\mathrm{S}+2 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow 3 \mathrm{SO}_2+2 \mathrm{H}_2 \mathrm{O}\)
3. \(\mathrm{C}+2 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CO}_2+2 \mathrm{SO}_2+2 \mathrm{H}_2 \mathrm{O}\)
4. \(\mathrm{CaF}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+2 \mathrm{HF}(\mathrm{NE}\)

Answer: 4. \(\mathrm{CaF}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+2 \mathrm{HF}(\mathrm{NE}\)

⇒ \(\mathrm{CaF}_2+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+2 \mathrm{HF}\)

Here, the oxidation state of every atom remains the same so, it is not a redox reaction.

Question 12.

1. \(\mathrm{H}_2 \mathrm{O}_2+\mathrm{O}_3 \longrightarrow \mathrm{H}_2 \mathrm{O}+2 \mathrm{O}_2\)
2. \(\mathrm{H}_2 \mathrm{O}_2+\mathrm{Ag}_2 \mathrm{O} \longrightarrow 2 \mathrm{Ag}+\mathrm{H}_2 \mathrm{O}+\mathrm{O}_2\)

The role of hydrogen peroxide in the above reactions is respectively

1. Oxidizing in (1) and reducing in (2)
2. Reducing in (1) and oxidizing in (2)
3. Reducing in (1) and (2)
4. Oxidizing in (1) and (2)

Answer: 3. Reducing in (1) and (2)

H₂O₂ acts as a reducing agent in both the reactions in which O₂ is evolved

Question 13. The pair of compounds that can exist together is

1. \(\mathrm{FeCl}_3, \mathrm{SnCl}_2\)
2. \(\mathrm{HgCl}_2, \mathrm{SnCl}_2\)
3. \(\mathrm{FeCl}_2, \mathrm{SnCl}_2\)
4. \(\mathrm{FeCl}_3, \mathrm{KI}\)

Answer: 3. \(\mathrm{FeCl}_2, \mathrm{SnCl}_2\)

Both FeCI₂ and SnCl₂ are reducing agents with low oxidation numbers.

Question 14. A mixture of potassium chlorate, oxalic acid and sulphuric acid is heated. During the reaction which element undergoes maximum change in the oxidation number?

1. S
2. H
3. Cl
4. C

Answer: 3. Cl

⇒ \(\stackrel{+1+5-2}{\mathrm{KClO}_3}+(\mathrm{COOH})_2+\mathrm{H}_2 \stackrel{+6}{\mathrm{~S}} \mathrm{O}_4 \longrightarrow\)\(\mathrm{K}_2^{+6} \mathrm{SO}_4+\mathrm{KCl}+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\)

Question 15. Oxidation numbers of P in PO₄^3-, of S in SO₄^2- and of Cr in Cr₂O₇ are respectively

1. +3, +6 and +5
2. +5, +3 and +6
3. -3, +6 and +6
4. +5, +6 and +6

Answer: 4. +5, +6 and +6

Let the oxidation number of P in \(\mathrm{PO}_4^{3-}\) be r.

∴ x+4(-2)=-3 = x=+5

Let the oxidation number of S in \(\mathrm{SO}_4^{2-}\) be y.

∴ y+4(-2)=-2 + y=+6

Let oxidation number of Cr in \(\mathrm{Cr}_2 \mathrm{O}_7^{2-}\) be z.

∴ 2z+7(-2)=-2 + z=*6

Question 16. The number of moles of MnO₄ required to oxidize one mole of ferrous oxalate completely in an acidic medium will be

1. 7.5 moles
2. 0.2 moles
3. 0.6 moles
4. 0.4 moles.

Answer: 4. 0.4 moles.

⇒ \(\left[5 e^{-}+\mathrm{MnO}_4^{-}+8 \mathrm{H}^* \rightarrow \mathrm{Mn}^{2+}+4 \mathrm{H}_2 \mathrm{O} \ldots\right.(1)] \times 2\)

⇒ \(\left[\mathrm{C}_2 \mathrm{O}_4^{2-} \rightarrow 2 e^{-}+2 \mathrm{CO}_2 \ldots \text { (2) }\right] \times 5\)

Question 17. Which is the best description of the behaviour of bromine in the reaction given below? \(\mathrm{H}_2 \mathrm{O}+\mathrm{Br}_2 \rightarrow \mathrm{HOBr}+\mathrm{HBr}\)

1. Proton acceptor only
2. Both oxidised and reduced
3. Oxidised only
4. Reduced only

Answer: 2. Both oxidised and reduced

Question 18. The oxidation states of sulphur in the anions \(\mathrm{SO}_3^2-\), \(\mathrm{S}_2 \mathrm{O}_4^{2-}\) and \(\mathrm{S}_2 \mathrm{O}_6^{2-}\) follow the order

1. \(\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{SO}_3^{2-}<\mathrm{S}_2 \mathrm{O}_6^{2-}\)
2. \(\mathrm{SO}_3^{2-}<\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{S}_2 \mathrm{O}_6^{2-}\)
3. \(\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{S}_2 \mathrm{O}_6^{2-}<\mathrm{SO}_3^{2-}\)
4. \(\mathrm{S}_2 \mathrm{O}_6^{2-}<\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{SO}_3^{2-}\)

Answer: 1. \(\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{SO}_3^{2-}<\mathrm{S}_2 \mathrm{O}_6^{2-}\)

⇒ \(\mathrm{SO}_3^{2-}: x+(-2) 3=-2\) or \(x-6=-2\) or \(x=+4\)

⇒ \(\mathrm{S}_2 \mathrm{O}_4^{2-}: 2 x+(-2) 4=-2\)

or \(2 x-8=-2\) or \(2 x=+6\)

∴ x = +3

⇒ \(\mathrm{S}_2 \mathrm{O}_6^{2-}: 2 x+(-2) 6=-2\)

or \(2 x-12=-2\) or \(2 x=+10\)

∴ x = +5

Oxidation states follow the order : \(\mathrm{S}_2 \mathrm{O}_4^{2-}<\mathrm{SO}_3^{2-}<\mathrm{S}_2 \mathrm{O}_6^{2-}\)

Question 19. The oxidation state of Fe in Fe₂O₃ is

1. 5/4
2. 4/5
3. 3/2
4. 8/3

Answer: 4. 8/3

⇒ \(\mathrm{Fe}_3 \mathrm{O}_4: 3 x+4(-2)=0 \Rightarrow x=+\frac{8}{3}\)

Question 20. Reaction of sodium thiosulphate with iodine gives

1. Tetrathionate ion
2. Sulphide ion
3. Sulphate ion
4. Sulphite ion.

Answer: 1. Tetrathionate ion

⇒ \(2 \mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3+\mathrm{I}_2 \rightarrow \underset{\mathrm{Sodium tetrathionate}}{\mathrm{Na}_2 \mathrm{~S}_4 \mathrm{O}_6}+2 \mathrm{NaI}\)

Question 21. The oxide, which cannot act as a reducing agent is

1. CO₂
2. ClO₂
3. NO₂
4. SO₂

Answer: 1. CO₂

Since carbon is in its maximum oxidation state of +4, therefore, carbon dioxide (CO₂) cannot act as a reducing agent.

Question 22. Which substance is serving as a reducing agent in the following reaction? \(14 \mathrm{H}^{+}+\mathrm{Cr}_2 \mathrm{O}_7^{2-}+3 \mathrm{Ni} \rightarrow 7 \mathrm{H}_2 \mathrm{O}+2 \mathrm{Cr}^{3+}+3 \mathrm{Ni}^{2+}\)

1. H⁺
2. Cr₂O^2-₇
3. H₂O
4. Ni

Answer: 4. Ni

Since the oxidation number of Ni increases from 0 to 2, therefore it acts as a reducing agent.

Question 23. The oxidation state of I in H₄IO₆^– is

1. +1
2. – 1
3. + 7
4. + 5

Answer: 3. + 7

Let r = Oxidation state of l. Since oxidation state of H = +1 and oxidation state of O = – 2, therefore for H₄IO₆^–, we get (4 x 1) + x + (6x – 2) = -1 or x = +7

Question 24. Consider the change in the oxidation state of bromine corresponding to different emf values as shown in the given diagram:

Then the species undergoing disproportionation is

1. BrO^–₃
2. BrO^–₄
3. Br₂
4. HBrO

Answer: 4. HBrO

For a reaction to be spontaneous, E°_cell, should be positive as ΔG° = -nFE°_cell

HBrO → Br₂; E° = 1.595 V, SRP (cathode)

HBrO → BrO^–₃; E° = -1.5 V, SOP (anode)

2HBrO → Br₂ + BrO^–₃

E°_cell = SRP (cathode) – SRP (anode)

= 1.595 – 1.5 = 0.095 V

E°_cell> 0 ⇒ ΔG° < 0 (spontaneous)