Important Redox Reactions Questions from NEET Previous Papers

Q 1 :

Consider the following compounds:
$\underline{K} O_{2}, H_{2} \underline{O_{2}} and H_{2} \underline{S} O_{4}$
The oxidation states of the underlined elements in them are, respectively, [2025]

+1, −2 and +4
+4, −4 and +6
+1, −1 and +6
+2, −2 and +6

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(3)

${KO}_{2}$ ionises as $K^{+}$ and $O_{2}^{-}$ ions as it is a superoxide.

Thus, oxidation state of K is +1.

$H_{2} O_{2}$ ionises as $H^{+}$ and $O_{2}^{2 -}$ as it is a peroxide.

Oxidation state of each O in $O_{2}^{2 -}$ is -1.

In $H_{2} {SO}_{4}$ , let the oxidation state of S be $x$ ; hence

$2 (+ 1) + x + 4 (- 2) = 0$

$2 + x - 8 = 0$

$x = + 6$

Q 2 :

Which reaction is not a redox reaction? [2024]

$Z n + C u S O_{4} \to Z n S O_{4} + C u$
$2 K C l O_{3} + I_{2} \to 2 K I O_{3} + C l_{2}$
$H_{2} + C l_{2} \to 2 H C l$
$B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$

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(4)

$\overset{+ 2}{Ba} \overset{- 1}{{Cl}_{2}} + {\overset{+ 1}{Na}}_{2} \overset{+ 6 - 2}{{SO}_{4}} \to \overset{+ 2}{Ba} \overset{+ 6}{S} \overset{- 2}{O_{4}} + 2 \overset{+ 1}{Na} \overset{- 1}{Cl}$

Here no change in oxidation numbers take place. Hence it is not a redox reaction.

Q 3 :

On balancing the given redox reaction,
$a {Cr}_{2} O_{7}^{2 -} + b {SO}_{3}_{(a q)}^{2 -} + c H_{(a q)}^{+} \to 2 a {Cr}_{(a q)}^{3 +} + b {SO}_{4}_{(a q)}^{2 -} + \frac{c}{2} H_{2} O_{(l)}$
the coefficients a, b and c are found to be respectively [2023]

1, 8, 3
8, 1, 3
1, 3, 8
3, 8, 1

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(3)

${Cr}_{2} {O_{7}}_{(a q)}^{2 -} + 3 {SO}_{3}_{(a q)}^{2 -} + 8 H_{(a q)}^{+} ⟶ 2 {Cr}_{(a q)}^{3 +} + 3 {SO}_{4}_{(a q)}^{2 -} + 4 H_{2} O_{(l)}$

Q 4 :

The oxidation states not shown by Mn in given reaction is

$3 {MnO}_{4}^{2 -} + 4 H^{+} ⟶ 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O$

A. +6 B. +2 C. +4 D. +7 E. +3

Choose the most appropriate answer from the options given below: [2024]

D and E only
B and D only
A and B only
B and E only

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(4)

$\overset{+ 6}{3 {MnO}_{4}^{2 -}} + 4 H^{+} \to \overset{+ 7}{2 {MnO}_{4}^{-}} + \overset{+ 4}{{MnO}_{2}} + 2 H_{2} O$

Q 5 :

Which of the following reactions is the metal displacement reaction? Choose the right option. [2021]

$2 Pb {({NO}_{3})}_{2} ⟶ 2 PbO + 4 {NO}_{2} + O_{2} ↑$
$2 {KClO}_{3} \overset{Δ}{\to} 2 KCl + 3 O_{2}$
${Cr}_{2} O_{3} + 2 Al \overset{Δ}{\to} {Al}_{2} O_{3} + 2 Cr$
$Fe + 2 HCl ⟶ {FeCl}_{2} + H_{2} ↑$

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(3)

In metal displacement reaction more reactive metal displaces the less reactive metal.

${Cr}_{2} O_{3} + 2 Al \overset{Δ}{\to} {Al}_{2} O_{3} + 2 Cr$

Q 6 :

What is the change in oxidation number of carbon in the following reaction? [2020]

${CH}_{4 (g)} + 4 {Cl}_{2 (g)} ⟶ {CCl}_{4 (l)} + 4 {HCl}_{(g)}$

+4 to +4
0 to +4
−4 to +4
0 to −4

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(3)

In ${CH}_{4}$ , oxidation number of carbon is −4 while in ${CCl}_{4}$ , oxidation number of carbon is +4. Thus, the change in oxidation number of carbon in the given reaction is from −4 to +4.

Q 7 :

The correct structure of tribromooctaoxide is [2019]

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(2)

Q 8 :

Which of the following reactions are disproportionation reactions?

(i)   $2 {Cu}^{+} ⟶ {Cu}^{2 +} + {Cu}^{0}$

(ii)   $3 {MnO}_{4}^{2 -} + 4 H^{+} ⟶ 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O$

(iii)   $2 {KMnO}_{4} \overset{Δ}{\to} K_{2} {MnO}_{4} + {MnO}_{2} + O_{2}$

(iv)   $2 {MnO}_{4}^{-} + 3 {Mn}^{2 +} + 2 H_{2} O ⟶ 5 {MnO}_{2} + 4 H^{+}$

Select the correct option from the following:   [2019]

(i) and (iv) only
(i) and (ii) only
(i), (ii) and (iii)
(i), (iii) and (iv)

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(2)

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

$2 {Cu}^{+} \to {Cu}^{2 +} + {Cu}^{0}$

$3 \overset{+ 6}{Mn} O_{4}^{2 -} + 4 H^{+} \to 2 \overset{+ 7}{Mn} O_{4}^{-} + \overset{+ 4}{Mn} O_{2} + 2 H_{2} O$

Q 9 :

The oxidation state of Cr in ${CrO}_{5}$ is [2019, 2014]

– 6
+ 12
+ 6
+ 4

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(3)

${CrO}_{5}$ has butterfly structure having two peroxo bonds.

Peroxo oxygen has - 1 oxidation state.

Let oxidation state of Cr be $' x'$

$C r O_{5} : x + 4 (- 1) + 1 (- 2) = 0 \Rightarrow x = + 6$

Q 10 :

The correct order of N-compounds in its decreasing order of oxidation states is [2018]

${HNO}_{3}, NO, N_{2}, {NH}_{4} Cl$
${HNO}_{3}, NO, {NH}_{4} Cl, N_{2}$
${HNO}_{3}, {NH}_{4} Cl, NO, N_{2}$
${NH}_{4} Cl, N_{2}, NO, {HNO}_{3}$

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(1)

$\overset{+ 5}{{HNO}_{3}}, \overset{+ 2}{N} O, \overset{0}{N_{2}}, \overset{- 3}{N} H_{4} Cl$

Q 11 :

For the redox reaction,

${MnO}_{4}^{-} + C_{2} O_{4}^{2 -} + H^{+} ⟶ {Mn}^{2 +} + {CO}_{2} + H_{2} O$

The correct coefficients of the reactants for the balanced equation are: [2018]

${MnO}_{4}^{-} - 16,$ $C_{2} O_{4}^{2 -} - 5,$ $H^{+} - 2$
${MnO}_{4}^{-} - 2,$ $C_{2} O_{4}^{2 -} - 5,$ $H^{+} - 16$
${MnO}_{4}^{-} - 2,$ $C_{2} O_{4}^{2 -} - 16,$ $H^{+} - 5$
${MnO}_{4}^{-} - 5,$ $C_{2} O_{4}^{2 -} - 16,$ $H^{+} - 2$

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(2)

The correct balanced equation is

$2 {MnO}_{4}^{-} + 5 C_{2} O_{4}^{2 -} + 16 H^{+} \to 2 {Mn}^{2 +} + 10 {CO}_{2} + 8 H_{2} O$

Q 12 :

Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour? [2016]

$Cu + 2 H_{2} {SO}_{4} ⟶ {CuSO}_{4} + {SO}_{2} + 2 H_{2} O$
$S + 2 H_{2} {SO}_{4} ⟶ 3 {SO}_{2} + 2 H_{2} O$
$C + 2 H_{2} {SO}_{4} ⟶ {CO}_{2} + 2 {SO}_{2} + 2 H_{2} O$
${CaF}_{2} + H_{2} {SO}_{4} ⟶ {CaSO}_{4} + 2 HF$

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(4)

${CaF}_{2} + H_{2} {SO}_{4} \to {CaSO}_{4} + 2 HF$

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

Q 13 :

(I) $H_{2} O_{2} + O_{3} ⟶ H_{2} O + 2 O_{2}$

(II) $H_{2} O_{2} + {Ag}_{2} O ⟶ 2 Ag + H_{2} O + O_{2}$

Role of hydrogen peroxide in the above reactions is respectively [2014]

oxidizing in (I) and reducing in (II)
reducing in (I) and oxidizing in (II)
reducing in (I) and (II)
oxidizing in (I) and (II)

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(3)

$H_{2} O_{2}$ acts as reducing agent in both the reactions in which $O_{2}$ is evolved.

Q 14 :

The pair of compounds that can exist together is [2014]

${FeCl}_{3}, {SnCl}_{2}$
${HgCl}_{2}, {SnCl}_{2}$
${FeCl}_{2}, {SnCl}_{2}$
${FeCl}_{3}, KI$

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(3)

Both ${FeCl}_{2}$ and ${SnCl}_{2}$ are reducing agents with low oxidation numbers.

Topic Question Set

Q 1 :

Consider the following compounds:
$\underline{K} O_{2}, H_{2} \underline{O_{2}} and H_{2} \underline{S} O_{4}$
The oxidation states of the underlined elements in them are, respectively, [2025]

Q 2 :

Which reaction is not a redox reaction? [2024]

Q 3 :

On balancing the given redox reaction,
$a {Cr}_{2} O_{7}^{2 -} + b {SO}_{3}_{(a q)}^{2 -} + c H_{(a q)}^{+} \to 2 a {Cr}_{(a q)}^{3 +} + b {SO}_{4}_{(a q)}^{2 -} + \frac{c}{2} H_{2} O_{(l)}$
the coefficients a, b and c are found to be respectively [2023]

Q 4 :

The oxidation states not shown by Mn in given reaction is

$3 {MnO}_{4}^{2 -} + 4 H^{+} ⟶ 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O$

A. +6 B. +2 C. +4 D. +7 E. +3

Choose the most appropriate answer from the options given below: [2024]

Q 5 :

Which of the following reactions is the metal displacement reaction? Choose the right option. [2021]

Q 6 :

What is the change in oxidation number of carbon in the following reaction? [2020]

${CH}_{4 (g)} + 4 {Cl}_{2 (g)} ⟶ {CCl}_{4 (l)} + 4 {HCl}_{(g)}$

Q 7 :

The correct structure of tribromooctaoxide is [2019]

(2)

Q 9 :

The oxidation state of Cr in ${CrO}_{5}$ is [2019, 2014]

Q 10 :

The correct order of N-compounds in its decreasing order of oxidation states is [2018]

Q 11 :

For the redox reaction,

${MnO}_{4}^{-} + C_{2} O_{4}^{2 -} + H^{+} ⟶ {Mn}^{2 +} + {CO}_{2} + H_{2} O$

The correct coefficients of the reactants for the balanced equation are: [2018]

Q 12 :

Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour? [2016]

Q 13 :

(I) $H_{2} O_{2} + O_{3} ⟶ H_{2} O + 2 O_{2}$

(II) $H_{2} O_{2} + {Ag}_{2} O ⟶ 2 Ag + H_{2} O + O_{2}$

Role of hydrogen peroxide in the above reactions is respectively [2014]

Q 14 :

The pair of compounds that can exist together is [2014]

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Topic Question Set

Q 1 : Consider the following compounds: K-O2,H2O2- and H2S-O4 The oxidation states of the underlined elements in them are, respectively, [2025]

Q 2 : Which reaction is not a redox reaction? [2024]

Q 3 : On balancing the given redox reaction, aCr2O72-+bSO3(aq)2-+cH(aq)+→2aCr(aq)3++bSO4(aq)2-+c2H2O(l) the coefficients a, b and c are found to be respectively [2023]

Q 4 : The oxidation states not shown by Mn in given reaction is 3MnO42-+4H+⟶2MnO4-+MnO2+2H2O A. +6 B. +2 C. +4 D. +7 E. +3 Choose the most appropriate answer from the options given below: [2024]

Q 5 : Which of the following reactions is the metal displacement reaction? Choose the right option. [2021]

Q 6 : What is the change in oxidation number of carbon in the following reaction? [2020] CH4(g)+4Cl2(g)⟶CCl4(l)+4HCl(g)

Q 7 : The correct structure of tribromooctaoxide is [2019]

(2)

Q 8 : Which of the following reactions are disproportionation reactions? (i) 2Cu+⟶Cu2++Cu0 (ii) 3MnO42-+4H+⟶2MnO4-+MnO2+2H2O (iii) 2KMnO4→ΔK2MnO4+MnO2+O2 (iv) 2MnO4-+3Mn2++2H2O⟶5MnO2+4H+ Select the correct option from the following: [2019]

Q 9 : The oxidation state of Cr in CrO5 is [2019, 2014]

Q 10 : The correct order of N-compounds in its decreasing order of oxidation states is [2018]

Q 11 : For the redox reaction, MnO4-+C2O42-+H+⟶Mn2++CO2+H2O The correct coefficients of the reactants for the balanced equation are: [2018]

Q 12 : Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour? [2016]

Q 13 : (I) H2O2+O3⟶H2O+2O2 (II) H2O2+Ag2O⟶2Ag+H2O+O2 Role of hydrogen peroxide in the above reactions is respectively [2014]

Q 14 : The pair of compounds that can exist together is [2014]

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Q 1 :

Consider the following compounds:
$\underline{K} O_{2}, H_{2} \underline{O_{2}} and H_{2} \underline{S} O_{4}$
The oxidation states of the underlined elements in them are, respectively, [2025]

Q 2 :

Which reaction is not a redox reaction? [2024]

Q 3 :

On balancing the given redox reaction,
$a {Cr}_{2} O_{7}^{2 -} + b {SO}_{3}_{(a q)}^{2 -} + c H_{(a q)}^{+} \to 2 a {Cr}_{(a q)}^{3 +} + b {SO}_{4}_{(a q)}^{2 -} + \frac{c}{2} H_{2} O_{(l)}$
the coefficients a, b and c are found to be respectively [2023]

Q 4 :

The oxidation states not shown by Mn in given reaction is

$3 {MnO}_{4}^{2 -} + 4 H^{+} ⟶ 2 {MnO}_{4}^{-} + {MnO}_{2} + 2 H_{2} O$

A. +6 B. +2 C. +4 D. +7 E. +3

Choose the most appropriate answer from the options given below: [2024]

Q 5 :

Which of the following reactions is the metal displacement reaction? Choose the right option. [2021]

Q 6 :

What is the change in oxidation number of carbon in the following reaction? [2020]

${CH}_{4 (g)} + 4 {Cl}_{2 (g)} ⟶ {CCl}_{4 (l)} + 4 {HCl}_{(g)}$

Q 7 :

The correct structure of tribromooctaoxide is [2019]

Q 9 :

The oxidation state of Cr in ${CrO}_{5}$ is [2019, 2014]

Q 10 :

The correct order of N-compounds in its decreasing order of oxidation states is [2018]

Q 11 :

For the redox reaction,

${MnO}_{4}^{-} + C_{2} O_{4}^{2 -} + H^{+} ⟶ {Mn}^{2 +} + {CO}_{2} + H_{2} O$

The correct coefficients of the reactants for the balanced equation are: [2018]

Q 12 :

Hot concentrated sulphuric acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour? [2016]

Q 13 :

(I) $H_{2} O_{2} + O_{3} ⟶ H_{2} O + 2 O_{2}$

(II) $H_{2} O_{2} + {Ag}_{2} O ⟶ 2 Ag + H_{2} O + O_{2}$

Role of hydrogen peroxide in the above reactions is respectively [2014]

Q 14 :

The pair of compounds that can exist together is [2014]