MCQs on Auto Oxidation and Disproportionation

The term oxidation refers to any chemical change in which there is an increase in oxidation number while the term reduction refers to any change resulting in a decrease in oxidation number. Thus oxidation proceeds with loss of electrons while reduction is accompanied by a gain of electrons. The oxidising agent is by definition the substance containing the atom which shows a decrease in oxidation number, while a reducing agent is a substance containing the atom which shows an increase in oxidation number. Oxidation and reduction must always occur together and must compensate for each other. The term oxidising or reducing agent refers to the entire substance and not to just one atom. If a reagent acts as both oxidising and reducing agent we say it undergoes autoxidation or disproportionation.

1. Which one of the following will not reduce an acidified solution of potassium dichromate (VI)?

1. (CH₃)₂C(OH)CH₃
2. FeSO₄
3. CH₃CH₂CH(OH)CH₃
4. Zn

Answer: (a)

2. 25.4g of iodine and 14.2g of chlorine is made to react completely to yield a mixture of ICl and ICl₃. Calculate the ratio of moles of ICl and ICl₃

1. 1:1
2. 1:2
3. 1:3
4. 2:3

Answer: (a)

3. Which of the following has been arranged in order of decreasing average oxidation number of sulphur?

1. H₂S₂O₇ > Na₂S₄O₆ > Na₂S₂O₃ > S₈
2. SO₂⁺ > SO₄^2- > SO₃^2- > HSO₄^–
3. H₂SO₅ > H₂SO₃ > SCl₂ > H₂S
4. H₂SO₄ > SO₂ > H₂S > H₂S₂O₈

Answer: (c)

4. The decomposition of KClO₃ to KCl and O₂ on heating is an example of

1. intermolecular redox change
2. intramolecular redox change
3. disproportionation or auto redox change
4. none of these

Answer: (b)

5. Disproportionation means

1. only oxidation
2. only reduction
3. both, oxidation and reduction of a reactant
4. neither oxidation nor reduction

Answer: (c)

6. The compounds behaving both oxidizing, as well as a reducing agent, is

1. H₂O₂
2. SO₂
3. HNO₂
4. HNO₃

Answer: (a,b,c)

7. A substance which can be oxidised as well as reduced easily is

1. HCl
2. HClO
3. HClO₃
4. HClO₄

Answer: (b,c)

8. Equivalent mass of oxidising agent in the reaction

SO₂ + 2H₂S → 3S + 2H₂O

1. 32
2. 64
3. 16
4. 8

Answer: (c)

9. The oxidation number of iron in Fe₃O₄ is

1. +2
2. +3
3. 8/3
4. ⅔

Answer: (a, b)

10. Oxidation numbers of chlorine in Cl₂O, Cl₂ and ClO₃^– are respectively

1. +1, -1, +5
2. +1, 0, -1
3. -1, 0, +1
4. +1, 0, +5

Answer: (d)

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