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+ > SO42- > SO32- > 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

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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