Redox Reactions MCQ - Practice Questions & Answers

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Types of Redox Reactions

The different types of redox reactions are:

Decomposition Reaction
Combination Reaction
Displacement Reaction
Disproportionation Reactions

Decomposition Reaction
This is the reaction which involves the breakdown of a compound into different compounds. Some examples of this type of reaction are:

$2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \stackrel{\Delta}{\longrightarrow} 2 \mathrm{H}_{2}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{g})$
$2 \mathrm{KClO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} 2 \mathrm{KCl}(\mathrm{s})+3 \mathrm{O}_{2}(\mathrm{g})$

This must be noted here that all decomposition reactions are not redox reactions. For example, decomposition of calcium carbonate is not a redox reaction.
$\mathrm{CaCO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{g})$

Combination Reaction
These types of reactions are the opposite of decomposition reaction and hence involve the combination of two compounds to form a single compound. Some examples include:

$\mathrm{C}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{g}) \stackrel{\Delta}{\longrightarrow} \mathrm{CO}_{2}(\mathrm{g})$
$\mathrm{CH}_{4}(\mathrm{g})+2 \mathrm{O}_{2}(\mathrm{g}) \stackrel{\Delta}{\longrightarrow} \mathrm{CO}_{2}(\mathrm{g})$

Displacement Reaction
Displacement reactions, also known as replacement reactions, involve compounds and the replacing of elements. They occur as single and double replacement reactions. In other words, in theses type of reactions, an atom or an ion in a compound is substituted by another element. The general representation of this reaction is as follows:

$\mathrm{X}+\mathrm{YZ} \rightarrow \mathrm{XZ}+\mathrm{Y}$

Disproportionation Reactions

Disproportionation reactions are those reactions in which a single element in one oxidation state is simultaneously oxidized and reduced. Some examples include:

$\mathrm{P}_{4}(\mathrm{s})+3 \mathrm{OH^{-}}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}$
$\begin{array}{r}{\mathrm{S}_{8}(\mathrm{s})+12 \mathrm{OH}(\mathrm{aq}) \rightarrow 4 \mathrm{S}^{2-}(\mathrm{aq})+2 \mathrm{S}_{2} \mathrm{O}_{3}^{2}(\mathrm{aq})} \: {+6 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})}\end{array}$