Redox Reactions - Practice Questions & MCQ

The different types of redox reactions are:

• Decomposition Reaction
• Combination Reaction
• Displacement Reaction
• Disproportionation Reactions

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

$2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \xrightarrow{\Delta} 2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g})$

$2 \mathrm{KClO}_3(\mathrm{~s}) \xrightarrow{\Delta} 2 \mathrm{KCl}(\mathrm{s})+3 \mathrm{O}_2(\mathrm{~g})$

This must be noted here that all decomposition reactions are not redox reactions. For example, decomposition of calcium carbonate is not a redox reaction.

$\mathrm{CaCO}_3(\mathrm{~s}) \xrightarrow{\Delta} \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$

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

$\mathrm{C}(\mathrm{s})+\mathrm{O}_2(\mathrm{~g}) \xrightarrow{\Delta} \mathrm{CO}_2(\mathrm{~g})$

$3 M g(s)+N_2(g) \rightarrow M g_3 N_2(s)$

Displacement Reaction
Displacement reactions, also known as replacement reactions, involve compounds and the replacement 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. 

Some examples include:

$\begin{aligned} & \mathrm{Zn}(\mathrm{s})+\mathrm{CuSO}_4(\mathrm{aq}) \rightarrow \mathrm{ZnSO}_4(\mathrm{aq})+\mathrm{Cu}(\mathrm{s}) \text { (Metal displacement) } \\ & \mathrm{Na}(\mathrm{s})+\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g})(\mathrm{Non}-\text { Metal displacement) }\end{aligned}$

Disproportionation Reactions

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

$\begin{aligned} & \mathrm{P}_4(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_3(\mathrm{~g})+3 \mathrm{H}_2 \mathrm{PO}_2^{-} \\ & \mathrm{S}_8(\mathrm{~s})+12 \mathrm{OH}^{-}(\mathrm{aq}) \rightarrow 4 \mathrm{~S}^{2-}(\mathrm{aq})+2 \mathrm{~S}_2 \mathrm{O}_3^{2-}(\mathrm{aq})+6 \mathrm{H}_2 \mathrm{O}(\mathrm{l})\end{aligned}$