Question

Which of the following is not a decomposition reaction?

• A. $CaC{O}_3\stackrel{\Delta }{\to }CaO+C{O}_2$
• B. $2KCl{O}_3\stackrel{\Delta }{\to }2KCl+3O_2$
• C. Digestion of food in body
• D. $H_2+C{l}_2\to 2HCl$

Answer 1

The correct option is D

$H_2+C{l}_2\to 2HCl$

Explanation for the correct option:

(D)

• The reaction in which a single reactant decomposes into two or more products is called a decomposition reaction.
• The given reaction is as follows:
• ${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{HCl}\left(\mathrm{g}\right)$
• The reaction in which a two or more substances combine to form a single product molecule is termed a combination reaction or synthesis reaction.
• In the given reaction, the formation of single product hydrogen chloride or hydrochloric acid $\left(\mathrm{HCl}\right)$ takes place from two reactants, hydrogen $\left({\mathrm{H}}_2\right)$ and chlorine $\left({\mathrm{Cl}}_2\right)$, thus, the given reaction is classified as a combination reaction.

Explanation for incorrect options:

(A) The given reaction is as follows:

${\mathrm{CaCO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$

Here, calcium carbonate or limestone $\left({\mathrm{CaCO}}_3\right)$ decomposes to produce calcium oxide or quicklime $\left(\mathrm{CaO}\right)$ and carbon dioxide $\left({\mathrm{CO}}_2\right)$. Therefore, it is an example of a decomposition reaction.

(B) The given reaction is as follows:

$2{\mathrm{KClO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }2\mathrm{KCl}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)$

Here, potassium chlorate $\left({\mathrm{KClO}}_3\right)$ decomposes to produce potassium chloride $\left(\mathrm{KCl}\right)$ and oxygen molecule $\left({\mathrm{O}}_2\right)$. Therefore, it is an example of a decomposition reaction.

(C) During digestion, large molecules in food like starch and proteins get decompose into smaller molecules like simple sugar and amino acids respectively. Therefore, it is an example of a decomposition reaction.

$\underset{(\mathrm{Complex}\mathrm{molecules})}{\mathrm{Carbohydrates},\mathrm{fats}\mathrm{or}\mathrm{proteins}}\to \mathrm{Simpler}\mathrm{molecules}$

Final Answer: Hence, option (D) is correct. ${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{HCl}\left(\mathrm{g}\right)$ is not a decomposition reaction.