Question

A chemical reaction in which heat energy is evolved are called

• A. endothermic reaction
• B. decomposition reaction
• C. displacement reaction
• D. exothermic reaction

Answer 1

The correct option is D

exothermic reaction

The explanation for the correct answer

Option (d). exothermic reaction:

• A chemical reaction in which heat is evolved during the product is called an exothermic reaction.
• Combustion reaction: Carbon burns with oxygen to form carbon dioxide and release heat and light energy.
• Example: $$\begin{gathered} \mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+\mathrm{heat} \\ \mathrm{Carbon}\mathrm{Oxygen}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

The explanation for the incorrect answer

Option (a). endothermic reaction: A chemical reaction in which heat is absorbed during the reaction is called an endothermic reaction.

• Nitrogen reacts with oxygen which gains heat energy to form nitrous oxide
• Example: $$\begin{gathered} {\mathrm{N}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{NO}\left(\mathrm{g}\right) \\ \mathrm{Nitrogen}\mathrm{Oxygen}\mathrm{Nitrous}\mathrm{oxide} \end{gathered}$$

Option (b). decomposition reaction:

• A chemical reaction in which one reactant breaks into two or more products is called a decomposition reaction.
• Calcium carbonate decays to form calcium oxide and carbon dioxide.
• Example: $$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\to \mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \mathrm{Calcium}\mathrm{carbonate}\mathrm{Calcium}\mathrm{oxide}\hspace{0.17em}\mathrm{Carbon}\mathrm{dixide} \end{gathered}$$

Option (c). displacement reaction

• A chemical reaction in which a more reactive element displaces the less reactive element is called a displacement reaction.
• The more reactive Iron displaces the less reactive copper from its compound to form Iron (II) sulphate.
• Example: $$\begin{gathered} \mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{CuSO}}_4\left(\mathrm{s}\right)\to {\mathrm{FeSO}}_4\left(\mathrm{s}\right)+\mathrm{Cu}\left(\mathrm{s}\right) \\ \mathrm{Iron}\mathrm{Copper}\mathrm{sulphate}\mathrm{Iron}\left(\mathrm{II}\right)\mathrm{sulphate}\mathrm{Copper} \end{gathered}$$

Therefore, a chemical reaction in which heat energy is evolved is called an exothermic reaction i.e Option (A)