Question

Choose the correct option.

Chemical reactions in which heat energy is evolved are called ________

• A. Endothermic reactions
• B. Decomposition reactions
• C. Displacement reactions
• D.
  • Exothermic reaction

Answer 1

The correct option is D

• Exothermic reaction

The explanation for the correct option -

Option (D): Exothermic reaction

• Exothermic reaction - Chemical reactions in which heat energy is evolved are called exothermic reactions.
• Example - When water is added to quicklime(calcium oxide)$\left(\mathrm{CaO}\right)$, a lot of heat energy is produced along with calcium hydroxide$\left[\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right]$.

$\underset{\mathrm{Calcium}\mathrm{oxide}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Calcium}\mathrm{hydroxide}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}+\mathrm{Heat}$

The explanation for the incorrect option -

Option (A): Endothermic reactions

• Endothermic reactions - Chemical reactions in which heat is absorbed are called endothermic reactions.
• Example - Nitrogen $\left({\mathrm{N}}_2\right)$combines with oxygen$\left({\mathrm{O}}_2\right)$ at a very high temperature of about 3000$°\mathrm{C}$ to form nitrogen monoxide(Nitric oxide)$\left(\mathrm{NO}\right)$.

$\underset{\mathrm{Nitrogen}}{{\mathrm{N}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\stackrel{3000°\mathrm{C}}{\to }\underset{\mathrm{Nitrogen}\mathrm{monoxide}}{2\mathrm{NO}\left(\mathrm{g}\right)}$

Option (B) Decomposition reactions

• Decomposition reactions - A reaction in which a single compound breaks down into two or more substances is known as a decomposition reaction.
• Example - When limestone is heated, it breaks up into calcium oxide(quick lime ) and carbon dioxide.

$\underset{\underset{\mathrm{Limestone}}{\mathrm{Calcium}\mathrm{carbonate}}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}\stackrel{\mathrm{Heat}}{\to }\underset{\underset{\mathrm{Quicklime}}{\mathrm{Calcium}\mathrm{oxide}}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$

Option (C) Displacement reactions

• Displacement reactions - A reaction in which a more active (reactive) element displaces a less active (reactive) element from its compound when reacted together is called a displacement reaction.
• Example - Iron is more reactive than copper, it displaces copper from copper (II) sulfate solution.

$\underset{\underset{\mathrm{Grey}}{\mathrm{Iron}}}{\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\underset{\mathrm{Blue}}{\mathrm{Copper}\mathrm{sulphate}}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)}\to \underset{\underset{\mathrm{Pale}\mathrm{green}}{\mathrm{Iron}\left(\mathrm{II}\right)\mathrm{sulphate}}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)}+\underset{\underset{\mathrm{Reddish}\mathrm{brown}}{\mathrm{Copper}}}{\mathrm{Cu}\left(\mathrm{s}\right)}$

Therefore, Option (D) i.e. Exothermic reaction is correct.