Question

Which of the following reactions is an endothermic reaction?

• A. Burning of coal.
• B. Decomposition of vegetable matter into compost.
• C. Process of respiration.
• D. Decomposition of calcium carbonate to form quick lime and carbon dioxide.

Answer 1

The correct option is D

Decomposition of calcium carbonate to form quick lime and carbon dioxide.

Explanation for correct option:

(D) Decomposition of calcium carbonate to form quick lime and carbon dioxide.

• This is an endothermic reaction. the word “endo” means “in” and the word “thermic” means heat. So an endothermic reaction requires heat.
• Heat is required for the decomposition of calcium carbonate $\left({\mathrm{CaCO}}_3\right)$ to form quick lime $\left(\mathrm{CaO}\right)$and carbon dioxide $\left({\mathrm{CO}}_2\right)$.

$$\begin{gathered} {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\stackrel{∆}{\to }\mathrm{CaO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ \left(\mathrm{Limestone}\right)(\mathrm{Quick}\mathrm{lime}) \end{gathered}$$

Explanation for incorrect options:

(A) Burning of coal.

• This is not an endothermic reaction. Because during the burning of coal heat is released. So it is an exothermic reaction.
• The word “exo” means out and thermic means “heat”.
• An exothermic reaction is a reaction in which heat is released.

(B) Decomposition of vegetable matter into compost.

• This is not an endothermic reaction. Decomposition of vegetable matter into compost is an exothermic reaction.
• So energy is released druing the decomposition of vegetables into compost.

(C) Process of respiration.

• To keep alive, humans require energy. The food we eat provides this energy. Food is broken down into simpler components during digestion.
• Carbohydrates can be found in rice, potatoes, and bread, for example, glucose is formed when these carbs are broken down. In our body cells, glucose mixes with oxygen to provide energy. Hence respiration is an exothermic process.

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{aq}\right)+6{\mathrm{O}}_2\left(\mathrm{aq}\right)\to 6{\mathrm{CO}}_2\left(\mathrm{aq}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{energy} \\ \left(\mathrm{Glucose}\right) \end{gathered}$$

Hence, option (D) is correct i.e. Decomposition of calcium carbonate to form quick lime and carbon dioxide.