Question

Which of the following is/are endothermic reactions/s.
A.${\text{CaC}}{{\text{O}}_{\text{3}}} \to {\text{CaO + C}}{{\text{O}}_{\text{2}}}$
B.${\text{C}}{{\text{H}}_{{\text{4;}}}} + {\text{2}}{{\text{O}}_{{\text{2;}}}} \to {\text{C}}{{\text{O}}_{\text{2}}} + {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$
C.${\text{2HgO}} \to {\text{2Hg + }}{{\text{O}}_{\text{2}}}$
D.${\text{2}}{{\text{H}}_{\text{2}}}{\text{ + }}{{\text{O}}_{\text{2}}}\; \to {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$

Answer 1

Endothermic reactions usually involve the formation of chemical bonds via the absorption of heat from the surroundings. These reactions absorb heat from surroundings and hence, lower the temperature of their surrounding area, thereby creating a cooling effect.

Complete step by step answer:
Endothermic reactions are chemical reactions in which the reactants absorb heat energy from the surroundings to form products. Now, analysing the options systematically.
The reaction in option A is a decomposition reaction and heat has to be supplied to break calcium carbonate into calcium oxide and carbon dioxide. In any reaction, when the heat is supplied to carry out that reaction, it is an endothermic reaction.
The type of reaction in option B is combustion in which hydrocarbon-methane is reacting with ${{\text{O}}_{\text{2}}}$ to yield ${\text{C}}{{\text{O}}_{\text{2}}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$. A combustion reaction is always an exothermic reaction.
Mercuric oxide ${\text{HgO}}$ decomposes on heating to give mercury ${\text{Hg}}$ and oxygen ${{\text{O}}_{\text{2}}}$. It is a decomposition reaction and heat has to be supplied. In any reaction, when the heat is supplied to carry out that reaction, it is an endothermic reaction.
The type of reaction in option D is combustion in which hydrogen gas is reacting with oxygen gas to yield water. As the combustion reaction is always an exothermic reaction, it is an exothermic reaction.

Considering the given options, we can conclude that the correct answers to this question are option A and option C.

Note:
The terms ‘Endo’ and ‘Exo’ mean ‘within’ and ‘out’, respectively. The primary difference between endothermic and exothermic reactions is that the former absorbs heat from the surroundings whereas the latter releases heat to the surrounding. Also, if the student is unaware of the exception, this question can easily be solved via elimination.