Question

Which of the following is/are endothermic reactions?
A. ${\text{CaC}}{{\text{O}}_3} \to {\text{CaO}} + {\text{C}}{{\text{O}}_2}$
B. ${\text{C}}{{\text{H}}_4} + 2{{\text{O}}_2} \to {\text{C}}{{\text{O}}_2} + 2{{\text{H}}_2}{\text{O}}$
C. $2{\text{HgO}} \to 2{\text{Hg}} + {{\text{O}}_2}$
D. $2{{\text{H}}_2} + {{\text{O}}_2} \to 2{{\text{H}}_2}{\text{O}}$

Answer 1

The change of one or more substances into other substances having different composition and properties is called a chemical reaction. Reactions that absorb energy are endothermic. Energy is absorbed from the environment, so there is a decrease in the temperature.

Complete step by step answer:
All chemical reactions involve bond breaking and bond forming. Energy is needed to break bonds and be released when bonds are formed. Chemical reactions are accompanied by a change in energy, mainly in the form of heat.
A reaction in which heat is given out is called exothermic while a reaction in which heat is taken in is endothermic.
A. Consider the reactions given below:
${\text{CaC}}{{\text{O}}_3} \to {\text{CaO}} + {\text{C}}{{\text{O}}_2}$ and $2{\text{HgO}} \to 2{\text{Hg}} + {{\text{O}}_2}$
These are examples of decomposition reactions. In this reaction, ${\text{CaC}}{{\text{O}}_3}$ breaks down into ${\text{CaO}}$ and ${\text{C}}{{\text{O}}_2}$ and ${\text{HgO}}$ decomposes to ${\text{Hg}}$ and ${{\text{O}}_2}$. In these reactions, energy in the form of heat is taken from surroundings. Thus it is said to be an endothermic reaction. Thus the enthalpy change value is positive. Products have higher energy than the reactants. The system determines the sign of enthalpy change and we are part of surroundings. This feels cold when touched.
While other reactions are combustion and combination reactions. Combustion reactions are generally exothermic. Heat is released from the system to surroundings.
Hence the correct options are A and C.

Note:
Exothermic reaction causes the reaction mixture and its surroundings to become hotter. While endothermic reaction causes the reaction mixture and its surroundings to become colder. Photosynthesis is the best example of endothermic reaction. During photosynthesis, energy is drawn from the surroundings. This fuels the start of the reaction.