Question

Which of the following is an endothermic reaction?
A. $2{H_2} + {O_2} \to 2{H_2}O$
B. ${N_2} + {O_2} \to 2NO$
C. $2NaOH + {H_2}S{O_4} \to N{a_2}S{O_4} + 2{H_2}O$
D. ${C_2}{H_5}OH + 3{O_2} \to 2C{O_2} + 3{H_2}O$

Answer 1

In the case of endothermic reactions, the heat gets absorbed by the reactants from the surrounding to form the product. Increase in heat that is internal energy takes place in an endothermic reaction. Dissolving ammonium nitrate in water is an example of endothermic reaction.

Complete answer:
Endothermic reactions are those reactions in which the heat gets absorbed by the reactants from the surrounding to form a product. Increase in heat that is internal energy takes place in an endothermic reaction. A closed system generally absorbs thermal energy from the surrounding that is the heat transfer into the system.
From the above option,
In the case of option A: $2{H_2} + {O_2} \to 2{H_2}O$ ,
In the formation of water molecules, heat is released during the process and that’s why it is an exothermic process. So, this is an incorrect option.
In the case of option B: ${N_2} + {O_2} \to 2NO$ ,
In this reaction heat is absorbed by reactants from surroundings to form products. That’s why it is an endothermic reaction.
In the case of option C: $2NaOH + {H_2}S{O_4} \to N{a_2}S{O_4} + 2{H_2}O$ ,
In the formation of products, heat is released during the process and that’s why it is an exothermic process. So, this is an incorrect option.
In the case of option D: ${C_2}{H_5}OH + 3{O_2} \to 2C{O_2} + 3{H_2}O$ ,
In the formation of products, heat is released during the process and that’s why it is an exothermic process. So, this is an incorrect option.

Hence, option B is correct.

Note:
Process of photosynthesis, melting, evaporation, sublimation, cracking of alkanes, thermal decomposition, hydrolysis and so many reactions are endothermic reactions. Physically, melting of ice is also an endothermic reaction as energy is absorbed by ice to get converted into liquid state.