Question

Which of the following is an endothermic reaction?
A. $2H_2(g)+O_2(g)\to 2H_{2}O(l)$
B. $N_2(g)+O_2(g)\to 2NO(g)$
C .$NaOH(aq.)+HCl(aq.)\to NaCl(aq.)+H_{2}O(l)$
D. $C_2{H}_5(aq.)+3O_2(g)\to 2C{O}_2(g)+3H_{2}O(l)$

Answer 1

Hint: To solve this type of question, it is required to have knowledge about the concept of exothermic and endothermic reactions. Mostly the reactions in which the reactants have similar electronegativities, i.e. they are either both electronegative or both electropositive show endothermic reactions. This is because such reactions are not spontaneous and require extra energy in the form of heat to be absorbed for the reaction to occur.

Complete step by step answer:
As we know that for an endothermic process, heat is absorbed by the reactants for the reaction to occur. As heat is absorbed by the reactants, $\mathrm{\Delta }{H}_{reaction}>0$. For exothermic processes, heat is released during the formation of products, i.e. $\mathrm{\Delta }{H}_{reaction}<0$.
In option A, we know that hydrogen and oxygen are both oppositely charged in ionic form, hydrogen being highly electro-positive and oxygen being highly electro-negative. When present together, they both rapidly react to form water. This is also the reason why free hydrogen gas is very rare and also inflammable, as it readily reacts with oxygen to form water and heat is released during this process. Thus, it is an exothermic process.
In option B, we know nitrogen is an inert gas as it is present as $N\equiv N$ in the atmosphere and has very high bond dissociation energy. Oxygen is present as $O=O$ in the atmosphere and the product $NO$ is present as $N=O$. Nitrogen being an inert gas requires extra energy to break its triple bonds and form new bonds with oxygen and thus absorbs heat to convert to $NO$. Thus, it is an endothermic process.
In option C, we can clearly see that the reaction occurring is between a strong acid and a strong base which is a neutralisation reaction to form water and salt. We know that when a strong acid reacts with a strong base, a high amount of heat is released. Thus, the reaction is highly exothermic.
In option D, ethane gas reacts with oxygen to form carbon dioxide and water and clearly we can see that this is a combustion reaction. It involves burning of a hydrocarbon to form carbon dioxide, water and heat. Thus, this reaction is an exothermic reaction.

$\therefore$The endothermic reaction from the given options is option B .

Additional Information:
 Mostly for endothermic processes, the heat is absorbed from the surrounding environment and is considered as a reactant. So, it is written alongside the reactants in the reaction equation and for exothermic processes, heat is released in the environment and is written alongside the products in the reaction equation

Note:
 In most of the reactions, oxygen being a reactive gas readily reacts with other elements to form products through exothermic reactions. Sometimes, like in the case of nitrogen, as $N_2$ is also an electro-negative element and an inert gas as well, the reaction is not spontaneous at lower temperatures and requires high energy in the form of heat to occur.