Question

Identify the exothermic and endothermic reactions from the following:
(A) $HCl+NaOH\to NaCl+H_{2}O+heat$
(B) $2KCl{O}_3\left(s\right)\stackrel{\mathrm{\Delta }}{\to }2KCl\left(s\right)+3O_2\uparrow$
(C)$CaO+H_{2}O\to Ca{\left(OH\right)}_2+heat$
(D) $CaC{O}_3\left(s\right)\stackrel{\mathrm{\Delta }}{\to }CaO\left(s\right)+C{O}_2\uparrow$

Answer 1

Hint: The reactions where heat is on the product side are exothermic in nature. The reactions where heat is used on the reactant side to produce the products are endothermic in nature.

Complete step by step answer:
Understanding the concept of heat and its effects on both system and surrounding will give us a foundation from where we would be able to classify the reactions. The terms, exothermic and endothermic are both related to the exchange of heat between the system and surrounding. As the system is where any work takes place, so when describing the flow of heat, we assume the system to be the centre of our perception.
Taking the above assumptions into account, exothermic reactions can be defined as those where the system loses heat. In other words, the system releases heat into the surrounding and we tend to say that the chemical reaction responsible for this phenomena produces heat. Now, from the options you can easily identify the reactions which produce heat. The trick is to look for reactions which have heat as a product. Clearly, option (A) and (C) are exothermic reactions.

We take a look at endothermic reactions. As you might have guessed, it is fundamentally and functionally opposite to that of exothermic reactions. The system in this case is supplied with heat from the surrounding. The reactions that bring about this phenomenon tend to occur only after a threshold amount of heat has been absorbed by them.
To distinguish these reactions, one has to carefully spot-out those which have heat on the reactant side. In the options provided to us, we can clearly see that in some reactions the sign of $\mathrm{\Delta }$ is used on the arrow, which states that the reactants have used heat in order to produce the products.
So, the correct answer is “Option B and D”.

Note: Exothermic and endothermic are only valid when exchange of heat is considered. In case of energy, we say exergonic and endergonic. They are similar to the former in concept but are related to the exchange of energy between system and surrounding.