Question

Identify the endothermic and exothermic reaction.
a. $\text{HCl + NaOH }\to \text{ NaCl + }{{\text{H}}_{2}}\text{O + heat}$
b. $2\text{KCl}{{\text{O}}_{3}}\text{(s) }\xrightarrow{\Delta }\text{ 2KCl(s) + 3}{{\text{O}}_{2}}\uparrow $

Answer 1

Hint: An attempt to this question can be made by understanding exothermic and endothermic reactions. Now determine the enthalpy in these two types of reaction. This will help you get a clear idea about the intake/release of heat and thus determine the type of reaction as well.

Complete answer:
Enthalpy is a property of a thermodynamic system, that is a convenient state function used as a means of measurement in various chemical and biological systems at constant pressure.
A state function is a property such that its value does not depend on the path taken by the physical quantity to reach a specific value. Enthalpy is one such state function.
An exothermic reaction is defined as a reaction that involves the release of heat along with other products. On the contrary, endothermic reaction requires energy for the reaction to take place or occur in the first place.
Based on the above statements we can conclude that
$\text{HCl + NaOH }\to \text{ NaCl + }{{\text{H}}_{2}}\text{O + heat}$ is exothermic in nature
$2\text{KCl}{{\text{O}}_{3}}\text{(s) }\xrightarrow{\Delta }\text{ 2KCl(s) + 3}{{\text{O}}_{2}}\uparrow $ is endothermic in nature
Additional information: Thermodynamics is a branch of physics that deals with the physical quantities heat, work and temperature and their relation to radiation and energy released.
The behaviour of these quantities is in accordance with the laws of thermodynamics. The 3 laws of thermodynamics give a quantitative description using the above physical quantities at a microscopic level.
In contrast to state function, functions that depend on the path taken by the quantity to go between two values are called path functions. Both these functions are often used in thermodynamics.

Note: We can never define the absolute enthalpy of a substance or a reaction. This is the reason why we consider change in enthalpy for a reaction like enthalpy of formation, enthalpy of atomisation etc.