Question

What does one mean by exothermic and endothermic reactions? Give examples.

Answer 1

Hint: Both exothermic and endothermic deal with the flow of heat between a given system and its perceived surrounding. To differentiate between them we have to attach ourselves with a particular system and think accordingly.

Complete step by step solution:
Let’s start with an example. Ice cubes are the most chilled ones in our homes and experimenting with them has been an integral part of our childhood. When an ice cube melts, it undergoes a physical process in which the final result is a change in its state of matter, i.e. from solid to liquid. The kinetic energies of the molecules in a liquid are far greater than solid, which makes solid rigid and liquids to flow. The change of state in above case is brought about by the heat which is present in the surroundings. In other words heat is absorbed by the ice cubes breaking their intermolecular attractions and converting them to the more loosely packed liquid state. This is an example of endothermic reaction- a reaction in which heat is absorbed by a body or system.
A common laboratory reaction of ammonium chloride with water absorbs the surrounding heat and thus is endothermic in nature. The products are ammonium ion and chloride ion. The reaction is as follows:

\[N{{H}_{4}}Cl(s)\xrightarrow{{{H}_{2}}O}N{{H}_{4}}^{+}(aq)+C{{l}^{-}}(aq)\]

Exothermic reactions on the other hand are opposite, i.e. they release heat. The direction of flow of heat in this case is from the system to surrounding. This is what happens when water freezes to form ice. The molecular vibrations in water lose their energy and due to the decreasing inter-atomic distance, forces such as van der waals’ come into play fixing them in definite places and hence producing a crystallized structure called ice.
In our laboratories we have many times diluted a concentrated solution of sodium hydroxide ($NaOH$). This dilution process is accompanied by the release of heat which increases the temperature of the beaker holding the solution. This is therefore an exothermic process. The dissociation of $NaOH$into its constituent ions ($N{{a}^{+}}\And O{{H}^{-}}$) and their subsequent interaction with the water molecules makes them more stable hence releasing heat. The reaction is as follows:

\[NaOH(l)\xrightarrow{{{H}_{2}}O}N{{a}^{+}}(aq)+O{{H}^{-}}(aq)\]

Note: A reaction can release or absorb one of the following: heat, light and energy. Exothermic and endothermic only deal with heat and therefore are crucial clues when exchange of heat between the system and surrounding far outweighs any other physical quantity. It is very common that heat, light and energy are always associated with each other but care should be taken when assigning specific terms to each one of them.