Question

What is an exothermic reaction ?

Answer 1

Hint: These reactions have the standard enthalpy change of reactions a negative value. It is the opposite of endothermic reactions that involve the absorption of heat. These reactions usually lead to an increase in the entropy of the surroundings.


Complete step by step answer:
The exothermic reaction can be defined as the reactions for which overall standard enthalpy change is negative. These in general terms are known by the reactions which release heat and weak bonds are replaced by strong bonds. Most of the reactions that we perform in the lab are exothermic reactions.

For example – When we combine a strong acid with a strong base. What we see is that the beaker becomes hot. This is because the acid base reaction is exothermic in nature. A large amount of heat is released when acid combines with base giving salt and water.
Another example we see is the combustion of hydrocarbon. A large amount of energy in the form of heat is released during combustion of hydrocarbon.
Example: The combustion of methane is as-
$C{H_4} + {O_2} \to C{O_2} + {H_2}O + heat$
A large amount of heat is released in the reaction.

Additional Information:
In thermodynamics, an exothermic process may be defined as the one in which energy is released in the form of sound or light. In chemistry, actually the chemical bond is converted into thermal energy i.e. heat. By this we mean that the weaker bonds with high energy are broken and strong bonds are formed which need less energy for formation and as a result, energy is released.

Note: The exothermic reactions are sometimes confused with exergonic reactions. Now, what are these exergonic reactions?
These are the reactions for which the overall standard Gibbs energy change is negative. It is true that any strong exothermic reaction will be exergonic in nature.