Question

What is a spontaneous reaction?

Answer 1

Before defining spontaneity one must know about entropy and enthalpy. Entropy of a system can be defined by the orderness or diorderness of any system. Enthalpy is the measure of change in heat that comes in or out of the system. Temperature is always measured in Kelvin.

Complete answer:
For any system when entropy increases and enthalpy decreases in a system, this system is said to be spontaneous and when these conditions occur the reaction is said to be spontaneous reaction. Free energy that is associated with the reaction tells whether a reaction is spontaneous or nonspontaneous.
Gibbs free energy equation can be represented as:
$\Delta G{\text{ }} = \;\Delta H - T\Delta S$
Where, $\Delta G$ is Gibbs free energy
$\Delta H$ is change in enthalpy
$\Delta S$ is change in entropy
$T$is temperature
The reaction conditions are as follows:
If a reaction is exothermic, the value of Gibbs free energy is always negative thus the reaction is said to be spontaneous.
If a reaction is endothermic, the value of Gibbs free energy is always positive thus the reaction is said to be non-spontaneous.
If entropy change and enthalpy change both positive and negative, then the spontaneity will depend on the given temperature.
Every reaction is accompanied by net energy. A reaction which is spontaneous is always measured by the net energy released and if certain extra energy is required it is known as activation energy.

Note:
We have to remember that the non-spontaneous system is the one in which the energy is absorbed that means the reaction must be endothermic in nature. The Gibbs free energy for this type of reaction is positive whereas for spontaneous reaction it is negative.