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**Hint:**In thermodynamics, entropy is generally referred to as a measure of spontaneous changes that occur in a system. Simply, it is a tendency of the universe towards disorder.

**Complete Solution :**

Let us study what we mean by entropy and then look forward towards spontaneous and non-spontaneous reactions;

Entropy-

Entropy is a thermodynamic function defined as the measure of disorder of the thermodynamic system. It is represented by ‘S’

Characteristics-

-Entropy is called an extensive property because its value is dependent upon the amount of substance present in the system.

-As the change in value of entropy depends upon change in temperature, volume, pressure and number of moles of substance present in the system it is said to be state function.

-The total change in entropy of an isolated system and the change in entropy of the surrounding is always equal.

-The sum of change in entropy of the system and change in entropy of surrounding is called the entropy change of the universe. Mathematically it is represented as –

$\Delta {{S}_{universe}}=\Delta {{S}_{system}}+\Delta {{S}_{surrounding}}$

-The change in entropy for the cyclic process is 0.

-Reversible process is the process in which both system and surrounding can be returned to its original state by reversing the path.

$\Delta {{S}_{universe}}$ for a reversible process is 0 and hence $\Delta {{S}_{system}}=-\Delta {{S}_{surrounding}}$

- Irreversible process is the process in which the system and surrounding cannot be returned back to their original state.

$\Delta {{S}_{universe}}$ is greater than 0 for an irreversible.

Now, Free energy change-

The free energy change or Gibbs free energy is the amount of energy released while formation of products from the reactants. It is denoted by $\Delta G$.

Formula for Gibbs free energy is $\Delta G=\Delta H-T\Delta S$

Spontaneous process- The process in which free energy change is negative and energy is released.

- In short, if $\Delta G$ is negative for a reaction then it is considered as the energy is released and thus, the reaction is spontaneous. This means that the Gibbs energy of the system is decreased during the process.

$\Delta G$ < 0, the process is spontaneous.

Non-spontaneous process- the process in which free energy change is positive and energy is absorbed.

- In short, if $\Delta G$ is positive for a reaction then it is considered as the energy is not released and products are not formed thus, the reaction is non spontaneous.

$\Delta G$ > 0, the process is non spontaneous.

At equilibrium, the free energy change is zero i.e.

$\Delta G$ = 0, the process is at equilibrium.

**Note:**Since, to determine the actual total entropy content of a given system is difficult and is practically not possible so, it is described as the change in entropy accompanying change in the state of the system.

- The change in entropy is represented by $\Delta S$ and $\Delta S{}^\circ $ is the change in entropy under standard conditions. SI unit is J/Kmol and CGS unit is cal/Kmol.

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