Entropy (S)

Entropy is a measure of the degree of disorder or randomness of a system. It is represented by the letter ‘S’ and a change in Entropy is ∆S.

Mathematically; 

Entropy change, ∆S = S (products) – S (reactants)

The standard entropy change is the difference between the standard entropy of the product and the reactants. The standard entropy change ∆S^Ꝋ is a state function because it depends on the initial and final state of the system. 

∆S^Ꝋ = ∆S₂^Ꝋ – ∆S₁^Ꝋ

The S.I. unit for entropy is JK^-1 mol^-1. i.e. JoulesJoule is the SI (International System of Units) unit of energy and work. It is equal to the amount of work done when a force of 1 newton displaces a mass... More per Kelvin per mole

Entropy increases from the solid to liquid and to gaseous state because as we go from solid to liquid to gaseous states randomness or disorderliness increases i.e. ∆S tends to be positive similarly in the reverse case i.e. gaseous to liquid to solid states, entropy decreases i.e. ∆S tends to be negative.

In a solid state, the molecules are held together, so they are highly ordered. A perfectly ordered system has an entropy value of zero.

Example 7.3.1:

Nitrogen gas reacts with hydrogen to form ammonia gas. Given that the standard entropies of nitrogen, hydrogen and ammonia gases are 192, 131 and 193 KJ/mol respectively. Calculate the standard entropy change for the reaction.

N_2[g] + 3H_2[g]  → 2NH_3[g]

Solution: