Entropy is the quantitative measure of disorder in a system or randomness of a substance. It is expressed by symbol ‘S’.
Entropy is an extensive property of the substance and at definite temperature and pressure, entropy of one mole of a substance has only one constant value.
Entropy increases = randomness increases
Entropy decreases = randomness decreases
If the state of a system changes, entropy will also changes.
ΔS = S Final – S Initial
ΔS = q rev / T
Here q rev = heat gained or lost reversibly by a system and T = Temperature
Unit of S = calorie / °C
Entropy of a system (ΔSsystem) and Entropy of surroundings (ΔS surroundings) helps to decide whether chemical reaction occurs spontaneously or not.
ΔSsystem + ΔS surroundings > 0 reaction = spontaneous
ΔSsystem + ΔS surroundings < 0 reaction = not spontaneous
ΔSsystem + ΔS surroundings = 0 reaction = equilibrium