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Entropy (Thermodynamics)

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
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