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Spontaneity - Entropy and Second Law of Thermodynamics

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Estimated time: 6 minutes
CBSE: Class 12

Definition: Entropy

A useful state variable that measures the change in heat divided by the temperature of the system, where the combined entropy of the system and its environment remains constant if the process approaches reversibility, is called entropy.

CBSE: Class 12

Formula: Entropy

ΔS = \[\frac {ΔQ}{T}\]

CBSE: Class 12

Key Points: Entropy and Second Law of Thermodynamics

Entropy (S): A thermodynamic property that measures the degree of randomness or disorder of a system.

\[\Delta S=\frac{q_{rev}}{T}\]

Second Law: The entropy of the universe always tends to increase during any spontaneous process.

Δ Suniverse > 0 (for spontaneous processes)

Total entropy change:

Δ Stotal = ΔSsystem + ΔSsurroundings
 
Condition Process
ΔStotal = +ve Spontaneous
ΔStotal = −ve Non-spontaneous
ΔStotal = 0 Equilibrium

Entropy of mixing:

\[\Delta S_{mix}=-R\sum x_i\ln x_i\]

(ΔS for mixing is always positive, since ΔS is always fractional/positive)

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