Second Law of Thermodynamics

Theory

Consider two processes:

•Dissolution of sugar in water at room temperature: Itself

•Burning of coal in air or oxygen: Needs initiation

•Spontaneity can be defined as process having the potential to proceed without the assistance of external agency.

•Spontaneous process cannot be reversed on its own.

•Spontaneous process does not mean it is instantaneous.

Entropy change and sponteneity:

Entropy (S) is used to measure the extent of disorder or randomness. More the disorder, more is entropy.

The second law of thermodynamics states that:
 For any spontaneous process, the entropy of the universe increases (ΔS_univ > 0)

ΔS_univ = Î”S_sys + Î”S_surr > 0

ΔS = S_final - S_initial

ΔS is defined as the ratio of heat absorbed by the system isothermally and reversibly to the temperature at which it is absorbed.

∆S=q_rev/T

At equilibrium, ΔS = 0