Question

Entropy changes during phase transformations.

Answer 1

a) 

∆_fus H = enthalpy of fusion per mole

T_m = melting temperature in Kelvin

b) 

∆_fusH = enthalpy of vapourization per mole

T_b = boiling temperature in kelvin

c)

∆_sub H = enthalpy of sublimation at the temperature T.

• For all spontaneous processes, the total entropy change must be positive

∆S_total = ∆S_system +∆S_surr > O

∆S_total = +ve ⇒ Process is spontaneous

S_total = -ve ⇒ Direct process is non-spontaneous; the reverse process is spontaneous.

• When a system is in equilibrium, the entropy is maximum and the change in entropy, ∆S= O.

• ∆U does not discriminate between reversible and irreversible process whereas ∆Sdoes.

∆U = O (for both reversible & irreversible expansion for an ideal gas; under isothermal conditions)

∆S_total > O (for irreversible process) and ∆S_total = O (for reversible process)