Question

All statements are thermodynamically correct, EXCEPT:

a) The Gibbs free energy variation for vaporization under T and P constants is always positive
b) If a saturated liquid undergoes reversible adiabatic expansion to a lower pressure, then some of the liquid will vaporize
c) The heat capacity at constant pressure of a monocomponent system consisting of liquid and vapor in equilibrium is infinite
d) The heat capacity at constant volume of a monocomponent system consisting of liquid and vapor in equilibrium is infinite

Answer 1

Statements & Explanations :

a)Gibbs' free energy change is zero for phase change since both phases are in equilibrium.

Moreover, Considering fundamental property relation for free energy:

dG= VdP – SdT

At constant Pressure and temperature during phase change, dG=0.

Hence, Gibbs free energy change for Vaporization is 0 ( Not Positive).

b) Adiabatic means dQ =0. Since It is a reversible process, dW= -pdV

From first law of thermodynamics, dU= -pdV. Since it is expansion process, Work will be done by system and System Temperature will decrease. This temperature decrease will cause the further cooling of liquid instead of Vaporization. Hence, This statement is incorrect.

c) Cp ( Heat capacity at constant pressure) = rate of change of enthalpy with temperature at constant pressure = dH/dT. Since system is at equilbrium, there is no change of temperature & dT=0.

Cp=dH/dT=dH/0= Infinity. Hence, this statement is correct.

d) Cv( Heat capacity at constant Volume) = rate of change of Internal Energy with temperature at constant pressure = dU/dT. Since system is at equilbrium, there is no change of temperature & dT=0.

Cp=dU/dT=dU/0= Infinity. Hence, this statement is correct.