For the vaporization of a liquid at a given pressure: A.ΔG is positive at all temperatures. B.ΔG is negative at all temperatures. C.ΔG is positive at low temperatures, but negative at high temperatures (and zero at some temperature). D.ΔG is negative at low temperatures, but positive at high temperatures (and zero at some temperature). E. None of these.

C. ΔG is positive at low temperatures, but negative at high temperatures (and zero at some temperature).

Explanation:

Since we need to give energy in the form of heat to vaporize a liquid, the enthalpy is positive. In a gas, molecules are more separated than in a liquid, therefore the entropy is positive as well.

Considering the Gibbs free energy equation:

ΔG= ΔH - TΔS

          +        +

When both the enthalpy and entropy are positive, the reaction proceeds spontaneously (ΔG is negative) at high temperatures. At low temperatures, the reaction is spontaneous in the reverse direction (ΔG is positive).