You set the pure component molar enthalpies of pure n-propanol and pure n-butanol equal to zero at T = 325 K, P = 0.1 MPa. What can you say about the molar enthalpy of an equimolar mixture at 325 K and 0.1 MPa?

Question

A. Nothing. You cannot fix the reference molar enthalpy for both substances to zero at the same temperature and pressure.
B. The molar enthalpy of the mixture is probably close to zero since the mixture is a good candidate to behave as an ideal solution.
C. You need to know the heat capacity of the pure components to make a reasonable estimation of the molar enthalpy of the mixture.
D. Since the heat of vaporization of both substances is near 42 kJ/mol, the solution enthalpy should be around 42 kJ/mol for an equimolar mixture.
E. You must account for the increased number of available microstates to make an accurate estimation of the molar enthalpy.

Answer 1

A. Incorrect. This is a perfectly legitimate thing to do. What you cannot do is to fix the reference of a substance at more than one point. Setting a pure component reference for both a mixture and the pure substance would cause inconsistencies in calculations.
B. Correct. Because the components are of similar size and are both straight-chain alcohols, we expect them to interact in a similar way.
C. Incorrect. The heat capacity will allow you to calculate changes in the pure component enthalpy, but does not tell you about the mixture.
D. Incorrect. The heat of vaporization is different from the enthalpy. If a liquid-to-vapor phase change is taking place, you know the enthalpy is increasing by the amount of the heat of vaporization, but otherwise you cannot relate ?Hvap to H.
E. Incorrect. This would be more important for calculations of entropy and Gibbs free energy.