Question

What is an example of a thermochemistry practice problem?

Answer 1

The boiling point of methanol is 64.7 C, and its enthalpy of vaporization is 37 kJ/mol. Calculate the equilibrium vapor pressure of methanol at 32 C.

Hint:
Use the phase equilibrium expression
`ΔS_(vap) = (ΔH_(vap))/(T_b)`
with the data provided to compute `K_(eq)`. Remember that the activity of the liquid is 1, so `K_(eq)=P_(vap)`

Answer: 0.2438 bar

Solution:
`ΔS_(vap) = (ΔH_(vap))/(T_b) = 109.5 J/(mol-K)`
`ΔG_(vap)(T) = ΔH_(vap) - TΔS_(vap) = 3581. J/(mol)` at `T = 305.15 K`

The equilibrium constant `K_(eq) = P_(vap) = e^((-ΔG_(vap))/(RT)) = 0.2438` (bar)