Why does Heat of Vap go down?
Why does Heat of Vap go down?
(OP)
Never noticed this but it sparked some discussion today. Why does the heat of vaporization of water go down as the pressure goes up? Seems counter-intuitive.
Onwards,
Matt





RE: Why does Heat of Vap go down?
Good luck,
Latexman
RE: Why does Heat of Vap go down?
As Latexman says the heat of vaporization decreases monotonically from the triple point to the critical point.
The answer to ChEMatt's question is found in the Clausius-Clapeyron equation for the latent heat of evaporation of pure substances:
where ΔV is the molar volume difference between the saturated vapor and the saturated liquid.
As T increases, (dP sat/dT) drops over the range of interest, so does ΔV, resulting in a narrower ΔH as T and P increase.
It should be noted that at low pressures the latent heat of vaporization is about constant since the slope (d ln P sat/d(1/T) in the Clausius-Clapeyron equation for the heat of vaporization:
is about constant.
RE: Why does Heat of Vap go down?
RE: Why does Heat of Vap go down?
25362, thank you for your valuable post as well!
Onwards,
Matt