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Why does Heat of Vap go down?
2

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?

Not really.  At the critical pressure (and temperature), ΔHv = 0, right?

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:

ΔH = T ΔV (dP sat/dT)

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:

ΔH = -R [d lnP sat/d (1/T)]

is about constant.

RE: Why does Heat of Vap go down?

Another way to look at it is that as the V/L system approaches the critical point the properties of the two phases must become the same (volumes, enthalpies, compressibilities, viscosity, etc). Since the V & L enthalpies become the same, the heat of vaporization must become zero. Also, the surface tension must go to zero.

RE: Why does Heat of Vap go down?

(OP)
DickRussell, that's pretty much what I came up with after some more thought.

25362, thank you for your valuable post as well!

Onwards,

Matt

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