# [Physics] Does increasing pressure increase or decrease latent heat of vaporisation

pressurethermodynamics

I found some say increasing pressure increases the latent heat of vaporisation (What is the effect of an increase in pressure on latent heat of vaporization?). This doesn't make sense to me because increasing pressure increases the boiling point of liquid. More work is required to convert water to vapour under increased pressure. That is why water boils at lower temperature at higher altitude. Can someone please confirm whether my understanding is correct?

Higher temperature is needed for boiling at higher pressure, as saturated vapour pressure, raining exponentially with temperature, must match external pressure.

Heat of evaporation decreases with temperature and is zero at the critical point, as properties of gaseous and liquid phase converge to the common point.

The rate of this decreasing is given by the difference of specific(or molar) heat of the vapour and the liquid.

E.g water evaporation heat at $$20 ^\circ \mathrm{C}$$ is equal to

• the evaporation heat at $$100 ^\circ \mathrm{C}$$

• plus heat to warm liquid water $$20->100 ^\circ \mathrm{C}$$

• minus released heat by cooling water vapour $$100->20 ^\circ \mathrm{C}$$

For evaporation at the same temperature but higher pressure, the evaporation heat does not increase, but is effectively integrated with higher work done on gas.

Last but not least, enthalpy ( of evaporation or any other) is state function at constant pressure.For standard enthalpy, standard pressure is implied and any volume work is implicitly included as difference $$\Delta H - \Delta U$$.