Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Vapour Pressure vs Heat of Evaporation

Vapour Pressure vs Heat of Evaporation

Vapour Pressure vs Heat of Evaporation

Hi there,

I have a general theory question.
Am I right to assume a liquid with a relatively high vapour pressure will have a relatively low heat of evaporation? Are they directly related?
Say some liquid has a (high) 40 kPa vapour pressure at 25degC it means the liquid has weak inter-molecular forces and many of the molecules can transition from the liquid to gas phase because they do not require much energy to do so. This is occuring at room temperature where the air temperature remains relatively constant. Therefore the amount of energy the liquid takes in from the air (heat of evap) must be quite low when in comparison to a liquid that has a low vapour pressure?

Thanks for any feedback.

RE: Vapour Pressure vs Heat of Evaporation

The Clasius Clayperon eqn sets out how the latent heat of vaporisation is related to vapor pressure:

dP/dT = ΔH/(T.ΔV), where dP/dT is the rate of change of saturation vapor pressure with temp, T is the mean phase transition temp and ΔH and ΔV are the molar latent heat of vaporisation and change in molar phase volume respectively. So, from this, we see it is the rate of change of sat vap pressure which is directly proportional to the phase change enthalpy.

RE: Vapour Pressure vs Heat of Evaporation

Ok thanks for that,

It does make sense, you need more energy to get larger change in vapour pressures over a certain temperature range..

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close