# How does enthalpy vary with pressure? Doesn't it increase at higher pressure?

##### 1 Answer

I think the confusion is in considering the variation of

**Variation in** **with pressure at a constant temperature**

I calculated the

http://www.tlv.com/global/TI/calculator/steam-table-pressure.html

Here is a graph I constructed in Excel plotting data acquired from the above calculations for the variation in the **enthalpy of vaporization of water** at 10 - 2000 PSI:

**Enthalpy of vaporization** (

#s = k_H P#

The solubility of a gas increases with a higher vapor pressure of gas above the solution.

If you **increase** the pressure of the atmosphere, the gas becomes **more** soluble in solution (because it's pushed into the solution), so its vapor pressure above the solution **increases**.

Since higher vapor pressure means easier boiling, it also means easier vaporization and thus lower *decreases* at *higher* pressure for a *constant* temperature.

**Variation in** **with pressure at a constant temperature**

After some derivation, we would have:

#color(blue)(((delH)/(delP))_T = T((delV)/(delT))_P - V)#

This says that the variation in

- The variation in volume due to temperature changes at constant pressure
- The volume of the system

Let's say we have an ideal gas. Then:

#((delV)/(delT))_P = (del)/(delT)[(nRT)/P]_P = (nR)/P#

Since

But

This was one possibility for what you meant, but it's likely not what you were thinking.