Chapter 3 – Enthalpy and Internal Energy: The Importance of State Functions

Let’s examine the pressure of any system as a function of T and V

For a fluid system or a solid Isothermal Compressibility

For solids and fluid systems The coefficient of thermal expansion

Assume that we will write pressure as a function of T and V

In general, we write U as a function of T and V

Examine the second partial derivative

A T 1, V m,1, P 1 B Stirrer Valve Thermal insulation FF OO C O C O

The partial derivative is known as the Joule coefficient,  J.

The change in the internal energy under isothermal conditions is related to the Joule Coefficient

For an ideal gas In general

In general, we write H as a function of T and P

Define the constant pressure heat capacity, C P

Examine the second partial derivative

Porous Plug Thermal insulation T 1, P 1, V m,1 T 2, P 2, V m,2 FF OO C O C O FF OO C O C O

The partial derivative is known as the Joule-Thomson coefficient,  JT.

The change in the enthalpy under constant pressure conditions is related to the Joule-Thomson Coefficient