# Homework Help: 1st Law of thermo Qn

1. Mar 8, 2014

### sero2000

1. The problem statement, all variables and given/known data

Basically I just want to know why if I increase the pressure from 0 to 10MPa in a isothermal system containing oxygen, the enthalpy will decrease?

2. Relevant equations

Delta H = Delta U + delta PV

PV = nRT

PV = ZRT

3. The attempt at a solution

During a tutorial, I was actually told that because of the compression, the compressed molecules will have less space to move around which results in a decrease in internal energy. Honestly I dont get it.

If I have a compression taking place, doesn't that mean Work is done on the system? Wouldnt that mean in order to keep the system isothermal, energy would have to be removed from the system? which results in enthalpy being reduced?

In that case and also because at 10MPa the gas is not ideal, I used PV = ZRT but I cant find the link between this and Delta H = Delta U + delta PV.

Any help in clearing this doubt is really appreciated :D

2. Mar 8, 2014

### Staff: Mentor

Enthalpy is independent of pressure only for an ideal gas. For a real gas beyond the ideal gas region,
$$\frac{∂H}{∂p}=\left(V-T\frac{\partial V}{\partial T}\right)$$
Note, for an ideal gas, this is zero. There are generalized dimensionless graphs and tabulations in thermo books of the integral of this expression based on the corresponding states principal. You can use these to estimate the change in enthalpy for oxygen, from knowledge its critical properties. See Introduction to Chemical Engineering Thermodynamics by Smith and Van Ness.

Chet