# Change in Enthalpy when Internal Energy does not Change

1. Nov 7, 2012

### Luscinia

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

The internal energy of a perfect gas does not change when the gas undergoes isothermal expansion. What is the change in enthalpy?

2. Relevant equations
H=U+PV

3. The attempt at a solution

Since U, the internal energy, doesn't change, I assumed that the equation would be ΔH=PΔV. Why would PΔV equal 0 if there is an expansion (ΔV is not 0)? I'm guessing that I'm not seeing something here since this seemed to be a rather simple question.

2. Nov 7, 2012

### AGNuke

The enthalpy is actually $\Delta H = \Delta U + \Delta (PV)$. I don't think that PV will change.