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 The answer is 0 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.