# Find the dH for a system

1. Sep 26, 2009

### higherme

dH = dU + P dV

dU = q - P dV

I need to find the dH for a system (1mol gas)going from P=1.24 atm to P=0.62atm, with constant volume of 46.72 L. I already found the value of q = -4402 J, w=0 , and dU = -4402 J

I think dH = dU since its change in V is 0. So my answer was dH =dU = -4402J. But i dont' know why i can't get the answer.

2. Sep 26, 2009

### higherme

Re: enthalpy

nevermind, i figured out what i was missing.

3. Sep 26, 2009

### higherme

Re: enthalpy

if I want to find the enthalpy change of a whole cycle, do i add up all the enthalpy of each step? The system goes from A to B, from B to C, and from C back to A

4. Sep 27, 2009

### Mapes

Re: enthalpy

1. Is enthalpy a state variable? If so, what are the implications for changes over a complete cycle?

2. Your differential enthalpy equation isn't quite right.

5. Sep 27, 2009

### higherme

Re: enthalpy

ya, enthalpy is a state variable. so this means that it doesn't depend on the path, right? so if i want to find change in enthalpy, it will be the final enthalpy minus initial enthalpy?

6. Sep 27, 2009

### higherme

Re: enthalpy

if its not a state variable (i.e heat (q)), it would be path dependant, which means for a complete cycle, you would have to add up the heat at each step, correct?

7. Sep 28, 2009

### Mapes

Re: enthalpy

Agreed, enthalpy is a state variable (i.e., its value is determined only by the system's current state). So if the system returns to its original state at the end of a cycle, what is the enthalpy change?