# enthalpy

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### chiddler

##### Full Member
10+ Year Member
&#916;H = &#916;U + P&#916;V

when P is constant, then

&#916;H = q

Ok I understand that because you can use the internal energy equation and substitute stuff.

So what about when pressure is not constant. Can you not measure enthalpy?

&#916;H = &#916;U + P&#916;V

when P is constant, then

&#916;H = q

Ok I understand that because you can use the internal energy equation and substitute stuff.

So what about when pressure is not constant. Can you not measure enthalpy?

H=U+PV and U=Q-W and W=PV

So H=U-PV+PV, as you say.

If P is not constant, the change in volume is not easily predictable. I think this relates to PV work in the sense that when P is not constant, you need to use calculus to calculate the work done. So you can measure enthalpy, just not in a way that we would be able to/required to on the mcat.

now that i think about it,

internal energy, &#916;U = Q - P&#916;V. Rearranging this to: Q = &#916;U + P&#916;V

enthalpy, &#916;H = &#916;U + P&#916;V

Q = &#916;U + P&#916;V
&#916;H = &#916;U + P&#916;V

uh. what's the difference? it looks like &#916;H = &#916;Q under all conditions.

now that i think about it,

internal energy, &#916;U = Q - P&#916;V. Rearranging this to: Q = &#916;U + P&#916;V

enthalpy, &#916;H = &#916;U + P&#916;V

Q = &#916;U + P&#916;V
&#916;H = &#916;U + P&#916;V

uh. what's the difference? it looks like &#916;H = &#916;Q under all conditions.

Yea, U=Q-PV

and H=U+PV so H=Q-PV+PV. PV cancels out even if P is not constant it seems. I guess you can use H=Q even if P isn't constant? But you just can't calculate PV work easily if P isn't constant?

Members don't see this ad :)
Yea, U=Q-PV

and H=U+PV so H=Q-PV+PV. PV cancels out even if P is not constant it seems. I guess you can use H=Q even if P isn't constant? But you just can't calculate PV work easily if P isn't constant?

no there's a reason that they don't cancel out. but my calculus sucks to understand it conceptually. i asked this question a few days ago on reddit:

Me: If you substitute &#916;U = Q - P&#916;V into &#916;H = &#916;U + P&#916;V:
&#916;H = &#916;U + P&#916;V
&#916;H = (Q - P&#916;V) + P&#916;V = Q
&#916;H = Q

Them: It was bad math, work in differential form:
dU = dQ + dW
Assume we only have PV work
dU = dQ - PdV
H = U + PV
dH = dU + PdV + VdP
dH = dQ + VdP
Thus, dH = dQ only with constant pressure

no there's a reason that they don't cancel out. but my calculus sucks to understand it conceptually. i asked this question a few days ago on reddit:

Me: If you substitute &#916;U = Q - P&#916;V into &#916;H = &#916;U + P&#916;V:
&#916;H = &#916;U + P&#916;V
&#916;H = (Q - P&#916;V) + P&#916;V = Q
&#916;H = Q

Them: It was bad math, work in differential form:
dU = dQ + dW
Assume we only have PV work
dU = dQ - PdV
H = U + PV
dH = dU + PdV + VdP
dH = dQ + VdP
Thus, dH = dQ only with constant pressure

Told you it had something to do with calculus

I can't explain it though, I took calculus 6 years ago.

oh. i just realized i reworded my question and then answered myself...

thanks medpr