I'm studying for the Physics GRE and going over my thermo notes.(adsbygoogle = window.adsbygoogle || []).push({});

Ok, so for enthalpy change

(delta)H = (delta) U + P * (delta) V

H is enthalpy, U is just proportional to T

What pressure are they talking about?

If the volume and temperature are changing, then the internal pressure is definitely changing, but in this equation it seems to be constant through out the process. So I assume that this is supposed to mean the pressure of the surroundings.

But this is tricky.

If you have a box of gas in space, the surrounding pressure is zero. So it seems like the only enthalpy change would be due to U. But this doesn't seem right. So let's look at how it expands.

Some latch could get thrown during the process and allow the piston to slide out frictionlessly. Or the piston could be connected to a spring that would get compressed as the gas expanded. Or the piston could just be rusty and release energy as heat when it moves.

I feel like all of these scenarios would have a different enthalpy change, but the equation doesn't seem to account for this. I'm guessing there's something more interesting in the P factor. What does it really mean?

-thanks

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# Enthalpy (H=U+PV) What is P, really?

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