[trigger701]

Hi everyone,

I am trying to get my head around the equation for enthalpy and visualise it in terms of actual changes in volume. I want to know why the two pdV parts are opposites.

So the equation begins as:

H = dU + PV

As I understand it the PV is the work required to make room for the system as if it had been created from nothing.

Then the differential of this equation is taken, and the internal energy, dU is split up into pressure-volume work and heat, so that now there is a -pdV and a +pdV which cancel.

My question is, how can these two pdVs be the exact opposites of each other? If the +pdV comes from the idea of the system being brought into existence, and the other comes from any work done to the system. Why are these the same?? Is the volume change associated with the system's existence the same as the volume change concerning the internal energy?

[mjc123]

You appear to be confusing, as many people do, thermodynamic quantities with changes in those quantities. So, for a start,
H = U + PV (not dU + PV)

If the +pdV comes from the idea of the system being brought into existence

No, as you said before, "PV is the work required to make room for the system as if it had been created from nothing." PdV is the subsequent work done by making a small change in the volume, and is of course the same as "the other comes from any work done to the system".

[trigger701]

Thank you for your reply mjc. That's set me straight.

[difakozigi]

Thanx a lot, been having issues with this too