## equilibrium, reversible, delta U

isochoric/isometric: $\Delta V = 0$
isothermal: $\Delta T = 0$
isobaric: $\Delta P = 0$

Ivanna_Tang_3B
Posts: 22
Joined: Wed Sep 21, 2016 2:58 pm

### equilibrium, reversible, delta U

I thought that I heard from or read somewhere that equilibrium meant that the reaction was also reversible and so delta U was equal to 0. but apparently that's not true?
So how would we know when a reaction is reversible (without the problem specifically saying so) and therefore when delta U=0?

thank you!

Chem_Mod
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### Re: equilibrium, reversible, delta U

In a reversible process, the system is always at equilibrium. This is only possible theoretically, where after every infinitesimal step of the process, the system is allowed to reach equilibrium. Practically, it can be done by "slowly" carrying out the process.

For a reaction at equilibrium, ∆G = 0. Any infinitesimal change to the concentrations does not change G, and is reversible (it can be reversed to restore the system to its initial state).

For a reversible expansion or compression of an ideal gas, ∆U = 0 (and ∆G = 0 for the gas + surroundings). Any infinitesimal change in volume can be reversed.