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Internal Energy and Spontaneity

Posted: Sun Jan 28, 2018 1:16 pm
by RuchaDeshpande1L
When expanding the volume of a container full of gas in a vacuum, no work is done and so delta U is equal to zero. However, can someone explain why the gas expands spontaneously? Is it because of entropy, and why?

Re: Internal Energy and Spontaneity

Posted: Sun Jan 28, 2018 1:24 pm
by miznaakbar
Spontaneously, a gas wants to occupy the greatest number of positions/states, meaning the entropy increases. Thus, the gas expands its volume to reach its maximum entropy without any change in heat, work, or internal energy.

Re: Internal Energy and Spontaneity

Posted: Sun Jan 28, 2018 1:46 pm
by Tia Tomescu 2D
To add on to that, the system is at equilibrium when entropy is at a maximum.

Re: Internal Energy and Spontaneity

Posted: Fri Feb 02, 2018 1:49 pm
by Sirajbir Sodhi 2K
The spontaneity of a reaction doesn't have a lot to do with internal energy, which is the thermodynamic potential. Only when S and V are constant will a spontaneous reaction reduce internal energy -- not at constant P or T. Rather, we need to look at Gibb's Free Energy which accounts for a non-constant enthalpy and entropy.

In this case, the gas expands as it wants to increase its entropy -- this cannot be predicted by internal energy, only Gibbs Free Energy.