## Whether PV is significant

$\Delta U=q+w$

Hai-Lin Yeh 1J
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### Whether PV is significant

For the first law of thermodynamics in our notes, there is this part about constant volume and constant pressure. For constant pressure, can someone explain the significance of "for a reaction at constant pressure that involves changes in the number of moles of gas, then PdeltaV is significant and for a reaction at constant pressure that involves solids and liquids, then PdeltaV is insignificant". How does this affect the equation we will use? to find internal energy?

David Effio 1H
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Joined: Wed Feb 14, 2018 3:01 am

### Re: Whether PV is significant

This is because changes in the volume of the system affect a gas more than a liquid or solid. Most of the time we will be working with gases since they are capable of doing work by expansion.
However, when adding energy to a liquid or solid, they will probably not expand much, if at all (assuming they remain in that state), so w = -P∆V is insignificant since no work will actually be done.

Hai-Lin Yeh 1J
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### Re: Whether PV is significant

Does the entire reaction have to contain all gases to be significant and vice versa? So if the reaction have gas as reactants and solid as products, is PdeltaV significant or nonsignificant?