Effect of attractive interactions on Gibbs free energy

[wnvl2]

Will the presence of attractive interactions between gas molecules raise or lower the molar Gibbs energy of a gas relative to its ‘perfect’ value?

I would think that these attracting forces result in a lower energy state. A decrease in the energy state implies a decrease in the enthalpy. A reduction of the enthalpy implies a lower Gibbs-free energy.

But what will happen to the entropy?

I think that we will get less disorder as the molecules attract each other. And ther will be a little bit more order, even when there is no condensation. For the Gibbs free energy, which is equal to G=H-TS, this means that both effects have to be balanced. So we cannot say in advance what the direction is of the total effect of attractive forces is on the Gibbs-free energy.

Is that reasoning correct?

 

[trurle]

For the Gibbs free energy, which is equal to G=H-TS, this means that both effects have to be balanced.

Entropy and enthalpy effects of molecular clusters can be said to be balanced only if exactly 50% of clusters are dissociated. Generally, such balance do not exist. For most common gases (except for water vapour), entropy contribution is much larger and molecules do not cluster in gas phase at room temperature despite of existence of attraction between them

 

[wnvl2]

But does that mean that when the attractive interactions in a gas increase, but it is not enough to make the gas condensate, that the effect on the entropy is zero or is there a little decrease of the entropy as the molecules are a little bit attracted to each other?

 

[trurle]

But does that mean that when the attractive interactions in a gas increase, but it is not enough to make the gas condensate, that the effect on the entropy is zero or is there a little decrease of the entropy as the molecules are a little bit attracted to each other?

Latter is true. Entropy of non-ideal gas is (a little) lower compared to entropy of ideal gas. This would manifest in experiment as very minor decrease of isobaric thermal capacity below theoretical 0.5⋅(i+2)⋅R
Effect is really small (entropy change is below 1/200000 for xenon at ambient conditions), and i am not sure if it was ever measured. In diatomic gases, it completely swamped by thermal capacity changes due to activation of rotational degrees of freedom.