Effect of attractive interactions on Gibbs free energy

In summary, the presence of attractive interactions between gas molecules can result in a lower energy state and a decrease in enthalpy. However, the effect on entropy is not as clear and is dependent on the balance between attractive forces and the natural disorder of gas molecules. Generally, in non-ideal gases, the entropy is slightly lower than in ideal gases due to these attractive forces. This effect is small and may not have been measured, particularly in diatomic gases where other factors may outweigh it.
  • #1
wnvl2
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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?
 
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  • #2
wnvl2 said:
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
 
  • #3
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?
 
  • #4
wnvl2 said:
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.
 
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Related to Effect of attractive interactions on Gibbs free energy

1. What is the Gibbs free energy?

The Gibbs free energy is a thermodynamic property that measures the amount of energy available to do useful work in a system at constant temperature and pressure.

2. How do attractive interactions affect Gibbs free energy?

Attractive interactions, such as van der Waals forces or hydrogen bonding, can decrease the Gibbs free energy of a system by stabilizing the molecules and reducing their potential energy. This results in a more negative value for the Gibbs free energy.

3. Can attractive interactions increase the Gibbs free energy?

In some cases, attractive interactions can increase the Gibbs free energy of a system. For example, if the attractive interactions are too strong, they can cause the molecules to become too tightly bound, making it more difficult for them to move and decreasing the system's entropy. This results in a more positive value for the Gibbs free energy.

4. How do attractive interactions affect the equilibrium constant?

Attractive interactions can affect the equilibrium constant by shifting the balance between the reactants and products. If the attractive interactions favor the formation of products, the equilibrium constant will increase. On the other hand, if they favor the formation of reactants, the equilibrium constant will decrease.

5. Can attractive interactions affect the spontaneity of a reaction?

Yes, attractive interactions can affect the spontaneity of a reaction by changing the value of the Gibbs free energy. If the attractive interactions decrease the Gibbs free energy, the reaction will be more spontaneous. However, if they increase the Gibbs free energy, the reaction will be less spontaneous.

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