[qwerty12321]

Hi Everyone, I have been researching for days, and am still very confused, so thought I might turn here for assistance.

I know the minimum free energy represents the state of maximum stability for a system. Free energy is decreased when enthalpy decreases and entropy increases. I understand why the stability would increase as enthalpy decreases, due to the decline in potential energy.

What I would really like to know is: how does entropy affect the stability (thermodynamic stability)?

Thanks very much, I would really appreciate any assistance regarding this issue.

[rabolisk]

As a high school student, this is beyond anything you need to know. Nevertheless, let me clear some confusions.

Minimum free energy does represent the state of maximum thermodynamic stability, and it is decreased when enthalpy is minimized and entropy is maximized. However, the idea that stability increases as enthalpy decreases because that is a decline in potential energy is wrong.

According to conservation of energy, and decrease in the energy of the system has to be balanced out by an increase in the energy of the surroundings because the energy of the universe is constant. This means that part of the universe (system) has a decrease in energy while the other part of the universe (surroundings) has an increase in energy. Since we define the boundary between the system and the surroundings, it simply cannot be true that a decrease in energy is a driving factor for thermodynamic stability. Why would something favorable (decreasing energy) happen for one part of the universe while something unfavorable (increasing energy) happen for the rest of the universe?

What drives thermodynamic processes is entropy, which is really probability. Entropy of the universe can only stay constant or increase. Any process that happens in the universe results in an increase in entropy. This is why maximization of entropy is a favorable factor in minimizing free energy. The reason a decrease in enthalpy is favorable is because a decrease in enthalpy of a system is approximately an increase in entropy of the surroundings. If the entropy change of the surroundings is added to the entropy change of the system, then you get the entropy change of the universe. If this entropy change of the universe is positive, you get a spontaneous process, which is (approximately) the same thing as a negative change of Gibbs free energy of the system.

http://en.wikipedia.org/wiki/Gibbs_free_energy

Most of this probably didn't make sense to you, and that's ok. Thermodynamics (physical chemistry) is not something you study until late into your chemistry degree.

[qwerty12321]

I know it's complicated, but I really need to know it!

Basically, I am trying to discuss the ka for acids, which I known is dependent on their strength.

When I first started researching, I found a bunch of factors that affect the strength of the acid. The most prominent one was the stability of the conjugate base. If it was more stable, then dissociation was more likely to occur.

Then I found out more about free energy. I found that if the free energy decreases, then the reaction is more likely to occur. I also read that minimum free energy is the most stable state.

So I guess I've kind of come to see stability and free energy as one and the same, which is probably wrong!

Have you got any idea where I am coming from, or how to de-confuse this situation?

[rabolisk]

Where is the confusion? If you are trying to determine what makes an acid strong, then what you should focus on is the structure of the acid and its conjugate base.

[qwerty12321]

I've looked at this and mentioned factors such as the polarity of the bonds and conjugate base stability. Due to this, I've justified why HOCl is a weak acid.

However, I am studying the effect of temperature on the Ka of an acid, which is also a measure of strength. Isn't it also true that I could say it is weak because the reactants have a much lower free energy then the products at room temperature?

Is there a link between these two measures of strength?

[rabolisk]

They are not separate concepts. Saying that the reactant (acid form, HOCl) has a much lower free energy than the product (base form, OCl-) is the same thing as saying that change in Gibbs free energy is positive, which is the same thing as saying that Ka is small (<<<1), which is the same thing as say that HOCl is a weak acid, which is the same thing as saying that the acid (HOCl) is relatively more "stable" than the conjugate base (OCl-).

[qwerty12321]

So, would it be correct to say that because the reactants are more stable than the products (due to polarity, resonance effects etc.), the reactants have a lower free energy? But then, how does the enthalpy and entropy of the products and reactants come into it?

[rabolisk]

ΔG = ΔH - TΔS

ΔG is positive because ΔH - TΔS is positive. For acid dissociation, I assume ΔS has to be positive, since acid dissociation creates more species. My guess is that the primary determinant of ΔG is ΔH.

Here is some data.
http://en.wikipedia.org/wiki/Acid_dissociation_constant