I have just learned about Gibbs free energy and how its value indicates the change in entropy of the universe and can thus be used to find the spontaneity of a reaction. However, I found this other interpretation of Gibbs free energy on Wikipedia that I don't get at all.
The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a thermodynamically closed system (one that can exchange heat and work with its surroundings, but no matter). This maximum can be attained only in a completely reversible process. When a system transforms reversibly from an initial state to a final state, the decrease in Gibbs free energy equals the work done by the system to its surroundings, minus the work of the pressure forces.
What exactly does 'non-expansion work' mean in the context of, say, a gas? How can it even do work other than by expanding or contracting?
Secondly, I thought the extractable energy during a reaction was ΔH( heat transferred during a physical or chemical reaction carried out at constant pressure). If that is actually ΔG=ΔΗ-TΔS instead, what kind of energy does 'TΔS" represent? I know that it is equal to the heat transferred into or out of the system if the process is done reversibly but I don't understand how that's relevant here.
Lastly, Wikipedia mentions that this maximum can only be extracted by a reversible process. How exactly would you extract this? For example, if a reaction has a ΔΗ=-50 and ΔG=-75, how do you get the extra 25 units of energy?