The free energy change between two sides of a chemical reaction is like the temperature difference between the hot and cold sinks of a heat engine. Heat flows can occur until equilibrium is reached.

We're on the way to chemical thermodynamics with all of this, so let's have a little preview of the concept of free energy.

During a chemical reaction, the balance of the change in entropy in the surroundings due to the flow of heat (Δ H/T) in or out of the system is balanced against the change in the entropy of the system itself, (Δ S). This balance determines the free energy change (Δ G). Free energy is about the balance of two kinds of entropy change.

We previously mentioned the conception of the temperature as a potential function for the escaping tendency of heat. Think of the free energy change as the measure of the available work that can be realized from the escaping tendency of any poised configuration, any system outside of equilibrium.

Remember that spontaneity is ultimately about increasing total disorder, entropy. Chemical reactions move toward the equilibrium state. This is another way of saying that the entropy of the universe is always increasing. If there is a difference in free energy between two possible states of the system, we are saying that change is set to occur which increases the disorder of the universe, that spontaneous, irreversible change is set to occur.

If a system possesses free energy in a given state compared to equilibrium, that state is less probable compared to the equilibrium state. The free energy is expended until the equilibrium state is reached, and, now, there, at equilibrium, all heat flows are reversible (Δ H/T = Δ S) or (Δ G = 0). Heat can flow microscopically into the system from the surroundings, but it is just as likely to flow right back out.

Events in the equilibrium state are analogous to events in the Carnot cycle, where heat flow and work can occur without increasing the entropy of the universe and any changes are completely reversible.