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.
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.
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 spontaneously occur which increases the disorder of the universe.
In terms of the equations, which are for thinking about (not for plugging and chugging) if there is a difference in free energy comparing Δ H/T going one way with Δ S going the other way, then chemical change is occurring spontaneously in one direction or the other. If a system possesses free energy in a given state, that state is less probable compared to the equilibrium state, and heat flows in the direction will be more likely.
Equilibrium for the system is a way for the universe to maximize its own disorder. Probability dictates. It is about the particles finding a macrostate with many microstates, the more likely one. So 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 are completely reversible.
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