Now, let’s go ahead and define the change in free energy for this particular reaction. Now as is implied by this delta sign, we’re measuring Free Power change. So in this case, we’re measuring the free energy of our product, which is B minus the free energy of our reactant, which in this case is A. But this general product minus reactant change is relevant for any chemical reaction that you will come across. Now at this point, right at the outset, I want to make three main points about this value delta G. And if you understand these points, you pretty much are on your way to understanding and being able to apply this quantity delta G to any reaction that you see. Now, the first point I want to make has to do with units. So delta G is usually reported in units of– and these brackets just indicate that I’m telling you what the units are for this value– the units are generally reported as joules per mole of reactant. So in the case of our example above, the delta G value for A turning into B would be reported as some number of joules per mole of A. And this intuitively makes sense, because we’re talking about an energy change, and joules is the unit that’s usually used for energy. And we generally refer to quantities in chemistry of reactants or products in terms of molar quantities. Now, the second point I want to make is that the change in Free Power-free energy is only concerned with the products and the reactants of Free Power reaction not the pathway of the reaction itself. It’s what chemists call Free Power “state function. ” And this is Free Power really important property of delta G that we take advantage of, especially in biochemistry, because it allows us to add the delta G value from multiple reactions that are taking place in an overall metabolic pathway. So to return to our example above, we had A turning into Free Power product B.
The historically earlier Helmholtz free energy is defined as A = U − TS. Its change is equal to the amount of reversible work done on, or obtainable from, Free Power system at constant T. Thus its appellation “work content”, and the designation A from Arbeit, the Free Energy word for work. Since it makes no reference to any quantities involved in work (such as p and Free Power), the Helmholtz function is completely general: its decrease is the maximum amount of work which can be done by Free Power system at constant temperature, and it can increase at most by the amount of work done on Free Power system isothermally. The Helmholtz free energy has Free Power special theoretical importance since it is proportional to the logarithm of the partition function for the canonical ensemble in statistical mechanics. (Hence its utility to physicists; and to gas-phase chemists and engineers, who do not want to ignore p dV work.)