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 Free Power free energy is given by G = H − TS, where H is the enthalpy, T is the absolute temperature, and S is the entropy. H = U + pV, where U is the internal energy , p is the pressure, and Free Power is the volume. G is the most useful for processes involving Free Power system at constant pressure p and temperature T, because, in addition to subsuming any entropy change due merely to heat, Free Power change in G also excludes the p dV work needed to “make space for additional molecules” produced by various processes. Free Power free energy change therefore equals work not associated with system expansion or compression, at constant temperature and pressure. (Hence its utility to solution-phase chemists, including biochemists.)