In this article we will discuss about the equilibrium constant of a reaction and free energy charge.

Any chemical reaction will reach an equilibrium after a sufficient time.

A reversible reaction may be written:

The velocity of the reaction which will produce C and D is written:

v1 = k1 [A][B]

and that of the reaction producing A and B would be:

v2 = k2 [C][D]

k1 and k2 are the velocity constants of the reaction.

When equilibrium is reached, velocities v1 and v2 are evidently equal

The concentrations indicated in this formulation are those reached at equi­librium.

The capacity of a molecule to react is characterized by a parameter ex­pressed in kCal/mole and called free energy. The free energy change in a reaction of the type A + B ⇋ C + D when one mole of A and one mole of B give one mole of C and one mole of D, while their respective concentrations [A], [B], [C], and [D] are maintained constant, is written:

This change therefore depends on two parameters. First, a constant char­acteristic of the reaction (∆G0), the standard free energy change, which is the change in free energy in conditions called standard: temperature, 298°K; pressure, one atmosphere; pH = 0; and [A], [B], [C], [D], maintained at 1M. We have then ∆G = ∆G0.

∆G then depends on the concentration of the products and reagents. It is important to note that, theoretically, there are always concentrations of reagents and products such that ∆G < 0. In this case, the reaction is called exergonic and is always accompanied by a decrease in free energy. It can then yield energy.

For example the reaction:

C6H12O6 + 6O2 → 6CO2 + 6H2O

has a ∆G of – 686 kcal/mole of glucose, which means that in standard condi­tions, the oxidation of one mole of glucose will yield 686 kilocalories of free energy. This energy may be dissipated as heat or converted into mechanical energy (muscle contraction), electrical energy (transmission of nerve impulse or formation of ion gradients), or in certain cases, even radiant energy. It can also be conserved as chemical energy in molecules, the most important of which is ATP.

Lastly, if the reaction is in equilibrium, ∆G = 0 and one has the following equation:

There is therefore, a direct relationship between the standard free energy-change of the reaction and its equilibrium constant.

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