How can think about systems that are not at equilibrium?
Many systems are not at equilibrium. Living cells are an example of a system that is permanently not at equilibrium. A cell at equilibrium is a dead cell. But, in the natural world there are many situations where systems do not come to equilibrium for various reasons. We have seen that we can understand the presence of chemical species at various concentrations using the concept of the chemical potential. If the chemical potentials of the reactants and products are equal then the system is at equilibrium. However, if they are not equal then the system is not at equilibrium. In this video we consider how to combine the informtion of chemical potential with our understanding of hcemical reactions in general to obtain na equation that permits us to predict the driving force (i.e. the free energy change) even when a system is not at equilibrium. This must always done relative to the standard state. The standard state applies to free energy in exactly the same way as it applies to enthalpy. This should not be a surprise since the enthalpy is part of the definition of the free energy.
We have seen that the ratio of product to reactant concentrations is called the reaction quotient. That quotient can have any value. However, when equilibrium is reached then the reaction quotient has a unique value and we give it a special name, the equilibrium constant. This is sometimes confusing to students since the reaction quoeient and the equilibrium constant have the same mathematical form, i.e. they are both product concentrations raised to their stoichiometric powers divided by reactant concentrations raised to their stoichiometric powers. However, we must understand that the value is fixed equilibrium. Since Delta G = 0 at equilibrium, that value is determined by the standard free eenergy change.
This difference is illustrated in the following video.
