Chemists are taught that if molecules A and B are in dynamic exchange and at equilibrium, there is no net exchange (flux) between them; 1 every molecule of A that is converted to B per unit time is balanced by a molecule of B converted to A. This observation is then used to derive relationships between the rate constants of the exchange, the associated free energy change of converting A to B, and the corresponding equilibrium constant. Although not always explicitly highlighted, this is a consequence of microscopic reversibility 4the transition state for the forward reaction is also the lowest energy pathway for the reverse. These ideas extend to linked equilibria; if A is in equilibrium with B which is in equilibrium with C, the concentrations [A], [B] and [C] satisfy the equilibrium constants KAB, KBC and KAC.