4 Equilibrium positions and rates of reaction – an overview
So far you have seen that if a chemical reaction is to occur at a particular temperature, two conditions must be fulfilled.
What are these conditions?
The equilibrium constant must be sufficiently large, and reaction rate sufficiently great.
So there is a crucial distinction between the equilibrium constant and the rate and this reveals itself in Figure 2.
The figure shows two different pathways by which the reactants can change into the products, but both routes begin at the same reactant energy level, and finish at the same product energy level. Regardless of reaction pathway, the energy difference between reactants and products is the same.
It is an energy difference between reactants and products that determines the equilibrium constant of a reaction, and therefore the equilibrium position. The fact that both pathways have the same energy difference, and therefore the same equilibrium constant, shows that the equilibrium constant in a reaction is not affected by how reactants change into products. With equilibrium constants, the nature and energies of the initial and final states are everything; what happens in between is immaterial.
When we turn to reaction rates, this is not so. In Figure 2, both routes start with the same reactants, and end with the same products, but the intervening stages along each pathway are very different. Such sequences of intervening stages are called reaction mechanisms, and the mechanism in the presence of a catalyst delivers a smaller energy barrier and a faster rate than the one that pertains when the catalyst is absent. With rates of reaction, therefore, the mechanism is crucial.