D19.5 Product-favored Reactions

The magnitude of an equilibrium constant is a measure of the yield of a reaction when it reaches equilibrium. A very large value for K (K >> 1) indicates that partial pressures or concentrations of products are much larger than for reactants when equilibrium has been achieved: nearly all reactants have been converted into products. If K is large enough, the reaction has gone essentially to completion when it reaches equilibrium.

We define a product-favored reaction as a reaction that proceeds spontaneously in the forward direction when all concentrations (or partial pressures) have the  value of 1 M (or 1 bar). If K > 1, then the concentrations of products are greater than the concentrations of reactants at equilibrium. Therefore, when all concentrations are at 1 M, the reaction would move towards producing greater concentrations of products in order to reach equilibrium. That is, the reaction proceeds in the forward direction. Thus, K > 1 means that a reaction is product-favored at equilibrium.

Similarly, a reactant-favored reaction is a reaction that proceeds spontaneously in the reverse direction when all concentrations (or partial pressures) have the  value of 1 M (or 1 bar). A very small value of K, (K << 1) indicates that equilibrium is achieved when only a small fraction of the reactants have been converted into products. If K is small enough, essentially no reaction has occurred when equilibrium is reached. By an argument similar to the one in the paragraph above, K < 1 means that a reaction is reactant-favored at equilibrium.

When K ≈ 1, both reactant and product concentrations are significant and it is necessary to use the equilibrium constant to calculate equilibrium concentrations.

Clearly it would be useful to know whether a reaction is product-favored, that is, whether K >>> 1, because such a reaction results in almost all reactants being converted to products when equilibrium has been reached. If you want to synthesize a vaccine that will prevent COVID-19 infection, the reactions used must produce products in high enough concentrations for the products to be easily separated from the reaction mixture. This Unit will develop ideas that enable such predictions.