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.

Reaction Directions and Standard State

An important goal of this course is to enable you to predict, for given values of temperature, concentrations, and partial pressures, whether reactants are changed to products or products are changed to reactants as a chemical reaction proceeds toward equilibrium. When reactants change to products we say that the reaction is spontaneous, or spontaneous in the forward direction. When products change to reactants, we say the reaction is not spontaneous, or that it is spontaneous in the reverse direction. In this context, the word “spontaneous” does not imply that the reaction is fast or slow, just that reactants change to products. Even if it takes millions of years for a process to occur, if there is an overall change of reactants to products we call the process spontaneous.

A standard state is a commonly accepted set of conditions used as a reference point. For chemists, the standard state refers to gases at a pressure of 1 bar, solutions at a concentration of 1 M (1 mol/L), pure solids, or pure liquids. (Note that some older thermochemical tables may list values with a standard state of 1 atm. Because 1 bar = 0.987 atm, thermochemical values are nearly the same under both sets of standard conditions; however, for accurate work the standard state should be checked.) The standard state does not specify a temperature.

It is also useful to define reaction direction terms that relate to standard-state conditions. If, when all substances are at the standard-state conditions, reactants change to products, we call a reaction product-favored. In contrast, if products change to reactants under standard-state conditions, a process is reactant-favored. That is, a product-favored process is spontaneous under the specific conditions of standard-state pressures or concentrations and a reactant-favored process is not spontaneous under those specific conditions. Note that the terms “product-favored” and “reactant-favored” are the same terms we defined above when discussing equilibrium constants.