D20.1 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 earlier when discussing equilibrium constants.
