In most of the chemical reactions we have discussed so far, the reactants have been completely transformed into products. Such a reaction is said to go to completion. Not all chemical reactions are like this, though. Quite often the reactants are only partially converted into products. An obvious example of a reaction which does not go to completion is the solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. of a weak acid (such as acetic acid) in water. The acid molecules donate their protons to water according to the equation
However, measurements of the conductivity of a solution of acetic acid indicate that the concentrations of hydrogen ions and acetate ions are much smaller than we would have expected, had the proton-transfer reaction gone to completion. By repeated conductivity measurements, these concentrations may be shown to remain constant over a long periodThose elements from a single row of the periodic table. of time. A system such as we have just described, in which appreciable concentrations of both reactants and products are present and in which concentrations do not change with time, is called an equilibrium mixture. The fact that the reaction does not go to completion is usually indicated, as in Eq. (1) by a double arrow. In such a mixture the reactants and products are said to be in equilibrium with each other.
The equilibrium state can be reached in several ways, beginning with reactants or products or from a higher or lower temperatureA physical property that indicates whether one object can transfer thermal energy to another object. or with or without a catalystA substance that increases the rate of a chemical reaction but that undergoes no net change during the reaction.. In all cases, however, the equilibrium concentrations at a given temperature will be related by the mathematical equilibrium-constant expression Kc. According to the law of chemical equilibrium, the equilibrium-constant expression contains the equilibrium concentration of each product raised to a power equal to the coefficient of that product in the chemical equationA representation of a chemical reaction in which chemical symbols represent reactants on the left side and products on the right side.. This is divided by the equilibrium concentration of each reactant raised to its appropriate power. Once the value of Kc has been measured at a given temperature, we can use it to calculate the equilibrium concentrations of reactants and products. The equilibrium constant may also be expressed in terms of partial pressures in the case of reactions involving gases.
In addition to its use in quantitative calculations, the equilibrium law permits qualitative predictions. A very large equilibrium constant corresponds to a reaction which goes nearly to completion, while a small equilibrium constant suggests that almost no reaction takes place. For a specific equilibrium reaction, Le Chatelier's principle can be used to predict the effect of a change in conditions of temperature, pressure, volume, or concentration of the species involved. A system always adjusts to a new equilibrium so as to counteract such a change of conditions to some degree. On a molecular level there are two factors which affect the position of an equilibrium reaction. The first of these is energyA system's capacity to do work.. The lower the energy of a molecule, the more likely the occurrence of that molecule, and therefore the greater its concentration will be in an equilibrium mixture. The second factor has to do with the number of structural arrangements possible for a given species. The greater the number of ways of arranging the atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. of a given molecule in three-dimensional space, the greater the probability that that particular molecule will exist. Both this probability factor and the energy factor mentioned earlier affect the size of any equilibrium constant. At low temperatures the energy factor predominates, while at high temperatures the probability factor is most important.