Collections:Texts:Chemistry:Chapter 13 Summary
From ChemEd Collaborative
SUMMARY
Many reactions do not go to completion but instead reach a state of dynamic equilibrium in which the concentrations of all participating substances remain constant. The equilibrium state can be reached in several ways, beginning with reactants or products or from a higher or lower temperature or with or without a catalyst. 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 equation. 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 energy. 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 atoms 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. In Chap. 16 we will discuss these two factors further and learn how they can be measured on the macroscopic level.
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