In the laboratory portion of your chemistry course you have had the opportunity to observe a number of chemical reactions. After a certain amount of time many of these reactions appear to "stop"--colors stop changing, gases stop evolving, and so forth. In several of these instances the process apparently stops before the reaction is complete, leading to a mixture of reactants and products.

For example, we can consider the interconversion of the gaseous nitrogen oxides shown in Figure 15.1. When pure frozen N₂O₄ is warmed above its boiling point (21.2°C), the gas in the sealed tube turns progressively darker as colorless N₂O₄ gas dissociates into brown NO₂ gas (Figure 15.2):

Eventually the color change stops even though there is still N₂O₄ in the tube. We are left with a mixture of N₂O₄ and NO₂ in which the concentrations of the gases no longer change.

The condition in which the concentrations of all reactants and products cease to change with time is called chemical equilibrium. Chemical equilibrium occurs when opposing reactions are proceeding at equal rates: The rate at which the products are formed from the reactants equals the rate at which the reactants are formed from the products. For equilibrium to occur, neither reactants nor products can escape from the system.

We have already seen several instances of equilibria. For example, the vapor above a liquid is in equilibrium with the liquid phase. The rate at which molecules escape from the liquid into the gas phase equals the rate at which molecules in the gas phase strike the surface and become part of the liquid. As another example, in a saturated solution of sodium chloride, the solid sodium chloride is in equilibrium with the ions dispersed in water. The rate at which ions leave the solid surface equals the rate at which other ions are removed from the liquid to become part of the solid.

Chemical equilibria are of importance in explaining a great many natural phenomena, and they play important roles in many industrial processes. In this and the next two chapters we will explore chemical equilibria in some detail. Here we will learn how to express the equilibrium position of a reaction in quantitative terms, and we will study the factors that determine the relative concentrations of reactants and products at equilibrium. We begin by exploring the relationship between the rates of opposing reactions and how this relationship leads to chemical equilibrium.

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