Collections:Texts:Chemistry:Chapter 13 - Chemical Equilibrium

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CHEMICAL EQUILIBRIUM

13.1 The Equilibrium State

13.2 The Equilibrium Constant

13.3 Calculating the Extent of a Reaction

13.4 Le Chatelier’s Principle

13.5 The Molecular View of Equilibrium

Summary 13

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 solution of a weak acid (such as acetic acid) in water. The acid molecules donate their protons to water according to the equation

CH₃COOH + H₂O CH₃COO^– + H₃O⁺      (13.1)

However, measurements of the conductivity of a solution of acetic acid (Sec. 11.3) 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 period 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. (13.1) by a double arrow. In such a mixture the reactants and products are said to be in equilibrium with each other.

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