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Unformatted text preview: USM Chemistry Workshop CHY 115 “ Any change in one of the variables that determines the state of a system in equilibrium causes a shift in the position of equilibrium in a direction that tends to counteract the change in the variable under consideration.” Henri Le Chatelier Unit 14 Equilibrium Concepts ome chemical reactions go to completion; that is, the reactants are changed into products until the reactants are completely consumed. Other reactions are reversible. As products are formed, the products react to reform reactants. For these reversible reactions, an equilibrium exists when the rates of the forward and reverse reactions become equal. You probably have already studied two examples of physical equilibria: the formation of a saturated solution and the equilibrium between a liquid and its vapor. Now we will begin to study the concept of chemical equilibria. Chemical equilibria abound in nature, especially in living organisms. Right now, there are a number of critical chemical equilibrium systems operating in your body that allow you to live. Oxygen molecules bind to hemoglobin molecules in your lungs, where the concentration of oxygen is high, and the oxygen is released in other parts of your body, where oxygen concentration is low. The acidity of your blood is controlled by a chemical equilibrium between carbonic acid and hydrogen carbonate ion. S Liquid–Vapor Equilibrium Consider a sealed container partially filled with water, as shown in Figure 1. We will assume that the container was completely empty before the water was poured in. The rate at which the water evaporates, which can be represented by the equation H 2 O (l ) H � 2 O( g ), depends on the temperature and the surface area of the liquid. If we hold temperature constant, the rate of evaporation in our container will be constant. This is represented by the upward-pointing arrows in Figure 1. Notice that all arrows have the same size, indicating that the evaporation rate does not change with time. 14. 1 USM Chemistry Workshop CHY 115 0 1 2 3 4 Time Rates of Evaporation and Condensation Evaporation Rate Condensation Rate Figure 14.1. Liquid–Vapor Equilibrium. Water is placed in a closed flask and comes to equilibrium with its vapor at constant temperature. Upward-pointing arrows represent evaporation rate, and downward-pointing arrows represent condensation rate. Equilibrium is established at Time 3, the time at which the evaporationarrows represent condensation rate....
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