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SGCh14 - Chapter Fourteen CHEMICAL EQUILIBRIUM Chemical...

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274 Chapter Fourteen CHEMICAL EQUILIBRIUM Chemical Equilibrium and the Equilibrium Constant Writing Equilibrium Constant Expressions Calculating Equilibrium Constants What the Equilibrium Constant Tells Us Factors Affecting Equilibrium CHEMICAL EQUILIBRIUM AND THE EQUILIBRIUM CONSTANT STUDY OBJECTIVES 1. Describe the affect of a reverse reaction on the amount of product formed. 2. Describe how equilibrium is established in a chemical reaction system. Reversible Chemical Reactions. In this chapter we will discuss how equilibrium is established in several types of chemical reaction systems. We will define the equilibrium constant and show how to calculate equilibrium concentrations of reactants and products. A state of chemical equilibrium exists when the concentrations of reactants and products are observed to remain constant with time. When a mixture of SO 2 and O 2 , for instance, is introduced into a reaction vessel at a temperature of 700 K, a reaction that produces SO 3 occurs: 2SO 2 + O 2 2SO 3 When a specific concentration of SO 3 is reached, no additional SO 3 is formed even though some SO 2 and O 2 remain. From that time on, the concentrations of SO 2 , O 2 , and SO 3 stay constant, and we say the system has reached chemical equilibrium. The constant concentrations are the result of a reversible chemical reaction. In the reverse reaction some SO 3 decomposes back into SO 2 and O 2 . 2SO 3 2SO 2 + O 2 When the rates of forward and reverse reactions are the same, no net chemical change occurs and a state of chemical equilibrium exists. The equilibrium state is referred to as dynamic because of the continual conversions of reactants into products, and products into reactants at the molecular level. A reversible reaction is represented by the opposing arrows in the chemical equation: 2SO 2 + O 2 2SO 3 The three compounds are said to be in equilibrium. At 700 K all three species exist together in the reaction vessel. Equilibrium can be reached by the reverse reaction as well. That is, if only SO 3 is added to the reaction vessel at 700 K, it will form an equilibrium amount of SO 2 and O 2 .
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Chemical Equilibrium / 275 The Equilibrium Constant. In quantitative terms, the equilibrium state is described by the equilibrium constant expression which is also called the mass action expression. K c = [SO 3 ] 2 [SO 2 ] 2 [O 2 ] Here the brackets [ ] indicate concentration in moles per liter. In this expression, note that the concentration of the product is in the numerator, and the concentrations of the reactants appear in the denominator. Since there are two reactants their concentrations are multiplied . Also, the concentration of each component is raised to a power equal to its coefficient in the balanced equation. The value of this expression is called the equilibrium constant .
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