Chapter 9 (1 of 2) - CHAPTER 9 Chemical Equilibrium 1 Basic...

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1 CHAPTER 9 Chemical Equilibrium 2 Basic Concepts ! Reversible reactions do not go to completion. " They can occur in either direction " Symbolically, this is represented as: • The reactions proceed until the composition of the reaction mixture reaches a state of minimum free energy. ! " ! " ! " ! " # $ g g g g D d + C c B b + A a 3 Basic Concepts ! Chemical equilibrium exists when two opposing reactions occur simultaneously at the same rate. " It is a stage in a chemical reaction when there is no further tendency for the concentrations or the partial pressures of a reaction mixture composition to change.
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4 Basic Concepts ! Chemical equilibria are dynamic equilibria. " Molecules are continually reacting, even though the overall composition of the reaction mixture does not change. " The criteria that identify chemical equilibrium are: 1. The forward reaction and its reverse reaction are both taking place. 2. The forward and reverse reactions are occurring at equal rates – there is no net change. 5 Basic Concepts ! To determine if a chemical reaction is at equilibrium, change the conditions in which the reaction is occurring. " If the reaction is at equilibrium, the composition of the reaction will change to reestablish equilibrium. 6 Basic Concepts ! One example of a dynamic equilibrium can be shown using radioactive 131 I as a tracer in a saturated PbI 2 solution. 2 * 2 2 H O 2 - 2(s) (aq) (aq) [1] Place solid PbI in a saturated PbI solution. PbI Pb 2 I [2] Stir for a few minutes, then filter the solution. Some of the radioactive iodine will go into soluti . on % # %
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7 9.2 Equilibrium and the Law of Mass Action ! Mathematician Cato Guldberg and chemist Peter Waage (from Norway) determined a mathematical relationship that summarizes the composition of a reaction mixture at equilibrium. 8 Equilibrium and the Law of Mass Action 3 2 2 2 2 ( ) ( ) SO SO O P K P P Using different initial concentrations of reactant gases, they were able to determine that the quantity of the below equation was the same for every experiment. 2 SO 2 (g) + O 2 (g) ! 2 SO 3 (g) 9 Equilibrium and the Law of Mass Action The quantity was the same. Different initial compositions of reactants were allowed to establish equilibrium.
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10 Equilibrium and the Law of Mass Action ! The constant “K” is characteristic of the composition of a reaction mixture at equilibrium at a given temperature. " It is called the equilibrium constant tants reac of pressures partial products of pressures partial K 11 Equilibrium and the Law of Mass Action ! The Law of mass action states that " For any reaction between ideal gases, the composition of the reaction mixture can be expressed in terms of an equilibrium constant . " Each partial pressure of the reactants and products
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Chapter 9 (1 of 2) - CHAPTER 9 Chemical Equilibrium 1 Basic...

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