1. Chemical Equilibrium Objectives: 1. Describe a reversible reaction. 2. Define chemical equilibrium and explain how it is achieved. 3. Explain Le Chatelier's principle. Key Terms reversible reaction, chemical equilibrium, Le Chatelier's principle

2. Equilibrium Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction.

3. Reversible Reactions A reversible reaction is one where the reactants and products exist in a state of equilibrium. Most reactions exist in a state of equilibrium. the creation of products is called the forward reaction the creation of reactants is called the reverse reaction in equilibrium the rates are equal (forward =reverse) the equilibrium position describes where equilibrium lies. – A + B  AB If this reaction favors the forward reaction or products, then equilibrium lies to the right or favors the forward reaction.

4. Equilibrium Constant The equilibrium constant (K eq ) is the ratio of product divided by the reactants – aA + bB  cC+ dD – equation: [C] c x [D] d =K eq [A] a x[B] b Note: the coefficients become exponents – What this means...? K eq >1 favors products K eq <1 favors reactants

5. Le Chatelier The equilibrium of a chemical reaction can be altered by changing the conditions that surround it. Le Chatelier described this with his principle that stated: If a change in conditions is imposed on a system in equilibrium, the equilibrium will shift in the direction that tends to reduce that change in conditions. Consider the following reaction: 2 NO 2  N 2 O 4 +  Pressure+  temperature – When the reaction is in equilibrium, a ratio is established between the products and the reactants. According to Le Chatelier we can manipulate the reactions by manipulating the conditions. – removing products drives the forward reaction – removing reactants drives the reverse reaction – decreasing the temperature drives the forward reaction