Equilibrium Chapter 19-2

Reversible Reactions A reaction in which the conversion of reactants into products and the conversion of products into reactants occur simultaneously

Reversible Reactions The reactions occur simultaneously in both directions. 2SO2 + O2 2SO3 The double arrows in the equation show that the reaction is reversible.

Chemical equilibrium It is a state in which the forward reaction and reverse reaction takes place at the same rate.

Le Châtelier’s Principle When stress is applied to a system at equilibrium, the system changes to relieve the stress. It is used to predict the changes that occur when a system at equilibrium is disturbed.

Le Châtelier’s Principle: Factors Affecting Equilibrium Concentration Changing the amount, or concentration, of any reactant or product in a system at equilibrium disturbs the equilibrium

Le Châtelier’s Principle: Factors Affecting Equilibrium Example: H2CO3 CO2 + H2O Take away CO2 More CO2 will be produced

Le Châtelier’s Principle: Factors Affecting Equilibrium 2.- Temperature Increasing the temperature causes the equilibrium position of a reaction to shift in the direction that absorbs heat.

Le Châtelier’s Principle: Factors Affecting Equilibrium Example: 2SO2 + O2 2SO3 + Heat Heat is a product. In this example, heating the reaction mixture at equilibrium the product yield decreases. Cooling, or removing heat, the product yield increases

Le Châtelier’s Principle: Factors Affecting Equilibrium 3.- Pressure A change in the pressure on a system affects only an equilibrium that has an unequal number of moles of gaseous reactants and products.

Le Châtelier’s Principle: Factors Affecting Equilibrium Increasing pressure favors the formation of product. There are less total moles in the product side. N2 + 3H2 2NH3 P more products

Le Châtelier’s Principle: Factors Affecting Equilibrium Increasing pressure favors the formation of reactant. There are less total moles in the reactant side. PCl5 PCl3 + Cl2

Equilibrium Constant Symbol for equilibrium constant = Keq Is the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to the power equal to the number of moles of that substance in the balanced chemical reaction.

Equilibrium Constant aA + bB cC + dD [C]c x [D]d Keq = [A]a x [B]b

Equilibrium Constant The [ ] square brackets indicate the concentrations of substances in moles per liter (mol/L) mol/L = Molarity mol/L = M

Equilibrium Constant 2SO2 + O2 2SO3 [SO3]2 Keq = [SO2]2 [O2]

Equilibrium Constant If Keq > 1 products favored at equilibrium. If Keq < 1 reactants favored at equilibrium.

Sample Question Calculate Keq for this reaction if the equilibrium concentrations are: [SO2] = 0.42 M, [O2] = 0.21 M, [SO3] = 0.072 M 2SO3 2SO2 + O2