Chemical Equilibrium Unit 8

Reversible Reactions Most reactions continue until all the reactants are used up; however some reactions appear to stop after a certain period of time This is because these reactions can proceed in both directions: Forward: N2 + 3H2  2NH3 Reverse: N2 + 3H2  2NH3

Chemical Equilibrium When the rate of the forward reaction equals the rate of the reverse reaction, the reaction is said to be in chemical equilibrium In other words, as soon as some product is formed it reacts to produce more reactant

The Concept of Equilibrium Consider the following reversible reaction: N2O4(g) 2NO2(g) Because the reaction is reversible, it has a rate for both the forward and backward reaction

Achieving Equilibrium At equilibrium, the rate of the forward reaction equals the rate of the backward reaction Neither reaction stops, they are just in dynamic equilibrium

The Equilibrium Constant The equation below represents an equilibrium expression for the generic reaction: The equilibrium constant is expressed with no units aA + bB cC + dD Example: N2(g) + 3H2(g) 2NH3(g)

Evaluating the Equilibrium Constant, K Write the equilibrium expression and determine the value of the equilibrium constant for the following reaction. The equilibrium concentrations are listed below: [SO2] = 0.44 M, [O2] = 0.22 M, [SO3] = 0.11 M 2SO3(g) 2SO2(g) + O2(g)

Equilibrium Constants Each reaction has a certain equilibrium constant that tells you to what extent the reaction proceeds to produce product Reactions with K values > 1 favor products Reactions with K values < 1 favor reactants

Le Châtelier's Principle Le Châtelier's principle essentially states that if a system is at equilibrium and that equilibrium is altered, the system will shift so as to reestablish equilibrium Most common methods of altering equilibrium: Adding reactant or product Temperature changes

Addition of Reactant or Product at Equilibrium N2(g) + 3H2(g) 2NH3(g) After addition of H2:

Effect of Temperature on Equilibrium Consider two different types of reactions: Exothermic & Endothermic Exothermic: Heat can be considered a product Endothermic: Heat can be considered a reactant Endothermic: Increase in T shifts reaction to the right Exothermic: Increase in T shifts reaction to the left

Summary of Shifts in Equilibrium Some sulfur trioxide is sealed in a container and allowed to equilibrate at a particular temperature. The reaction is endothermic. SO3(g) SO2(g) + ½ O2(g) In which direction will the reaction proceed (a) if more SO3 is added to the system? (b) if oxygen is removed from the system? (c) if SO2 is added to the system? (d) if the temperature is increased? (e) if the temperature is decreased?