Chemical Equilibrium & Le ChÂtelier’s Principle

Reversible Reactions Reversible reaction – a reaction in which the conversion of reactants to products and the conversion of products to reactants occurs simultaneously Most reactions are reversible

Reversible Reaction As the reaction proceeds, you begin with none of the product, SO3 As soon as you create any of the product, the product begins to decompose back to the reactants, SO2 & O2

Chemical Equilibrium Chemical equilibrium – when the rates of the forward and reverse reactions are equal Occurs in a closed system at a constant temperature No net change occurs in the actual amounts of the components of the system Can be established by starting with reactants only, products only, or equal amount of products and reactants

Chemical Equilibrium Have both reactions stopped? Dynamic NO! Dynamic Both the forward and reverse reactions continue!! Forward and reverse reaction rates are equal Concentrations remain constant Not necessarily equal

Chemical Equilibrium

Le ChÂTelier’s Principle Le Châtelier’s Principle – If a stress is applied to a system at dynamic equilibrium, the system changes in a way that relieves the stress Applies to chemical reactions at equilibrium Stresses: Concentration of reactants & products Changes in temperature Changes in pressure

Concentration Changing the concentration of any reactant or product at equilibrium disturbs equilibrium If you increase the concentration of a reactant, the equilibrium shifts to the products If you decrease the concentration of a reactant, the equilibrium shifts to the reactants If you increase the concentration of a product, the equilibrium shifts to the reactants If you decrease the concentration of a product, the equilibrium shifts to the products

Concentration H2CO3 CO2 + H2O At Equilibrium: Add H2CO3 (reactant), need to make more products H2CO3 CO2 + H2O CO2 + H2O H2CO3 H2CO3 H2CO3 H2CO3 H2CO3 CO2 + H2O CO2 + H2O

Pressure CO(g) + 3H2(g) CH4(g) + H2O(g) Reaction containing all gases If you increase pressure, the volume available for the reaction decreases Le Châtelier’s Principle states that the reaction will shift to produce the fewest moles of gas to alleviate the pressure Pressure – the number of gas particles colliding with the walls of the container

Pressure N2(g) + 3H2(g) 2NH3(g) 4 moles gas 2 moles gas Increase pressure: Equilibrium shifts to the right Fewer moles of gas Relieves pressure

Temperature Every chemical reaction is either exothermic or endothermic -ΔH Forward reaction: exothermic (heat is a product) Reverse reaction: endothermic (heat is a reactant) +ΔH Forward reaction: endothermic (heat is a reactant) Reverse reaction: exothermic (heat is a product) Le Châtelier’s Principle states that if heat is added to an equilibrium system, the equilibrium shifts in the direction in which heat is used up If heat is taken away from a system, equilibrium shifts in the direction in which heat is produced

Temperature Exothermic Reaction Equilibrium shifts to the left: CO(g) + 3H2(g) CH4(g) + H2O(g) + heat Equilibrium shifts to the right: Endothermic Reaction Equilibrium shifts to the right: heat + N2O4(g) 2NO2(g) Equilibrium shifts to the left: heat Raise the temperature heat Raise the temperature Lower the temperature Lower the temperature

Temperature

Summary