Chemical Equilibrium Collision theory Rates of reactions Reversible reactions Chemical equilibrium

A. Collision Theory Reaction rate depends on the collisions between reacting particles. Successful collisions occur if the particles... collide with each other have the correct orientation when they collide have enough kinetic energy when they collide

Unsuccessful Collisions A. Collision Theory Particle Orientation Required Orientation Unsuccessful Collisions Successful Collision

Activation energy: minimum energy required for a reaction to occur Exothermic Endothermic Activation energy Time Energy Time Energy Energy of reaction

B. Reaction Rates SURFACE AREA 2. CONCENTRATION of reactants The more collisions, the faster the reaction rate Increase collisions by changing the… SURFACE AREA More surface area = more collisions Increase surface area by crushing or dissolving 2. CONCENTRATION of reactants More particles = more collisions

50 mph “high-speed crash” 3. TEMPERATURE (T) of reactants Increase temperature; particles collide with more energy; increases the rate of the reaction. 5 mph “fender bender” 50 mph “high-speed crash” 4. Presence of a CATALYST speeds up a reaction, but is not consumed in the reaction. Lowers the activation energy. Activation energy Time Energy Activation energy with catalyst

C. Reversible Reactions Some reactions are reversible: They go forward (“to the right”) : A + B → C + D and backwards (“to the left”) : A + B ← C + D Written with a two way arrow: A + B ↔ C + D Examples: Boiling and condensing Freezing and melting

Chemical Equilibrium

C. Reversible Reactions At Equilibrium The rates of the forward & reverse reactions are equal The AMOUNTS of the reactants and product DO NOT HAVE TO BE THE SAME *In fact they can be dramatically different. A + B ↔ C + D A B + C D +

D. Equilibrium Expression We can write an expression for the position of equilibrium Consider this hypothetical rxn… The EQUILIBRIUM EXPRESSION is written wA + xB yC + zD K = [C]y [D]z [A]w [B]x The concentration (Molarity) or pressure of each substance is raised to a power equal to it’s coefficient Products / reactants

D. Equilibrium Expression Keq= [C]y [D]z [A]w [B]x NOTE: pure solids and liquids ARE NOT included in the equilibrium expression. Only gases and aqueous solutions are used.

H2O (g) + C (s) ↔ H2 (g) + CO (g) Practice: Write the equilibrium expression for the following reaction: H2O (g) + C (s) ↔ H2 (g) + CO (g)

E. Equilibrium Constant The equilibrium constant or Keq is the numerical value obtained when you solve the expression a value of Keq > 1 means that products are favored (there will be more product than reactant) Keq < 1 than reactants are favored (there will be more reactant than product)

Sample Problem 1 N2O4(g) 2NO2(g) A liter of gas mixture at 10C at equilibrium contains 0.0045mol N2O4 & 0.030 mol NO2. Write the Keq expression Calculate Keq for the reaction.

Sample Problem 1 [NO2]2 Keq= [N2O4]1 [.030M]2 Keq= [.0045M]1 The equilibrium expression is: [NO2]2 Keq= [N2O4]1 Calculating Keq is equal to: Keq= 0.20 Keq < 1, therefore reactants are favored [.030M]2 [.0045M]1 Keq=

CO2(g)+ H2(g)<==> CO(g) + H2O(g) Sample Problem 2 A mixture at equilibrium at 827°C contains 0.552 M CO2, 0.552 M H2, 0.448 M CO, and 0.448 M H2O. CO2(g)+ H2(g)<==> CO(g) + H2O(g) Write the equilibrium expression for the above reaction. Calculate Keq at this temp Are the reactants or products favored in this reaction?

F. Le Chatelier’s Principle * Le Chatelier’s Principle is about reducing stress – a stress applied to a chemical equilibrium Relax! Reduce stress brought on by chemical equilibrium with me, Henri Le Chatelier! (1850 – 1936)

F. Le Chatelier’s Principle When a stress is applied to a system (reaction) at equilibrium, the system responds to relieve the stress. A stress is a change in: Concentration Temperature Volume

F. Le Chatelier’s Principle: Concentration Ex: Co(H2O)62+(aq) + 4 Cl-(aq) ↔ CoCl42- (aq) + 6 H2O (g) (pink) (blue) Stress Result Add Cl- •Forward reaction favored • Uses up extra Cl- • More CoCl42- will form Add H2O • Reverse reaction favored • Use up extra H2O • More Co(H2O)62+ will form

F. Le Chatelier’s Principle: Temperature Ex: Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (g) (pink) (blue) ∆H = +57.5 kJ/mol This reaction is endothermic. Consider “heat” as a chemical. Heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (g) (pink) (blue) Stress Result Increase Temp •Forward reaction favored; shifts forward to reduce extra heat •More CoCl42- will form Decrease Temp •Backward rxn favored; shifts backward to replace “lost” heat •More Co(H2O)62+ will form

F. Le Chatelier’s Principle: Volume Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g) (1 + 3 = 4 moles of gas) ↔ (2 moles of gas) Stress Result Decrease Volume •Forward reaction favored •shifts to side with fewer moles of gas Increase Volume •Backward reaction favored • shifts to side with more moles of gas

F. Le Chatelier’s Principle: Catalysts MnO2 Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g) A catalyst increases the forward and backward rates equally, It will not shift the equilibrium.

Practice: Le Châtelier’s Principle Explain how the following changes will affect the position of the equilibrium below A (g) + B(aq) ↔ C(s) ∆H = -453 kJ/mol Increase pressure of A Increase temperature Add a catalyst Increase concentration of substance B Increase amount of substance C

Shift to product side Shift to reactant side No shift No shift; solids do not affect equilibrium