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Chemical Equilibrium Collision theory Rates of reactions Catalysts Reversible reactions Chemical equilibrium Le Chatelier’s Principle Concentration Temperature Volume Catalysts 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 have enough kinetic energy to break bonds A. Collision Theory Particle Orientation Required Orientation Unsuccessful Successful Collision Collisions Activation energy: minimum energy required for a reaction to occur Exothermic Endothermic Activation energy Energy Energy Time Time Energy of reaction A. Collision Theory Activation Energy depends on reactants low Ea = fast rxn rate Ea 16.2: Rates of Reactions Chemical kinetics: the study of the rate (the speed) of a reaction Rate of a chemical reaction depends on: 1. SURFACE AREA 2. CONCENTRATION of reactants 3. TEMPERATURE (T) of reactants 4. Presence/absence of a CATALYST SURFACE AREA Surface Area high SA = fast rxn rate more opportunities for collisions Increase surface area by… • using smaller particles • dissolving in water Effect of Concentration on Rate Concentration: increasing concentration of reactants results in more collisions. More collisions = increased rate of reaction Effect of Temperature on Rate Temperature: Increasing T increases particle speed. Faster reactants means more collisions have the activation energy, which increases the rate of the reaction. Temperature Analogy: 2-car collision 5 mph “fender bender” 50 mph “high-speed crash” Effect of Catalysts on Rate A catalyst: A chemical that influences a reaction, but is not consumed in the reaction. (It can be recovered unchanged at the end of the reaction.) Lowers the activation energy of the reaction. Activation energy Energy Activation energy with catalyst Time Catalysts Enzyme Catalysis 16.1: Reversible Reactions * Thus far, we have considered only one-way reactions: A + B → C + D 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 & condensing Freezing & melting Reversible Reactions o At chemical equilibrium there is no net change in the actual amounts of the components of the system. o And although the rates of the forward & reverse rxns are equal at chemical equilibrium, the concentrations of the components on both sides of the chem- ical eqn are not necessarily the same. *In fact they can be dramatically different. o Consider a set of escalators as being like the double arrows in a dynamic equilibrium. o The # of people using the up escalator must be the same as the # of people using the down escalator for the # of people on each floor to remain at equilibrium • However, the # of people upstairs do not have to equal the # of people downstairs • Just the transfer between floors must be consistent Examples of irreversible reactions: Striking a match / burning paper Dropping an egg Cooking (destroys proteins) 16.3: Chemical Equilibrium For a reversible reaction, when the forward rate equals the backward rate, a chemical equilibrium has been established. Both the forward and backward reactions continue, but there is a balance of products “un-reacting” and reactants reacting. A+B↔C+D A + B C + D Equilibrium Expression Chemist’s generally express the position of equilibrium in terms of numerical values These values relate the amounts of reactants to products at equilibrium Consider this hypothetical rxn… wA + xB yC + zD • Where “w” mols of reactant A and “x” mols of reactant B react to give “y” mols of product C and “z” mols of product D at equil. Equilibrium Expression We can write a mathematical expression to show the ratio of product concs to reactant concs called an equilibrium expression y z [C] [D] w x [A] [B] o The concentration of each substance is raised to a power equal to the # of mols of that substance in the balanced rxn eqn. o The square brackets indicate concentration in Molarity (mol/L) Equilibrium Expression [C]y [D]z Keq= w x [A] [B] The resulting ratio of the equilibrium is called the equilibrium constant or Keq The Keq is dependent on the temp If the temp changes so does the Keq NOTE: this is only for gases!!! Equilibrium Constant Equilibrium constants provide valuable chemical information They show whether products or reactants are favored in a rxn always written as a ratio of products over reactants a value of Keq > 1 means that products are favored Keq < 1 than reactants are favored Sample Problem 1 Dinitrogen tetroxide (N2O4), a colorless gas, and nitrogen dioxide (NO2), a brown gas, exist in equilibrium with each other according to the following eqn: 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 and calculate Keq for the reaction. Analyze: list what we know Known: [N2O4] = .0045mol/L [NO2] = .030mol/L Unknown: Keq expression = ? Keq = ? o At equilibrium, there is no net change in the amount of N2O4 or NO2 at any given instant Calculate: solve for unknowns The only product of the rxn is NO2, which has a coefficient of 2 in the balanced eqn The only reactant N2O4 has a coefficient of 1 in the balanced eqn The equilibrium expression is: [NO2]2 [.030M]2 Keq= Keq= [N2O4] 1 [.0045M]1 o Keq is equal to: Keq= 0.20 o Keq < 1, therefore rxn doesn’t favor products Classwork: • 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) a. Write the equilibrium expression for the above rxn. b. Calculate Keq at this temp? c. More CO2 is added to the system, which direction will the reaction shift? d. Are the reactants or products favored in this reaction? * 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) 16.4: Le Chatelier’s Principle Le Chatelier’s Principle: When a stress is applied to a system (i.e. reactants and products) at equilibrium, the system responds to relieve the stress. The system shifts in the direction of the reaction that is favored by the stress. A stress is a change in: Concentration Temperature Volume 16.5: Stress: Change Concentration Ex: Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (pink) (blue) Stress Result Add Cl1- Forward rxn favored Shifts forward to reduce extra Cl1- More CoCl42- will form Add H2O Backward rxn favored Shifts backward to reduce extra H2O More Co(H2O)62+ will form 16.7: Stress: Change Temperature Ex: heat + Co(H2O)62+ + 4 Cl1- ↔ CoCl42- + 6 H2O (pink) (blue) This reaction is endothermic. For Le Chatelier’s principle, consider “heat” as a chemical. Stress Result Increase T Forward rxn favored; shifts forward to reduce extra heat More CoCl42- will form Decrease T Backward rxn favored; shifts backward to replace “lost” heat More Co(H2O)62+ will form 16.6: Stress: Change Volume Ex: 1 N2 (g) + 3 H2(g) ↔ 2 NH3(g) (1 + 3 = 4 moles of gas) ↔ (2 moles of gas) Stress Result Decrease V Forward rxn favored; shifts forward to side with fewer moles of gas (reduces # of molecules packed into this smaller volume) Increase V Backward rxn favored; shifts backward to side with more moles of gas (to fill the larger volume with more molecules) 16.7: Catalysts & Equilibrium MnO2 Ex: 2 H2O2 (aq) ↔ 2 H2O (l) + O2 (g) Since a catalyst increases the forward and backward rates equally, it will not shift the equilibrium.