Chapter 19 Reaction Rates and Equilibrium

I.Rates of reaction A. Collision Theory 1. rates : measure the speed of any change during a time interval 2. collision theory : particles react to form products when they collide

3.

4. activation energy: minimum amount of energy that particles must have in order to react 5. activated complex: arrangement of atoms at the peak of the activation – energy barrier 6. transition state: refers to the activated complex

B. Factors affecting reaction rates 1. Temperature a. Increase temperature 1. increase the number of particle collisions 2. enough energy to slip past the activation-energy barrier 3. allows more products to form faster

b. Decrease temperature: slows down a reaction 2. Concentration a. Increasing the number of particles in a fixed volume causes more collisions (increases the amount of products) b. decreasing the number of particles slows the reaction

3. Particle size a. Smaller the particle size, the larger the surface area b. Increase in surface area 1. increases the amount of reactant exposed 2. increases the number of collisions, increases the amount of products

c. How do you increase particle size? 1. dissolve solid particles 2. crush solids into a powder 4. Catalysts a. Increases the rate of a reaction without being used up b. 2H 2 (g) + O 2 (g)  2H 2 O (l) Pt

c. d. Inhibitor 1. a substance that interferes with the action of a catalyst 2. slows the reaction

II.Reversible Reactions and Equilibrium A. Reversible reactions 1. happens simultaneously in both directions 2. 2SO 2 (g) + O 2 (g) 2SO 3 (g)

B. Chemical equilibrium : when the rate of the forward reactions equals the rate of the reverse reaction C. Equilibrium position 1. changes with the concentration of the substance 2. the arrow shifts towards the higher concentration

D. Le Chatelier’s Principle 1. If a stress is applied to a system in dynamic equilibrium, the system changes to relieve stress. 2. Concentration a. Add more products, the shift is towards the reactants

b. When a reactant is removed, the shift is towards the products 3. Temperature a. Increase in temperature causes a shift that absorbs heat b. Heating the reaction, favors the reactants c. Cooling the reaction, favors the products

4. Pressure a. Affects only gaseous systems b. Favors the least volume

E. Examples: N 2 + 3H 2 2NH 3 + heat stress results increase amount N 2 increase pressure decrease pressure increase temp. decrease temp.

F. Equilibrium constant (K eq ) 1. ratio of product concentration to reactant concentration 2. a A + b B c C + dD a,b,c,d = # moles 3. K eq = [C] c x [D] d [A] a x [B] b

4. [ ] indicate moles/L 5. K eq > 1, products are favored at equilibrium 6. K eq < 1, reactants are favored at equilibrium 7. [ ] are for gases

8. Ex. A liter of gas mixture at 10 °C at equilibrium contains mole N 2 O 4 and mole NO 2. Write the expression for the equilibrium constant and calculate the equilibrium constant (K eq ) for the reaction. N 2 O 4 (g) 2NO 2 (g)

9. Ex. At a certain temperature, K eq = 11.1, and the equilibrium mixture contains 4.00 mole Cl 2. How many moles of Br 2 and BrCl are present in the equilibrium mixture? BrCl (g) Cl 2 (g) + Br 2 (g)

III.Determining whether a reaction will occur A. Free energy (G) 1. energy available to do work 2. only will happen if the reaction can take place

3. Spontaneous reactions a. Reactions that occur naturally b. Favor the formation of products c. Release free energy d. Ex. Colorful fireworks display

4. Nonspontaneous reactions a. Does not favor the formation of products B. Entropy (S) 1. measure of the randomness of a system

2. Law of disorder : the processes move in the direction of maximum disorder or randomness a. Increasing entropy solid  liquid  gas b. Increasing entropy crystalline solid  solute particles

c. Increasing entropy the total number of product molecules is greater than the total number of reactant molecules 2 H 2 O (l)  2 H 2 (g) + O 2 (g) d. Increasing entropy happens when temperature increases, molecules move faster and faster

Heat, Entropy and Free Energy How changes in heat and entropy affect reaction spontaneity Heat change Entropy Spontaneous reaction? Decreases increases yes (exo) Increases decreases only if unfavorable heat change is offset by favorable entropy change

decreases decreases only if favorable (exo) entropy change is offset by favorable heat change increases decreases no (endo)

 Calculating Entropy and Free Energy 1. Entropy calculations a. S b. Units J/k c. S o is the symbol for solids, liquid or gases d. ΔS o = S o (products) – S o (reactants)

e. Ex.1 Calculate the standard entropy change (ΔS o ) that occur when 1 mole H 2 O (g) at 25 o C and kPa condenses to 1 mole H 2 O (l) at the same temperature. 2. NO (g) reacts with O 2 to form NO 2 (g). What is the standard change in entropy for this reaction when reactants and products are in the specified physical states at kPa and 25 o C? NO (g) + O 2 (g)  NO 2 (g)

2. Gibbs Free Energy (ΔG) a. The maximum amount of energy that can be attached to another process to do work. b. ΔG = ΔH – TΔS where ΔH : enthalpy T : temperature ΔS: entropy

c. ΔG o = ΔG f o (products) – ΔG f o (reactants) d. Summary: Reaction is ΔG ΔS spontaneous - - nonspontaneous + -

e. Examples: Using the values for ΔH and S, determine whether this reaction is spontaneous at 25 °C. C (s) + O 2 (g)  CO 2 (g) graphite Substance ΔH(kJ/mol) S (J/K  mole) C (s) O 2 (g) CO 2 (g)

Ex. 2 Using the date from Table 19.4 and ΔG° f =ΔG° f (products) - ΔG° f (reactants) determine whether the following equation is spontaneous Cl 2 (g) + H 2 O (g)  2HCl (g) + ½ O 2 (g)