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.

5. activation energy: minimum amount of energy that particles must have in order to react 6. activated complex: arrangement of atoms at the peak of the activation – energy barrier 7. 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. Results: If more reactant is added, shifts towards the product. If the reactant is removed, shifts towards the left.

For gases, pressure increased on reactants, shifts toward the side with the least volume. If temperature is increased, shifts in such a way an endothermic reaction is favored.

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

4. nonspontaneous reactions a. Does not favor the formation of products B. Entropy (S) 1. measure of the randomness of a system 2. ΔS° = S°(products) – S°(reactants)

3. The second law of thermodynamics: The entropy of the universe does not change for reversible processess and increases for spontaneous processes. 4. Reversible (ideal): Δs univ = Δs system + Δs surroundings =0 Irreversible (real,spontaneous) Δs univ = Δs system + Δs surroundings > 0

5. entropy on a molecular scale: a. Increases in this order solid, liquid, gas b. increases when a solid dissolves in water c. Increases when the total number of particles of product molecules is greater than the total number of reactant molecules 2H 2 O (l)  2H 2 (g) + O 2 (g)

d. Increase when the temperature increases 6. Law of disorder a. The spontaneous process always proceeds in such a way that the entropy of the universe increases b. If ΔS > 0 increase in disorder c. If ΔS < 0 decrease in disorder

C. Gibbs Free Energy (ΔG) 1. The maximum amount of energy that can be attached to another process to do work. 2. ΔG = ΔH – TΔS where ΔH : enthalpy T : temperature

3. Summary: reaction is ΔG ΔS spontaneous - - nonspontaneous + -

4. 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 CO

Ex. 2 Using the date for 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)