Reaction Energy and Reaction Kinetics

1. Reaction Energy and Reaction Kinetics Reaction Rate

2. Objectives • Define the rate of reaction and describe how it can be determined. • List and explain factors that affect the rate of reaction. • Define a catalyst and describe how it can affect the rate of reaction.

3. Reaction Rate Reaction rate– change in concentration of reactants per unit time as a reaction occurs. Chemical kinetics – study of reaction rates and reaction mechanisms.

4. Rate-Influencing Factors Nature of the Reactants H2(g) + Cl2(g) 2 HCl(g) (fast) 3 H2(g) + N2(g)  2 NH3(g) (slow) 4 Na(s) + O2(g)  2 Na2O(s) (fast) 4 Fe(s) + 3 O2(g)  2 Fe2O3(s) (slow)

5. Rate-Influencing Factors Surface Area • Heterogeneous reactions – reactions with reactants in two different phases • Heterogeneous reactions depend on contact between the two phases. • Increases in surface area result in the increase in the rate of heterogeneous reactions.

6. Rate-Influencing Factors Temperature An increase in temperature causes: • An increase in collision energy • An increase in collision frequency Both result in increased reaction rates • In general, a 10oC increase in temperature causes the reaction rate to double.

7. Rate-Influencing factors Concentration • Increasing the concentration of the reactants may cause an increase in the rate of reaction for homogeneous reactions.

8. Rate-Influencing Factors Presence of a Catalyst • Catalysts lower the activation energy for a reaction and, thus, increase the reaction rate. • Homogeneous vs. Heterogeneous catalysts – same or different phase as reactants.

9. Rate Laws for Reactions Objectives: • Explain the rate law for a chemical reaction. • Determine the rate law for a particular reaction given appropriate kinetics data. • Discuss the relationship between the rate law and the reaction mechanism.

11. Rate Law Rate Law – an equation that relates reaction rate and concentration of reactants R = k[A]n[B]m… R = reaction rate k = rate constant that depends on temperature n,m = must be determined experimentally

12. Rate Laws for Reactions Example: 2 H2(g) + 2 NO(g) N2(g) +2 H2O(g) • Doubling the amount of H2 , while keeping the amount of NO constant, causes the reaction rate to double. So… R [H2] • When the amount of NO is doubled, while keeping the amount of H2 constant, causes the reaction rate to increase fourfold. So… R [NO]2 Overall rate law: R = k[H2][NO]2

13. Rate Laws for Reactions For reactions that occur in a single step: A + B  2C R = k[A][B] For the reverse reaction…. 2C  A + B R = k[C]2

14. Rate Laws for Reactions For multi-step reactions, the rate law depends on the rate limiting step. Example: NO2(g) + CO(g) NO(g) + CO2(g) Step 1: NO2 + NO2  NO3 + NO (slow) Step 2: NO3 + CO  NO2 + CO2 (fast) Rate Law: R = k[NO2]2

15. Sample Problems • The rate law of a reaction is found to be R=k[X]3. By what factor does the rate increase if the concentration of X is tripled? The rate will increase by a factor of 27. • The rate of reaction involving two reactants, X and Z, is found to double when the concentration of X is doubled and to quadruple when the concentration of Z is doubled. Write the rate law for this reaction. R = k[X][Z]2 • The rate law for a single-step reaction that forms one product, C, is R = k[A][B]2. Write the balanced reaction of A and B to form C. A + 2B  C

16. Sample Problem • A particular reaction is found to have the following rate law: R = k[A][B]2 How is the rate affected by each of the following changes? • The initial concentration of A is cut in half. • The initial concentration of B is tripled. • The concentration of A is doubled, but the concentration of B is cut in half. • A catalyst is added.

17. Sample Problem A chemical reaction is expressed by the balanced chemical equation, A + 2B  C. Using the data below, answer the following: • Determine the rate law for the reaction • Calculate the value of the specific rate constant. • If the initial concentrations of both A and B are 0.30 M, at what initial rate is C formed?