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Reaction Energy and Reaction Kinetics Reaction Rate.

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Presentation on theme: "Reaction Energy and Reaction Kinetics Reaction Rate."— Presentation transcript:

1 Reaction Energy and Reaction Kinetics Reaction Rate

2 Objectives 1.Define the rate of reaction and describe how it can be determined. 2.List and explain factors that affect the rate of reaction. 3.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 H 2(g) + Cl 2(g)  2 HCl (g) (fast) 3 H 2(g) + N 2(g)  2 NH 3(g) (slow) 4 Na (s) + O 2(g)  2 Na 2 O (s) (fast) 4 Fe (s) + 3 O 2(g)  2 Fe 2 O 3(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 10 o C 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: 1.Explain the rate law for a chemical reaction. 2.Determine the rate law for a particular reaction given appropriate kinetics data. 3.Discuss the relationship between the rate law and the reaction mechanism.

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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 H 2(g) + 2 NO (g)  N 2(g) +2 H 2 O (g) Doubling the amount of H 2, while keeping the amount of NO constant, causes the reaction rate to double. So… R  [H 2 ] When the amount of NO is doubled, while keeping the amount of H 2 constant, causes the reaction rate to increase fourfold. So… R  [NO] 2 Overall rate law: R = k[H 2 ][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: NO 2(g) + CO (g)  NO (g) + CO 2(g) Step 1: NO 2 + NO 2  NO 3 + NO(slow) Step 2: NO 3 + CO  NO 2 + CO 2 (fast) Rate Law: R = k[NO 2 ] 2

15 Sample Problems 1.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 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 3.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? a.The initial concentration of A is cut in half. b.The initial concentration of B is tripled. c.The concentration of A is doubled, but the concentration of B is cut in half. d.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: a.Determine the rate law for the reaction b.Calculate the value of the specific rate constant. c.If the initial concentrations of both A and B are 0.30 M, at what initial rate is C formed? Experiment #Initial [A]Initial [B]Initial rate of formation of C M 2.0x10 -4 M/min M0.40 M8.0x10 -4 M/min 3O.40 M0.40 M1.6x10 -3 M/min


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