Presentation on theme: "Reaction Energy and Reaction Kinetics"— Presentation transcript:
1 Reaction Energy and Reaction Kinetics Reaction Rate
2 ObjectivesDefine 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 RateReaction rate – change in concentration of reactants per unit time as a reaction occurs. Chemical kinetics – study of reaction rates and reaction mechanisms.
5 Rate-Influencing Factors Surface AreaHeterogeneous reactions – reactions with reactants in two different phasesHeterogeneous 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 TemperatureAn increase in temperature causes:An increase in collision energyAn increase in collision frequencyBoth result in increased reaction ratesIn general, a 10oC increase in temperature causes the reaction rate to double.
7 Rate-Influencing factors ConcentrationIncreasing the concentration of the reactants may cause an increase in the rate of reaction for homogeneous reactions.
8 Rate-Influencing Factors Presence of a CatalystCatalysts 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 LawRate 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]2Overall 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 ProblemsThe 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]2The 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 ProblemA particular reaction is found to have the following rate law:R = k[A][B]2How 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 Initial rate of formation of C Sample ProblemA 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 reactionCalculate 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?Experiment #Initial [A]Initial [B]Initial rate of formation of C10.20 M2.0x10-4M/min20.40 M8.0x10-4M/min3O.40 M1.6x10-3M/min