Dr. Mihelcic Honors Chemistry1 Chemical Kinetics Rates and Mechanisms of Chemical Reactions

2 KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows. We can use thermodynamics to tell if a reaction is spontaneous. Only KINETICS will tell us HOW FAST the reaction happens! KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows. We can use thermodynamics to tell if a reaction is spontaneous. Only KINETICS will tell us HOW FAST the reaction happens! Chemical Kinetics

3 Meaning of Rate of Reaction rate – the change per interval of time Ex—speed (distance/ time) is a RATE

4 Types of rates - –initial rate, average rate, and instantaneous rate Reaction Rates

5 Units [ ] = Molarity = moles/L Time= hr, min, sec Overall= M/time or moles/L. time

6 C. Measurement General reaction rate: calculated by dividing rate expressions by stoichiometric coefficients

7

8 Instantaneous rate – the slope of the tangent line at a given point Average rate – the rate over a given time interval Initial rate – rate at “time zero”

9 Example 2: Sample Calculation of Average Rate Rate of decomposition of N 2 O 5 (g) at 67 o C: Time(min) [ N 2 O 5 ] What is the average rate of decomposition of dinitrogen pentoxide during the first three minutes?

10 II. Reaction Rate and Concentration A. Reactant concentration and collision theory *Reactions occur when molecules collide to exchange or rearrange atoms *Effective collisions occur when molecules have correct energy and orientation

11 1.Concentration and physical state of reactants and products 2. Temperature 3. Catalysts Rate Law: relates the rate of the reaction to the concentration of the reactants Factors Affecting Rate:

12 B. Rate Expression and Rate Constant The rate law is: R = k[A] m [B] n [C] p EOS The exponents in a rate law must be determined by experiment. They are not the stoichiometric coefficients and k is the rate constant.

13 B. continued m, n, and p are the reaction order can be 0, 1, 2 or fractions must be determined by experiment! Overall Order = Sum of m, n and p

14 Rate Constant, k Relates rate and concentration at a given temperature General Formula M (1- overall order) /time

15 -For one reactant aA  products Rate = k [A] m m= order of reaction with respect to A

16 If m = 0 (zero order) Rate = k [A] 0 rate is independent of [A] If m = 1 (1 st order) Rate = k [A] 1 If [A] doubles, then rate doubles If m = 2 (2 nd order) Rate = k [A] 2 If [A] doubles, then rate quadruples

17 C. Determining Rates Use data from experiments 1 and 2 Use a ratio of rate expressions for the 2 chosen points.

18 The initial rate of decomposition of acetaldehyde, CH 3 CHO, was measured at a series of different concentrations and at a constant temperature. Using the data below, determine the order of the reaction (m) in the equation: Rate = k[CH 3 CHO] m CH 3 CHO(g)  CH 4 (g) + CO(g) CH 3 CHO (mol/L) Rate (mol/L*min)

19 Consider the rate data for the decomposition of CH 3 CHO given in the above example. Knowing that the reaction is 2nd order, determine the a. value of the rate constant (k)

20 b. the rate of the reaction when [CH 3 CHO] = 0.452mol/L

21 Order of a Reaction for more than one reactant aA +bB  products Rate = k[A] m [B] n Overall order of reaction = m + n Use one equation to determine each reactant’s order, where the other reactant concentration is held constant.

22 The data below are for the reaction of nitrogen (II) oxide with hydrogen at 800 o C. 2NO(g) + 2H 2 (g)  N 2 (g) + 2H 2 O(g) Determine the order of the reaction with respect to both reactants, calculate the value of the rate constant, and determine the rate of formation of product when [NO]=0.0024M and [H 2 ]=0.0042M

23 Ex The initial rate of a reaction A + B  C was measured with the results below. State the rate law, the value of the rate constant, and the rate of reaction when [A] = 0.050M and [B] = 0.100M x x x10 -5

24 Ex The following data were collected for this reaction at constant temperature: 2NO(g) + Br 2 (g)  2NOBr(g) State the rate law, and determine the rate of this reaction when [NO]=0.15M and [Br2]=0.25M

25 Properties of Reactions OrderRate Expression 0 (zero) rate = k 1 (first)rate = k[A] 2 (second)rate = k[A] 2

26 Activation Energy the minimum amount of energy required to react, E a. activated complex - a short-lived molecule formed when reactants collide; it can return to reactants or form products.

27 Effective Collisions How can the number of effective collisions be increased? Increase concentration of reactants, Increase temperature of reaction, Increase surface area of reactants.

28 Potential Energy Diagrams and Catalysts

29 Potential Energy Diagrams and Catalysts

30 CATALYSIS A catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.

31 REACTION MECHANISMS mechanism: the elementary steps involved in a chemical reaction. Most chemical reactions DO NOT occur in a single step!

32 Reaction Mechanisms B. Rate Determining Step: the slowest step in the mechanism; thus determining the overall rate

33 C. Intermediates: A substance that is not an original reactant or a final product; it is produced in one step and consumed in another step of the mechanism

34 D. Catalysts: A substance that speeds up the reaction but is not permanently changed by the reaction; both an original reactant and a final product

35 Sample Problem H 2 O 2 (aq) + I 1- (aq)  H 2 O(l) + IO 1- (aq) Slow H 2 O 2 (aq) + IO 1- (aq)  H 2 O(l) + O 2 (g) + I 1- (aq) Fast Identify the following: Rate determining step? Are there any intermediates or catalysts in the reaction? If yes who?

36 Sample Problem O 3 (g) + Cl(g)  O 2 (g) + ClO(g) Slow ClO(g) + O(g)  Cl(g) + O 2 (g) Fast Identify the following: Rate determining step? Are there any intermediates or catalysts in the reaction? If yes who?