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Chemical Kinetics

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Rates and Mechanisms of Chemical Reactions Chemical Kinetics KINETICS the study of REACTION RATES and the MECHANISM (steps) the reaction follows. – PowerPoint PPT presentation

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Title: Chemical Kinetics


1
Chemical Kinetics
  • Rates and Mechanisms of Chemical Reactions

2
Chemical Kinetics
  • 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!

3
Meaning of Rate of Reaction
  • rate the change per interval of time
  • Exspeed (distance/ time) is a RATE

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

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 N2O5(g) at 67oC
  • Time(min) 0 1 2 3 4
  • N2O5 0.160 0.113 0.080 0.056
    0.040
  • 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
Factors Affecting Rate
  • 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

12
B. Rate Expression and Rate Constant
The rate law is R kAmBnCp
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 Am
  • m order of reaction with respect to A

16
  • If m 0 (zero order)
  • Rate k A0
  • rate is independent of A
  • If m 1 (1st order)
  • Rate k A1
  • If A doubles, then rate doubles
  • If m 2 (2nd order)
  • Rate k A2
  • 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, CH3CHO, 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 kCH3CHOm
  • CH3CHO(g) ? CH4(g) CO(g)

CH3CHO (mol/L) 0.162 0.195 0.273 0.410 0.518
Rate (mol/Lmin) 3.15 4.56 8.94 20.2 35.2
19
  • Consider the rate data for the decomposition
    of CH3CHO 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 CH3CHO
    0.452mol/L

21
Order of a Reaction for more than one reactant
  • aA bB? products
  • Rate kAmBn
  • Overall order of reaction m n
  • Use one equation to determine each reactants
    order, where the other reactant concentration is
    held constant.

22
  • The data below are for the reaction of nitrogen
    (II) oxide with hydrogen at 800oC.
  • 2NO(g) 2H2(g) ? N2(g) 2H2O(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 NO0.0024M and
    H20.0042M.

1 0.0010 0.0040 0.12
2 0.0020 0.0040 0.48
3 0.0030 0.0040 1.08
4 0.0040 0.0010 0.48
5 0.0040 0.0020 0.96
6 0.0040 0.0030 1.44
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.

1 0.1 0.1 4.0x10-5
2 0.1 0.2 4.0x10-5
3 0.2 0.1 16.0x10-5
24
  • Ex The following data were collected for this
    reaction at constant temperature
  • 2NO(g) Br2(g) ? 2NOBr(g)
  • State the rate law, and determine the rate of
    this reaction when NO0.15M and Br20.25M.

1 0.1 0.1 12
2 0.1 0.2 24
3 0.2 0.1 48
4 0.3 0.1 108
25
Properties of ReactionsOrder Rate Expression0
(zero) rate k1 (first) rate kA2
(second) rate kA2
26
Activation Energy
  • the minimum amount of energy required to react,
    Ea.
  • 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
  • H2O2(aq) I1-(aq) ? H2O(l)
    IO1-(aq) Slow
  • H2O2(aq) IO1-(aq) ? H2O(l) O2(g) I1-
    (aq) Fast
  • Identify the following
  • Rate determining step?
  • Are there any intermediates or catalysts in the
    reaction? If yes who?

36
Sample Problem
  • O3(g) Cl(g) ? O2(g) ClO(g) Slow
  • ClO(g) O(g) ? Cl(g) O2(g) Fast

Identify the following Rate determining
step? Are there any intermediates or catalysts in
the reaction? If yes who?
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