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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
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?