Chemical Kinetics

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Chemical Kinetics

• Rates and Mechanisms of Chemical Reactions

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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!

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Meaning of Rate of Reaction

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

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Reaction Rates

• Types of rates -
• initial rate, average rate, and instantaneous rate

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Units

• Molarity moles/L
• Time hr, min, sec
• Overall M/time or moles/L.time

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

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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?

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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

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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

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B. Rate Expression and Rate Constant
The rate law is R kAmBnCp
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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

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Rate Constant, k

• Relates rate and concentration at a given
  temperature

General Formula M(1- overall order) /time
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-For one reactant

• aA? products
• Rate k Am
• m order of reaction with respect to A

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• 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

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C. Determining Rates

Use data from experiments 1 and 2
Use a ratio of rate expressions for the 2 chosen
points.
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• 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
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• 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)

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• b. the rate of the reaction when CH3CHO
  0.452mol/L

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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.

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• 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
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• 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
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• 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
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Properties of ReactionsOrder Rate Expression0
(zero) rate k1 (first) rate kA2
(second) rate kA2
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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.

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Effective Collisions

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

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Potential Energy Diagrams and Catalysts
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Potential Energy Diagrams and Catalysts
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CATALYSIS

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

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REACTION MECHANISMS

• mechanism the elementary steps involved in a
  chemical reaction.
• Most chemical reactions DO NOT occur in a single
  step!

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Reaction Mechanisms

• B. Rate Determining Step
• the slowest step in the mechanism thus
  determining the overall rate

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• 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

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• 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

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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?

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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?