Reaction Rate Laws

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Reaction Rate Laws

• Podcast 16.3

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16.1-16.2 Review

• Factors affecting reaction rates include
• 1._____________
• 2._____________
• 3._____________

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Whats a Rate Law?

• A rate law is an expression that relates how the
  _______ of reaction changes as ___________________
  __ changes.

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Consider the decomposition of hydrogen
peroxide 2 H2O2 ? 2 H2O O2

• The reaction is _______________
• The rate law is rate k H2O21
• k is called the _________________
• The larger the value for k, the ________ the
  reaction. (k is ______________dependent)

H2O2
Initial Rate of Decomp
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What would a second order reaction look like?
Consider A? B

• The reaction is _______________
• The rate law is rate k A2

A
Initial Rate of Decomp
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How do we determine the rate law?

• You cannot figure out the rate law from the
  balanced equation.
• You must look at __________________
• -Alternatively, you can look at graphs or
  mechanism, but that will not be covered here. . .

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Method of Initial Rates

• Allows us to determine reaction order by
  comparing initial rates of a reaction carried out
  with varying reactant concentrations.
• For example if doubling X doubles the rate,
  the rate law must be first order with respect to
  X. If doubling X quadruples the rate, the rate
  law must be second order with respect to X.

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Example

• Given the following information, determine the
  overall rate law for the reaction A
  B ? C

Experiment A0 (M) B0 (M) Initial Rate (M/s)
1 1.0 1.0 0.0050
2 2.0 1.0 0.010
3 2.0 2.0 0.040
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A B ? C
Experiment A0 (M) B0 (M) Initial Rate (M/s)
1 1.0 1.0 0.0050
2 2.0 1.0 0.010
3 2.0 2.0 0.040

• When A doubles, and B is constant, rate
  ____________
• When B doubles, and A is constant, rate
  ____________
• The experiment is _____ order with respect to A,
  and _____ order with respect to B.
• The rate law is _______________________
• Do not compare experiments when both
  concentrations change, as they do in experiment 1
  and 3.

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Overall Order

• The overall order is the sum of the exponents in
  the rate law.
• Rate kA1B2
• represents a _____ order reaction

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A B ? C
Experiment A0 (M) B0 (M) Initial Rate (M/s)
1 1.0 1.0 0.0050
2 2.0 1.0 0.010
3 2.0 2.0 0.040

• We can also determine the rate constant, k, from
  this data, once the rate law is known.
• Choose any experiment and substitute values.

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Practice Problem

• What is the rate law and the value of rate
  constant for the reaction 2 AB2 C ? CB BA
  based on the data below?

Experiment AB20 (M) C0 (M) Initial Rate (M/s)
1 1.0 1.0 1.4 x 10-7
2 0.5 1.0 3.5 x 10-8
3 1.0 2.0 5.5 x 10-7
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Solution
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One last note

• A reaction can also be ________ order.
• This means that the rate does not depend on the
  concentration of that reactant.
• Mathematically x0 ________
• For a reaction A B ? C that is zero order in
  A, and 1st order for B, the rate law would be

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Chapter 16 -Summary Topics

• Collision Theory
• Average Rate
• Activation Energy
• Catalysis
• Rate Law
• Rate Constant, k
• Reaction Order
• Initial Rates Method
• Not covered - reaction mechanisms/16.4