Rate Law and Reaction Mechanisms

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Rate Law and Reaction Mechanisms

• Honors Chemistry
• Mrs. Coyle

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Reaction Rate

• Rate ?A
• ?t
• The change in concentration of a reactant or
  product over time.

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Number of Molecules of Reactants and Products vs
Time
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Reaction Rates and Stoichiometry
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Rate Expression (Rate Law)

• Rate k Ax By
• k is the specific rate constant.
• The exponents and k are determined from
  experimental data.
• x th order in A
• yth order in B
• Overall order of the reaction is xy

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Ex. 1 Determining Rate Law

• Trial A B Rate(M/s)
• 0.01 0.01 .004
• 0.02 0.01 .008
• 0.01 0.02 .008

Note Calculating Rate Law exponents from
Experimental Data for A Rate 2 kA2x
B2yRate 1 kA1x B2y
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Ex. 2 Determining the Rate Law

• Trial A2 B Rate(M/s)
• 0.1 0.1 .004
• 0.1 0.2 .004
• 0.2 0.2 .008

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Reaction Mechanisms
Reaction mechanism a series of elementary steps
that occur during a chemical reaction.
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Intermedeates

• Species that appear in a reaction mechanism but
  not in the overall balanced equation.
• Intermedeates are formed and consumed during the
  reaction.

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Rate Laws of Elementary Steps
rate k A
rate k AB
rate k A2

• NOTEIn the rate law expression of the
  elementary steps the exponents match with the
  coefficients of the reactants.

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Rules for Writing Possible Reaction Mechanisms

• The sum of the elementary steps must give the
  overall balanced equation for the reaction.
• The rate law (shown by the rate-determining step)
  should predict the same rate law that is
  determined experimentally.

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Rate Determining Step

• The slowest step in the reaction mechanism.