Rate Laws

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Slide 1
Rate Laws

• Reactions are reversible.
• As products accumulate they can begin to turn
  back into reactants.
• Early on the rate will depend on only the amount
  of reactants present.
• We want to measure the reactants as soon as they
  are mixed.
• This is called the Initial rate method.

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Types of Rate Laws

• Differential Rate law - describes how rate
  depends on concentration.
• Integrated Rate Law - Describes how concentration
  depends on time.
• For each type of differential rate law there is
  an integrated rate law and vice versa.
• Rate laws can help us better understand reaction
  mechanisms.

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Differential Rate law - how rate depends on
concentration.
Slide 2

• Example
• aA bB ? Products
• A and B are chemical formulas for the reactants
• a and b are the coefficients in the balanced
  equation

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Differential Rate law
Slide 3

• The dependence of the reaction rate on
  concentration is given by the rate law
• An equation that tell how the rate depends on the
  concentration of each reactant.
• For this reaction
• aA bB ? Products
• The rate law expression is
• Rate kAmBn

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Rate Law
Slide 4

• Rate kAmBn
• k is a proportionality constant called the rate
  constant
• m and n exponents determined by the experiment

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Determining m and n
Slide 5

• For simple reactions (the ones found in this
  class) an exponent of 1 means that the rate
  depends linearly on the concentration of the
  corresponding reactant.
• For example if m1 and the A is doubled, the
  rate doubles.
• If m 2 and the A is doubled, A2 quadruples
  and the rate increases by a factor of 4.
• If m0, the rate is independent of the A

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Slide 6

Rate Laws

• Two key points
• The concentration of the products do not appear
  in the rate law because this is an initial rate.
• The order must be determined experimentally,
• cant be obtained from the equation