Determining%20the%20Form%20of%20the%20Rate%20Law

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• Determining the Form of the Rate Law

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Rate Law

• The mathematical expression that allows
  calculations of reaction rate as a function of
  reactant concentration.
• Must be determined experimentally by altering the
  initial concentrations of all reactants and
  observing the effect on initial rate.

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How Data is created
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Rate Law Equation

• Since the concentrations within a reaction change
  the instant the reaction starts, chemists look at
  the effect of the initial concentrations.
• For any general reaction
• aA bB ? products
• The rate law equation may be expressed as the
  product of the initial concentrations raised to
  some exponential value.
• Rate kAmBn
• k, m and n must be experimentally determined

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Method of Initial Rates

• Used to find the form of the rate law
• Choose one reactant to start with
• Find two experiments where the concentration of
  that reactant changes but all other reactants
  stay the same
• Write the rate laws for both experiments
• Divide the two rate laws
• Solve for the order
• Follow the same technique for other reactants

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Example

• Choose one reactant to start with
• NH4
• Find two experiments where the concentration of
  that reactant changes but all other reactants
  stay the same
• Exp 2 3

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• Write the rate laws for both experiments
• Exp 2 2.70x10-7 k(0.100)x(0.010)y
• Exp 3 5.40x10-7 k(0.200)x(0.010)y
• Divide the two rate laws
• 0.50 0.50x
• Use log rules to solve for the order
• x 1 so the order for NH4 is one

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• Follow the same technique for other reactants
• NO2- Exp 1 2
• Exp 1 1.35x10-7 k(0.100)1(0.0050)y
• Exp 2 2.70x10-7 k(0.100)1(0.010)y
• 0.5 0.5y
• y 1
• So Rate kNH41NO2-1
• Overall Reaction Order