Chemistry 173

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Chemistry 17-3

• The Reaction Process

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Reaction Mechanisms

• Most reactions do not occur as simple one-step
  processes
• A reaction mechanism is the step-by-step sequence
  of reactions by which an overall chemical change
  occurs

• Ex) I2 H2 ? 2HI
• Step 1 I2 ? 2IStep 2 I H2 ? H2IStep
  3 H2I I ? 2HI
• I and H2I are known as intermediates, because
  they do not appear in the overall equation

3
Collision Theory

• In order for reactions to occur between
  substances, their particles must collide
• Certain assumptions must hold true in order for a
  collision to be effective
• FIRST, the collision must have enough energy for
  the molecules to react
• SECOND, the reactant molecules must be correctly
  oriented for the reaction

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Ineffective and Effective Collisions
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Activation Energy

• The minimum amount of energy required to
  transform reactants into products is called
  activation energy, Ea
• The transitional structure which exists
  temporarily as reactants are converted into
  products is called the activated complex

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Chemistry 17-4

• Reaction Rate

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Reaction Rate

• The change in concentration of the reactants as
  time proceeds during a reaction is known as the
  reaction rate
• Reaction rates and reaction mechanisms are
  related in an area of chemistry known as chemical
  kinetics
• As we look at factors influencing the reaction
  rate, think about rates of solution

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Rate-Influencing Factors

• 1) Nature of ReactantsSubstances vary greatly
  in their tendencies to react. So, reaction rate
  depends upon the particular reactants involved in
  the reaction
• 2) Surface AreaIncreasing the surface area
  ______ the reaction rate. (Why?? And how can we
  increase the surface area??)

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Rate-Influencing Factors, cont.

• 3) TemperatureIncreasing the temperature ______
  the reaction rate. (Why??)
• 4) ConcentrationIncreasing the concentration
  ______ the reaction rate. (Why??)

10
Rate-Influencing Factors, cont.

• 5) Catalysts any substance that changes the
  rate of reaction but is regenerated at the end of
  the reaction
• Catalysts do not appear in the balanced equation
  because they participate only in intermediate
  steps!

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Reaction pathways for one reaction using several
different catalysts
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Rate Laws

• An equation that relates the rate of the reaction
  to the concentrations of the reactants is called
  the rate law for the rxn
• Ex 2H2(g) 2NO(g) ? N2(g) 2H2O(g)
• The rate law for this reaction was experimentally
  determined to be R k H2 NO2 where k is a
  constant

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Rate Laws Rxn Pathway

• For a ONE STEP reaction mechanism
• the rate law is proportional to the product of
  the reactant concentrations, each raised to its
  stoichiometric coefficient

• What?????
• Example 3A B 2C ? 2D
• R k A3 B1 C2
• This relationship is only true for single step
  reactions!!!

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Rate Laws Rxn Pathway, cont.

• For a MULTI-STEP reaction mechanism
• the rate law is determined from the slowest step
  in the reaction, called the rate-determining step

• Example NO2(g) CO(g) ? NO(g) CO2(g)
• Step 1 (slow)NO2 NO2 ? NO3 NO
• Step 2 (fast)NO3 CO ? NO2 CO2

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Rate Laws Rxn Pathway, cont.

• Example NO2(g) CO(g) ? NO(g) CO2(g)
• Step 1 (slow)NO2 NO2 ? NO3 NO
• Step 2 (fast)NO3 CO ? NO2 CO2

• The first step in this reaction is the rate
  determining step
• The rate law for this reaction is R k NO22
• Notice that CO is not involved in the rate law!
  (Why not??)

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Order of Reaction

• The general form for a rate law isR k An
  Bm . . .
• The order of reaction is equal to the sum of the
  superscripts in the rate law
• Example
• R k H2 NO2

• This reaction is third order
• Thats nice but what does it mean??
• If the concentration of ALL reactants are
  doubled, the rate will be increased by a factor
  of ___.