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Rates of Chemical Reactions I A Clock Reaction


									Experiment 29
      Page 331
 To measure the effect of concentration upon the rate of
 the reaction peroxydisulfate and iodine
    S2O82  2 I   I 2  2SO4 

 To determine the order of the reaction with respect to
  the concentration of the reactants
 To obtain the rate law of the reaction
      [ S 2O82 ]
                    k[ S 2O82 ]x [ I  ] y
Factors that Affect Rates of
1.   Nature of Reactants
         The reactants must come into direct contact with each other
          and must collide with sufficient energy to result in a reaction
2. Concentration
         Changes the number of particles per unit volume
3. Temperature
         Changes the kinetic energy of the particles (i.e. velocity)
          which changes the number of collisions in a period of time
4. Catalyst
         Lowers the activation energy of the reaction while remaining
          chemically unchanged
Order of Reaction
                  A B C  D
The rate of the reaction may be expressed by:
                      rate  k A B 
                                  x    y

   [A] and [B] are the molar concentrations of A and B
   x and y are the powers which the concentration must
    be raised to describe the rate
   k is the specific rate constant for a reaction
       Independent of concentration and depends only on
       Once the rate is known the value of k can de calculated
Order of Reaction con’t.
                 A B C  D
 Suppose we found that x = 2 and y = 1 for the reaction
 The rate law would then be:
                         rate  k[ A]2 [ B]
 If [B] is doubled (keeping [A] the same)the rate will double
 If [A] is doubled (keeping [B] the same)the rate will
 The powers in the rate law are the order of the reaction
 The reaction above would be:
    Second order in A
    First order in B
    The overall order is the sum of the exponents or third-order
Experimental Procedure
 Do preliminary experiment on page 335 to observe color
   Prepare solution using instructions and set-up on page 335
    and 336
   Fill seven test tubes with -mL Na2S2O3 and put aside
   Add Ammonium persulfate (NH4)2S2O8 to solution as per
    procedure on 336 and begin timing IMMEDIATELY!
   The instant the color changes 2x10 -4 mol of S O 2- has
                                                  2 8
    reacted record time and add -mL Na2S2O3 and color will
   Record time for color to reappear and repeat for remaining
    6 aliquots of Na2S2O3 and for solutions 2, 3 and 4
Lab Report
 Do Calculations on page 337
    Plots may be done by hand or spreadsheet
 Answer all questions and fill out tables on pages 339
  and 340
 Due next week:
   Report Sheet for Experiment 29 – page 339-340
   Four plots from the calculation section (one for each
   Pre-Lab questions for experiments 29 (page 337) and 21
    (page 231)

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