Experiment 29
Page 331
Objective
 To measure the effect of concentration upon the rate of
the reaction peroxydisulfate and iodine
S2O82  2 I   I 2  2 SO42
 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
t
Factors that Affect Rates of
Reaction
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
temperature
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
quadruple
 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
overall
Experimental Procedure
 Do preliminary experiment on page 335 to observe color





change
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!
2-4
The instant the color changes 2x10 mol of S2O8 has
reacted record time and add -mL Na2S2O3 and color will
dissapear
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
solution)
 Pre-Lab questions for experiments 29 (page 337) and 21
(page 231)