Micro iodine clock

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					                            Microscale Chemistry Experiment
             Investigation of the overall order of reaction between hydrogen
                 peroxide and iodide in acidic medium (clock reaction)

                                     Student Handout

Purposes
To determine the kinetic order of the reaction between H2O2(aq) and I-(aq) in acidic
media with respect to
(i)    H2O2(aq),
(ii)   I-(aq) and
(iii)  H+(aq).

Introduction
The kinetics of the reaction:

               H2O2(aq) + 2I- (aq) + 2H+(aq)  I2(aq) + 2H2O(l)

can be investigated by the introduction of a small and fixed amount of S2O32- (aq) and
starch indicator.

     H2O2(aq) + 2I-(aq) + 2H+(aq)  I2(aq) + 2H2O(l) ……….. main reaction
               2S2O32-(aq) + I2(aq)  S4O62-(aq) + 2I-(aq) …….monitor reaction
          Starch solution + I2(aq)  blue complex ………… ..indicator reaction

The added S2O32-(aq) consumes the I2 (aq) produced from the main reaction. As long as
there are S2O32- (aq) ions in the reaction mixture, I 2(aq) formed from the main reaction
will be instantaneously consumed by the S 2O32-(aq) ions and the starch indicator will not
be affected. However, when all S 2O32-(aq) ions are consumed, the I2(aq) starts to form and
will immediately turn the starch indicator to deep blue.

The overall result is that upon mixing different amounts of H2O2(aq), I-(aq), H+(aq),
S2O32-(aq) and starch indicator, no change w ill be observed at the start of the experiment,
but the reaction mixture suddenly changes to deep blue after a period of time. The time
elapsed before the development of the blue colour depends on the amount of S 2O32 -(aq)
used. The greater the amount of S 2 O32-(aq) is used, the longer w ill be the time taken for
the development of colour. Thus the reaction responsible by the S 2O32-(aq) is also known
as the monitor reaction, as it controls the time taken for the development of colour.

Reactions using the above technique are collectively classified as „clock reactions‟. If
iodine is used to indicate the reaction time, it is called an iodine clock reaction. Likewise,
if bromine is used, it is called a bromine clock reaction. (The „clock‟ ca n be slowed down
by adding more S2O32-(aq).)

Kinetic interpretation of clock reactions
Time elapsed for colour development indicates the time (t) taken for the formation of a
certain amount of iodine from the main reaction. 1/t would be proportional to the rate of
formation of this amount of iodine. 1/t would also be proportional to the initial rate of
decrease in concentration of I -(aq) or H2O2(aq) if the amount of iodine formed is small or
if the amount of S2O32-(aq) used is small. Hence there is a need to use small amount of

                                              1
S2O32-(aq).

Order w.r.t. I- (aq) will be investigated by keeping concentrations of H 2O2(aq) and H +(aq)
constant while varying the concentration of I -(aq) in the ratio of 1 : 2 : 4 : 8. If the ratio of
(1/t) doubles each time, the order of reaction w.r.t. to I -(aq) will be determined as 1. If the
ratio of (1/t) remains unchanged, the order can be regarded as zero. The experiment is
then repeated for determining orders for H2O2(aq) and H+(aq).

Safety
Wear safety spectacles and avoid skin contact with the chemicals. Dispose
of chemical waste and excess materials according to your teacher‟s
instruction.                                                                           EYE PROTECTION
                                                                                        MUST BE WORN




Further information on the chemicals used in the experiment can be found in the
Material Safety Data Sheet (MSDS). Consult your teacher for details.

Materials and Apparatus
About 20 cm3 of each of the follow ing solutions in labelled plastic bottles:
0.60 M H2SO4(aq), 0.60 M KI(aq), 1.50% H2O2(aq)
                                                            HARMFUL
                                                             HARMFUL/
                                                             IRRITANT




0.08 M Na 2S2O3(aq), starch solution, deionized water.
Two 8-well reaction strips, micro-tip plastic pipette, stop watch, micro-stirrer or
toothpicks.

Experimental Procedure
(A)   Kinetic order w.r.t. iodide ion
 1. Using a fresh and clean micro-tip plastic pipette, transfer 1 drop each of 1.5%
    H2O2(aq), 0.6 M H2SO4(aq) and starch indicator solution to 4 separate wells of a
    8-well reaction strip (call it strip A) so that each well has a total volume of 3 drops

 2. Take another 8-well reaction strip (call it strip B), again using a fresh and clean
    micro-tip plastic pipette, transfer 1 drop of 0.08 M Na 2S2O3(aq) to each of the first 4
    wells.

 3. Into the same 8-well reaction strip, place 1 drop of 0.6 M KI(aq) to the first well, 2
    drops to the second, 4 drops to the third and 8 drops to the fourth. Add 7 drops of
    deionized water to the first well, 6 drops to the second and 4 drops to the third so
    that the total volume of reactant mixture in each of the 4 well of strip B is 9 drops.
    (see Table 1)

 4. Stir the solution mixture in each of the wells of strip B with micro-stirrer or
    toothpick.

 5. Invert strip B and stack it atop strip A so that the first 4 wells of strip B is directly
    above the first 4 wells of strip A.




                                                2
 6. Hold the two strips firmly together by means of two small pieces of rubber tubing,
    one at each end and lower the strip combination suddenly (“shake-down”
    technique) so that the two solution mixtures mix thoroughly (see Figure 2). Start the
    stop watch at the same time.

 7. Turn the strip combination upside down repeatedly and look for the sudden
    appearance of a deep blue colour. Record the time taken. Carry on recording time
    until all the 4 wells have developed colour in the correct sequence.

 8. Clean the reaction strips thoroughly with deionized water and empty the water in
    the wells.




         Fig. 1   Size of 8-well reaction strip         Fig. 2    The “shake-down” technique

Table (1)
                                         Number of drops
                            Strip A                                 Strip B
                                         Starch                                 0.08M
       Well     H2O2(aq) H2SO4(aq)                      I-(aq)      H2O(l)
                                        solution                              S2O32-(aq)
            1                                             1            7
            2                                             2            6
                    1          1           1                                      1
            3                                             4            4
            4                                             8            0

(B)   Kinetic order w.r.t. H 2O 2
9.    Repeat steps (1) to (8) according to Table (2).

Table (2)
                                         Number of drops
                            Strip A                                 Strip B
                            0.08 M       Starch
       Well       I-(aq)                               H2O2(aq)     H2O(l)    H2SO4(aq)
                           S2O32-(aq)   solution
            1                                             1            7
            2                                             2            6
                    1          1           1                                      1
            3                                             4            4
            4                                             8            0




                                                   3
(C)   Kinetic order w.r.t. H +
10. Repeat steps (1) to (8) according to Table (3).

Table (3):
                                          Number of drops
                             Strip A                                 Strip B
                              0.08M       Starch
       Well      I-(aq)                                 H2SO4(aq)    H2O(l)    H2O2(aq)
                            S2O32-(aq)   solution
          1                                                 1           7
          2                                                 2           6
                   1             1            1                                     1
          3                                                 4           4
          4                                                 8           0

Results

                                Relative                    Rel. initial    Deduced
                Variable                          t (s)
                              concentration                rate,1/t (s-1)    order

                                     1
                                     2
                 [I-(aq)]
                                     4
                                     8
                                     1
                                     2
               [H2O2(aq)]
                                     4
                                     8
                                     1
                                     2
                [H+(aq)]
                                     4
                                     8

               (If initial rate doubles when the concentration of a species is doubled,
                 the reaction is first order with respect to that species)

Discussion Questions
1.    With the help of an appropriate sketch, illustrate the meaning of “initial rate”.

2.    Why is it assumed that in order to obtain initial rate, time (t) has to be small?

3.    Why are initial rate preferred to rates at other times of a reaction, i.e.
      instantaneous rates?

4.    Why the amount of S2O32-(aq) added to the reaction mixture has to be small?

5.    From the deduced order of each of the reactants, give a rate equation for the
      reaction.

6.    A mechanism for the reaction consists of the following three elementary steps:


                                                    4
                 H2O2(aq) + I-(aq)  X + H2O(l) …………. (slow)
                        H+(aq) + X  Y ……………………. (fast)
                Y + H+(aq) + I-(aq)  I2(aq) + H2O(l) ……... (fast)

     Suggest species for X and Y in the above elementary reactions in order that the rate
     expression for the rate determining step fits with the experimental rate equation.

Reference
D. Ehrenkranz and J. Mauch Chemistry in Microscale, Kendall/Hunt Publishing Co. (1990)
p. 153




                                            5
                               Microscale Chemistry Experiment
              Investigation of the overall order of reaction between hydrogen
                  peroxide and iodide in acidic medium (clock reaction)

                                         Teacher Notes

Kinetic runs are normally performed in sequential order starting with the first one. Time
taken for the whole experiment is the sum of the time spent for each run. One can speed
up the experiment by starting all the kinetic runs at the same time and recording the
time taken for all the kinetic runs simultaneously in one go. Time spent for the whole
experiment is then equal to the time required for the longest run. Thus the experiment
can be completed in a much shorter time. This can be achieved by using micro-scale
equipment together with the “shake-down” technique.

Curriculum Links
*     Rate of chemical reaction
*     Factors influencing reaction rate
*     Rate equation and order of reaction
*     The interpretation of reaction rate at molecular level

Pre-Laboratory Talk
1.     Brief on the purposes, technique and advantages in using the micro-scale
       instrument, in particular the “shake-down” technique, perhaps using a
       demonstration.

2.     Remind students that there will be ample time in carrying out the experiment and
       that there is a need to repeat the experiment for a number of times to obtain
       concordant results.

Time Required
Pre-laboratory talk: 15 minutes;      Experiment: 40 minutes

Remarks
(i)       Advantages in using the micro-scale technique
        Besides the usual advantages enjoyed by performing an experiment in small scale,
        the follow ing three advantages cannot possibly be achieved by conventional
        methods:

        (a)   The first lies in expressing relative concentration in terms of number of drops
              of reagent rather than actual concentration in mol dm-3. This greatly simplifies
              the calculation.

        (b)    The second is that the experiment allow s 4 kinetic runs to be performed at the
               same time instead of spending time on 4 different experiments. This shortens
               the time required for each order determination to as little as 1 minute.

        (c)    The layout of the experiment enables students to understand the aim more
               coherently as order determinations are completed with minimal steps.



                                                6
(ii)     The “1248” method
        The method offers a non-graphical approach to study reaction kinetics. Judging
        from the name of the method, it literally means varying the concentration of the
        reacting species according to a ratio of 1: 2: 4: 8, or doubling the concentration of
        the species in succession. If the observed reaction rates also increase in the same
        order , i.e. 1: 2: 4: 8, then the kinetic order of the reaction w ith respect to the
        species must be one. On the other hand, if the observed reaction rates increase
        according to a ratio of (1) 2: (2)2: (4)2: (8)2, then the kinetic order of the reaction with
        respect to the species should be two. Lastly, if the reaction rate is insensitive to
        change in concentration, the reaction is obviously of zero order w ith respect to the
        species. Reaction examples encountered in AL Chemistry are mainly of first or
        zero order kinetics, rarely of second order kinetics.

(iii)   Sources of m aterial
        The crucial apparatus of the experiment is the 8-well reaction strip. Strips
        employed should be small enough so that contained liquids would not drop when
        they are turned upside down. The 8-well reaction strips required for the
        experiment are obtainable from: MicroChem Lab (Website: www.mcl.hk)


Sample results


                                 Relative                 Rel. initial
                 Variable                        t (s)                    order
                               concentration             rate,1/t (s-1)

                                    1            104       9.6 x 10 -3
                                    2             55       1.8 x 10 -2
                  [I-(aq)]                                                   1
                                    4             28       3.6 x 10 -2
                                    8             14       7.1 x 10 -2
                                    1            115       8.7 x 10 -3
                                    2             58       1.7 x 10 -2
                [H2O2(aq)]                                                   1
                                    4             29       3.4 x 10 -2
                                    8             14       7.1 x 10 -2
                                    1            102       9.8 x 10 -3
                                    2             72       1.4 x 10 -2
                 [H+(aq)]                                                   ---
                                    4             51       2.0 x 10 -2
                                    8             26       3.8 x 10 -2

        Since the “shake-down” mixing method is not as effective as conventional pouring
        and stirring, repeated “shake-down” is necessary to achieve a close to
        homogeneous solution mixture. Thus each run has to be repeated two or three
        times to obtain concordant results. For both variations of I -(aq) and H2O2(aq),
        doubling the concentration also doubles the reaction rate. The reaction is therefore
        first order with respect to I-(aq) and to H2O2(aq). The figures for the H+(aq) ion
        variation showed no strong indication for a first order or a zero order kinetics w ith
        respect to H+(aq).




                                                  7
Suggested Answers to Discussion Questions
1.   With the help of an appropriate sketch, illustrate the meaning of “initial rate”.
Answer:
    According to the main reaction,
                   H2O2(aq) + 2I- (aq) + 2H+(aq)  I2(aq) + 2H2O(l)

     formation of iodine is due to the consumption of iodide ions. The formation of a
     small amount of iodine is determined by the extent of decrease in concentration of
     iodide ions. As shown by the follow ing decay curve, initial rate or the rate of
     decrease in concentration of iodide ions at the start of the experiment can be
     achieved by considering [I-] o, [I-]t1 and t1 . A longer time interval, t 2, gives average
     rate.

                         [iodide] o




                         [Iodide]
                                    t1
                                         t1


                         [Iodide]
                                 t2
                                              t2


                                                           Time


2.   Why is it assumed that in order to obtain initial rates, t has to b e small?
Answer:
    From the above sketch, initial rate can be achieved only if a short time interval is
    considered. A longer time interval will result in average rates which are not
    regarded as instantaneous rates. Rates considered in rate equations are
    instantaneous rates.

3.   Why are initial rates preferred to rates at other times of a reaction, i.e.
     instantaneous rates?
 Answer:
    Initial rates rather than instantaneous rates are generally considered. This is because
    initial rates can be measured more easily and less reaction complications are
    expected at the start of the reaction.

4.   Why the amount of S2O32-(aq) added to the reaction mixture has to be small?
Answer:
    The added S2O32-(aq) consumes the iodine formed and controls the initial rate of
    decrease of iodide ions of the decay curve. For a decent initial rate of decrease of
    iodide ions, amount of S 2O32-(aq) used has to be small so that the time taken for the
    appearance of the blue colour is short enough to warrant for an initia l rate.



                                                   8
5.   From the deduced order of each of the reactants, give a rate equation for the
     reaction.
Answer:
         d[I - (aq)]
     -                k [I - (aq)][ H 2 O 2 (aq)]
              dt

6.   Answer:         X = OI-(aq),        Y = HOI(aq)

Reference
D. Ehrenkranz and J. Mauch Chemistry in Microscale, Kendall/Hunt Publishing Co. (1990)
p. 153




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