1. Exp 13: The Rate of an Iodine Clock Reaction
Intro: Reaction Kinetics
Some reactions proceed so rapid, that they appear to occur instantaneously
Many reaction proceed much slower and take minutes to years to occur
Can you think of examples?
Differences in reaction rates are dependent on molecular characteristics of the reactants
Factors that affect the reaction rate are
Concentration of reactants
Temperature
Presence of a catalyst

2. Exp 13: The Rate of an Iodine Clock Reaction
Purpose
Determine the rate law for the iodine clock reaction
Evaluate the effect of a catalyst on the rate law
See: Chang, Chapter 13

3. Exp 13: The Rate of an Iodine Clock Reaction
Series of redox reactions
Oxidation of iodide to iodine by potassium persulfate
Reaction of iodine with starch gives a dark-blue color
See: Chang, Chapter 13
Rxn 1 S2O I-  2SO42- + I2
Rxn 2 I2 + 2S2O32-  2I- + S4O62-
Rxn 3 I2(aq) + starch (aq)  blue color
Modified Landolt Reaction

4. Iodine Starch Clock Reaction
S2O82-(aq) + 2 I-(aq)  I2(aq) SO42-(aq)
I2(aq) S2O32-(aq)  2 I-(aq) S4O62-(aq)
(3) I2(aq) starch (aq)  blue color
The reaction can be described by the following mechanism Reaction step 1:
Potassium persulfate reduces sodium iodide to form iodine and sulfate
This reaction proceeds very slowly. It is the rate determining step.
Reaction step 2:
To study the rate of the reaction, we need to be able to determine how fast one of the products is formed or how fast one of the reactants is consumed.
The timing method used in this experiment makes this reaction a "clock reaction".
To time the rate of reaction (1), the reaction (2) will occur simultaneously in the same reaction tube as reaction (1)
Reaction step 3:
Iodine liberated is instantaneously retransformed by to iodide and tetrathionate.
This is an "immeasurable fast" reaction.
The blue starch iodine complex becomes visible after complete consumption of the thiosulfate ions, since then a reduction of the iodine formed cannot take place according to equation (3) any longer.

5. EXPERIMENTAL PROCEDURE
S2O I-  2SO42- + I2 (slow, rate-determining reaction)
I2 + 2S2O32-  2I- + S4O62- (very fast reaction)
excess I2 + starch  dark-blue complex (very fast)
Test 3 different concentrations of reactants
See reaction table (next slide)
Do reaction at increased temperature (35-37oC)
Do reaction in the presence of a catalyst (Cu2+ ions)

6. EXPERIMENTAL PROCEDURE
Record temperature in the lab
Mark 3 test tubes
Rinse pipets twice with 1-mL of NaI solution
Pipet required volume of NaI in each tube (see reaction table, next slide)
Add NaCl to required tubes
Add Na2S2O3 to required tubes
Add starch to required tubes
Add K2SO4 to required tubes
DO NOT ADD K2S2O8 to the test tubes until you are ready to start the reaction as this will start the reaction!!

7. EXPERIMENTAL PROCEDURE Reaction Mixture Table
Solution
used
0.20 M
NaI
NaCl
0.010 M
Na2S2O3
2% starch
K2SO4
K2S2O8
Reacting
ion: I-
“ions”
 S2O3-2
“indicator”
SO4-2
 S2O8-2
Tube #
Volume 1
2.0 mL
1.0 mL
2.0 ml 2
0 mL
4.0 ml 3
4.0 mL
4 (at 35oC)
5 (with Cu2+)
5.0 mL
Total volume per group 12 8 10 5 17
Repeat
Do Once
Total volume in each tube: 11.0 mL

8. EXPERIMENTAL PROCEDURE
Note time or use stopwatch
Add 2.0 mL K2S2O8 to tube #1
this will start the reaction!!
Quickly cover tube with Parafilm and invert ~ 3-5 times to mix
Record the time that the dark color appears
The color appears suddenly!!
Exp #1 should take about 4-8 minutes
Experiment #2
Repeat the experiment with tube #2
Add 4.0 mL K2S2O8 to tube #2
Mix as before and record how long it takes for the solution to turn blue
Experiment #3
Repeat the experiment with tube #3
Add 2.0 mL K2S2O8 to tube #3
Clean and dry the tubes and repeat the experiments
Differences in time should be less than 10 s

9. EXPERIMENTAL PROCEDURE
Experiment #4: temperature effect
Put warm water from the faucet in a beaker
Pipet ~ 5 mL of K2S2O8 in tube #0
Pipet the volumes in tube #4 as indicated in the reaction table, except K2S2O8
Put the tubes in the warm water bath
Allow to equilibrate for ~ 5 min
Start the reaction by adding 2 mL K2S2O8 from tube #0 into tube #4
Note time and mix as before
Put tube back into water bath
Record time to turn blue
Experiment #5
Fill tube #5 with the same amounts as tube #1, except K2S2O8 (see reaction table)
Add 1 drop of 0.2 M CuSO4 solution
Shake gently to mix
Initiate reaction by adding 2.0 mL K2S2O8 (room temp, not warm!!)
Mix as before and record how long it takes for the solution to turn blue

10. EXPERIMENTAL PROCEDURE Reaction Mixture Table
Solution
used
0.20 M
NaI
NaCl
0.010 M
Na2S2O3
2% starch
K2SO4
K2S2O8
Reacting
ion: I-
 S2O32-
SO42-
 S2O82-
Trial #
Concentration  1
3.6 x 10-2
1.82 X 10-3
1.0 mL 2
0 mL
7.3 x 10-2 3
4 (at 35oC)
5 (with Cu2+)

11. Reaction Rate and Product Concentration
In general, for the reaction
aA + bB cC + dD
rate = 1 a -
= -
[A] t b
[B] c
[C]
= + d
[D]
The numerical value of the rate depends upon the substance that serves as the reference
The rest is relative to the balanced chemical equation

12. Reaction Rate and Product Concentration
S2O I-  2SO42- + I2
I2 + 2S2O32-  2I- + S4O62-
Net Reaction: S2O S2O32-  2SO S4O62-
For every molecule of S2O82- that is consumed, 2 molecules of S2O32- are consumed or
D [S2O82-] = ½ D[S2O32-]  D[S2O32-] = 2 D [S2O82-]
Look at the reaction concentration table:
[S2O32-] = 1.82 X 10-3 M: limiting reactant compared to
[S2O82-] = 3.6 x 10-2 M

13. Next Week: Exp 14A – Le Chatelier’s Principle
Lab Report for Exp 13” “The Rate of an Iodine Clock Reaction”
Due Monday Oct 15:
Data Sheets, calculations
Post-lab question 1a-e, 3
Post-lab question 2 (5 bonus points)
Prelab Preparations for Exp 14A: “Le Chatelier’s Principle”
Due: Prelab questions: #1, 2, 3, 4, 5
For more info, see Chang, Chapter 14
Lab preparations as usual: Read and look up the following
Introduction and purpose of the experiment
Experimental procedure
Properties of chemicals
Ammonia, NH3 . Nickel nitrate, Ni(NO3)2 .
Sodium hydroxide, NaOH . Cobalt nitrate, Co(NO3)2
Hydrochloric acid, HCl . Calcium nitrate,Ca(NO3)2
Next Week: Exp 14A – Le Chatelier’s Principle