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
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
Exp 13: The Rate of an Iodine Clock Reaction
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 S2O82- + 2I-  2SO42- + I2
Rxn 2 I2 + 2S2O32-  2I- + S4O62Rxn 3 I2(aq) + starch (aq)  blue color
Modified Landolt
Reaction
Iodine Starch Clock Reaction
(1)
S2O82-(aq) + 2 I-(aq)  I2(aq) + 2 SO42-(aq)
(2)
I2(aq) + 2 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.
EXPERIMENTAL PROCEDURE
S2O82- + 2I-  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)
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!!
EXPERIMENTAL PROCEDURE
Reaction Mixture Table
Solution
used
0.20 M
NaI
0.20 M
NaCl
0.010 M
Na2S2O3
2% starch
0.20 M
K2SO4
0.20 M
K2S2O8
Reacting
ion:
I-
“ions”
S2O3-2
“indicator”
SO4-2
S2O8-2
Do Once
Repeat
Tube #
Volume
1
2.0 mL
2.0 mL
2.0 mL
1.0 mL
2.0 mL
2.0 ml
2
2.0 mL
2.0 mL
2.0 mL
1.0 mL
0 mL
4.0 ml
3
4.0 mL
0 mL
2.0 mL
1.0 mL
2.0 mL
2.0 ml
4 (at 35oC)
2.0 mL
2.0 mL
2.0 mL
1.0 mL
2.0 mL
2.0 ml
5 (with Cu2+)
2.0 mL
2.0 mL
2.0 mL
1.0 mL
2.0 mL
2.0 ml
0
Total volume per
group
5.0 mL
12
8
10
5
Total volume in each tube: 11.0 mL
8
17
EXPERIMENTAL PROCEDURE
•
Experiment #1
– 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
– Mix as before and record how long it takes for the solution to turn blue
•
•
Clean and dry the tubes and repeat the experiments
Differences in time should be less than 10 s
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
EXPERIMENTAL PROCEDURE
Reaction Mixture Table
Solution
used
0.20 M
NaI
Reacting
ion:
I-
Trial #
0.20 M
NaCl
0.010 M
Na2S2O3
2% starch
S2O32-
0.20 M
K2SO4
0.20 M
K2S2O8
SO42-
S2O82-
Concentration
1
3.6 x 10-2
3.6 x 10-2
1.82 X 10-3
1.0 mL
3.6 x 10-2
3.6 x 10-2
2
3.6 x 10-2
3.6 x 10-2
1.82 X 10-3
1.0 mL
0 mL
7.3 x 10-2
3
7.3 x 10-2
0
1.82 X 10-3
1.0 mL
3.6 x 10-2
3.6 x 10-2
4 (at 35oC)
3.6 x 10-2
3.6 x 10-2
1.82 X 10-3
1.0 mL
3.6 x 10-2
3.6 x 10-2
5 (with Cu2+)
3.6 x 10-2
3.6 x 10-2
1.82 X 10-3
1.0 mL
3.6 x 10-2
3.6 x 10-2
Reaction Rate and Product Concentration
In general, for the reaction
aA + bB
rate =
-
1
[A]
a
t
= -
cC + dD
1
[B]
b
t
= +
1
[C]
c
t
= +
1
[D]
d
t
The numerical value of the rate depends upon the substance that
serves as the reference
The rest is relative to the balanced chemical equation
Reaction Rate and Product Concentration
S2O82- + 2I-  2SO42- + I2
I2 + 2S2O32-  2I- + S4O62Net Reaction:
S2O82- + 2S2O32-  2SO42- + S4O62-
For every molecule of S2O82- that is consumed, 2
molecules of S2O32- are consumed
or
 [S2O82-] = ½ [S2O32-]  [S2O32-] = 2  [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
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)
Next Week: Exp 14A – Le Chatelier’s Principle
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
.