Introduction
This experiment is designed to study the kinetics of a popular iodine/starch clock reaction:
IO
3
-
+ S
2
O
5
-2
→ I
2
+ SO
4
-2
This reaction starts by taking 0.20 M KIO
3
and reacting it with 0.20 M Na
2
S
2
O
5
. This produces a Redox reaction and the production of an acidic environment. Starch is added to the mix, which reacts with the
I
2
to produce a dark blue starch/iodine complex. The starch acts as an indicator for the production of the
I
2
. Students will determine the rate law based on the time it takes for the reaction to change, and they will study the effects of a catalyst on reaction rates.
PreLab Questions
1. What is the general rate law expression for this reaction?
2. Determine the balanced redox equation (acidic solution).
3. How does a catalyst affect a chemical reaction – give details?
Materials
0.20 M KIO
3
, 0.20 M Na
2
S
2
O
5
, Starch solution, 0.2 M H
2
SO
4
, Deionized water
100 mL beaker + disposable pipette for adding KIO
3
solution
10 mL graduated cylinders (1 for solution A, 1 for solution B) stopwatch
100 mL beaker for mixing solution B Test tube brush
125/250 mL Erlenmeyer flask for mixing solution A and as a serving as a reaction vessel.
Procedure
PART ONE – DETERMINATION OF THE RATE LAW AND RATE CONSTANT
1. Clean and dry all glassware at the beginning of this lab and after each chemical reaction.
2. Get approximately 75 mL KIO
3
solution in your 100 mL beaker.
3. Use separate graduated cylinders for making solution A (which will be added directly to the
Erlenmeyer flask) and solution B (that will then be mixed in the 100 mL beaker prior to being added to the flask).
4. Carry out the following five experiments; measuring the time (in seconds) it takes for the solution to create a dark blue/black precipitate. Start timing the instant the solutions are added together
5. Use the times generated as a way of measuring the rate of the reaction.
6. If time permits, repeat any of the five experiments if you feel any of your results are very flawed.
Record your times on the class data table and calculate a class average time for each experiment.
7. Determine the reaction rate and rate constant (k) using data from Experiment 1 Class Average. a. Calculate the number of moles of IO
3
-1
in 10.0mL of 0.20
M KIO
3
. b. The combined volumes of Solutions A and B in Experiment 1 = 56.0mL (10+30+2+6+8)mL.
Calculate your initial [IO
3
-
] for Experiment 1 using a total volume of 56.0mL. c. Determine the rate of the reaction using the formula Rate = ∆[IO
3
] / ∆t. d. Calculate the value of the rate constant (k) by using the data from Experiment 1 (note: you will need to also determine the initial [S
2
O
5
-2
] using the same techniques for calculating [IO
3
-
] outlined above.
8. Determine the order of each reactant, and the rate law expression using the Class Averages.

DATA TABLE ONE
Exp.
1
2
3
4
Solution A
KIO
3
10.0mL
20.0mL
5.0mL
10.0mL
H
2
O Na
2
S
2
O
4
Starch
30.0mL 2.0mL
20.0mL 2.0mL
35.0mL 2.0mL
30.0mL
Solution B
1.0mL
6.0mL
6.0mL
6.0mL
6.0mL
H
2
O
8.0mL
8.0mL
8.0mL
9.0mL
Time (s) Class Ave (s)
5 10.0mL 30.0mL 4.0mL 6.0mL 6.0mL
PART TWO – CALCULATING THE RATE BASED ON SPECIFIED [REACTANT]
1. Clean and dry all glassware at the beginning of this lab and after each chemical reaction.
2. Based on your determined rate law, estimate (before carrying out the reaction!) the time it would take to carry out the following reaction. Show your calculations for your estimation.
DATA TABLE TWO
Exp.
6
Solution A
KIO
3
15.0mL
H
2
O Na
2
S
Solution B
2
O
25.0mL 3.0mL
4
Starch
6.0mL
H
2
O
7.0mL
Est. Time
(s)
Actual time
(s)
3. Complete the above reaction and then calculate your experimental error. Show your work.
PART THREE – THE AFFECTS OF A CATALYST ON A CHEMICAL REACTION
1.
Repeat the conditions of Experiment 1 from Part One, with the exception of adding 2.0 mL of 0.2 M
H
2
SO
4
to Solution A as shown in experiment 7.
DATA TABLE THREE
Exp.
Solution A
KIO
3
H
2
O
Solution B
H
2
SO
4
Na
2
S
2
O
4
Starch H
2
O Time (s)
7 10.0mL 28.0mL 2.0mL 2.0mL 6.0mL 8.0mL
2.
Compare the times of Experiment 1, and Experiment 7 and explain in your own words what would account for the differences in time.
DISCUSSION
1.
Why does reaction rate change as concentration changes?
2.
Explain the procedure used to find the rate law.
3.
How could this lab be done differently to get more precise measurements for our concentrations, and ultimately more precise time measurements (how could you improve the quality of your data)?
4.
Why does reaction rate change as temperature changes?
5.
How would you expect the activation energy to change when a catalyst is added to the reaction?

To calculate the rate for the reaction for all five experiments:
1.
Use the volume of KIO
3
to determine the moles of KIO
3
.
2.
Calculate the molarity of IO
3
-1
using the total volume of solution A and B.
3.
Use the volume of Na
2
S
2
O
5
to determine the moles of Na
2
S
2
O
5
.
4.
Calculate the molarity of S
2
O
5
-2
using the total volume of solution A and B.
5.
The rate = ∆[IO
3
] / ∆ time. So take your experiment 1 [IO
3
-
] and divide by the class average time for experiment 1 to get the rate for that trial. Do this for ALL FIVE trials.
To calculate the order for each reactant:
Do the differential rate law calculations using the MOLARITIES of either IO
3
-1 or S
2
O
5
-2 and the
RATES just calculated above. Find the order for each reactant concentration.
To calculate the value of the rate constant, k:
Plug in two sets of numbers into your rate law expression.
To estimate the time it takes to carry out Part 2 reaction:
Rate = k[IO
3
-
] m
[S
2
O
5
-2
] n
(obviously put what you get for m and n!)
Fill it all in but Rate . For the Rate
, you will put in ∆[IO
3
] / ∆ time using the molarity of IO
3
-
for reaction 6 with “x” for time. Solve for “x” in seconds.
LAB REPORT
One report per group – with the following included (TYPED!!!! Be sure to tell me who did what by initialing those parts of the lab you did.)
1.
Title page
2.
Prelab questions
3.
Data Table One (all class data including your own)
4.
Data Tables Two and Three
5.
Calculations (Part One #7, #8; Part Two #2, #3, Part Three #3)
6.
Discussion questions (one set per person in lab)