Name:_____________________________________________________ Date: ________
Background
One of the most important aspects of any chemical reaction is the rate of that reaction. Rate can be though of as the time it takes for reactants to change over a period of time. Subsequently, average rate can be found by:
Rate average = -Δ[
Reactant
] =
Δ [Product]
Δ time Δ time
The rate of a reaction is dependent on temperature and concentration of reactants. Rates can also be manipulated with the aide of a catalyst, since the role of a catalyst is to speed up the reaction. In this lab, you will investigate an iodine clock reaction. The reaction proceeds according to the following reactions:
Reaction 1 IO
3¯
+ 3 HSO
3
¯ → I¯ + 3 SO
42
¯ + 3 H +
Reaction 2
Reaction 3
Reaction 4
6 H
H
2
2 I
O + I
3
+ + IO
3
3¯
+ 8 I¯ → 3 I
¯ + HSO
3
¯ → 3 I¯ + SO
¯ + Starch → Starch-I
3
5
¯ + 3 H
2
42
O
¯ + 3 H +
¯ complex (blue) + I¯
When all of the sodium metabisulfite is consumed by the reaction, the starch-I
5
¯ complex forms, turning the solution a dark blue. You will determine the rate of the reaction by measuring the amount of time it takes for the formation of the dark blue color.
You will also determine how the concentrations of potassium iodate and sodium metabisulfite effect the rate of the reactions above.
Prelab Questions
1) Calculate the concentrations of each reactant listed in Table 3.
Record this information in your laboratory notebook.

Materials
0.10 M KIO
3
0.20 M Na
2
S
2
O
5
24 well plate starch solution
4 beral pipettes toothpicks stopwatch
Procedure
1. Using one wells of the 24 well plate, mix the potassium iodate, starch solution, and water in the proportions indicated on Data Table 1 for
Mixture 1.
2. Noting the time on your stopwatch, add the correct number of sodium metabisulfite drops to the first well. MAKE SURE YOU ADD THE
Na
2
S
2
O
5
LAST. Stir to thoroughly mix the reagents.
3. Continue stirring until the dark blue color appears. A piece of white paper placed beneath the plate is helpful. Record the time it takes for the color to appear.
4. Repeat steps 1-3 for Mixture 1 two more times. Record the time for each trial in Table 2.
5. Repeat steps 1-4 for Mixtures 2, 3, 4, and 5.
Analysis
Please show work, when appropriate, in your laboratory report.
1) Calculate the average time it took for the reaction to go to completion for each mixture tested. Record your answers in Table 2.
2) Approximate the average rate of reaction by dividing the change in concentration of sodium metabisulfite in the mixture by the time obtained in this experiment. Record your answers in Table 3. There is an assumption that the Na
2 5
concentration goes to zero. S
2
O
Therefore,
Rate ave =- - Δ[
Na
2
S
2
O
5 ]
Δ time
3) Calculate the order of each reactant.

4) Write the rate law for the reaction.
5) What was the overall order of the reaction?
6) Calculate the rate law constant, k, for each trial. Record your answers in Table 3. Remember to include appropriate units.
7) Calculate the initial rate of a reaction that would occur if the following mixture was tested in this experiment.
Mixture
Number
6
0.10 M
KIO
(Drops)
25
3
H
2
O
(Drops)
8
Starch
(Drops)
5
0.20 M
Na
2
S
2
O
5
(Drops)
2
Conclusions
1.
Was the rate dependent on the concentration of potassium iodate?
Use evidence from your experiment to support your answer.
2. Was the rate dependent on the concentration of sodium metabisulfite? Use evidence from your experiment to support your answer.
3. What would happen to the rate if 1.00 M KIO
3
were used in the experiment in place of 0.10 KIO
3
?
4. What would happen to the rate of the reaction if 4 M Na
2
S
2
O
5
was used in the experiment?
5. Why was it necessary to keep the number of total drops used in each mixture the same for each trial?
6. Why does reaction rate change as temperature changes?

Table 1. Experimental Design
Mixture
Number
0.10 M
KIO
3
(Drops)
H
2
O
(Drops)
Starch
(Drops)
1 10 23 5
0.20 M
Na
2
S
2
O
5
(Drops)
2
2
3
5
15
28
18
5
5
2
2
4
5
10
10
21
19
5
5
4
6
Table 2. Experimental Data – Reaction Times
Mixture
Number
Trial 1
Time
(seconds)
Trial 2
Time
(seconds)
Trial 3
Time
(seconds)
1
2
3
4
5
Average
Time
(seconds)

Data Table 3. Concentrations and Rates of Reactions
Mixture
Number
[KIO
3
] [Na
2
S
2
O
5
] Rate k
2
1
4
3
5