Reaction Kinetics Lab
Introduction
How fast do products form from reactants in a chemical reaction? The rate of a reaction depends
on a number of variables: the nature of the reactants, the temperature, the concentration of the reactants,
and the presence of catalysts.
In this investigation, you will determine the effect of the concentration of a reactant on the rate of
a reaction that follows the sequence of chemical changes described by the following equations:
IO3- (aq) + 3HSO3- (aq)  3HSO4- (aq) + I- (aq)
6H+ (aq) + IO3- (aq) + 5I- (aq)  3I2 (aq) +3H2O (l)
I2 (aq) + starch (aq)  I2 ∙ [starch] (aq)
In this reaction, iodate ion, IO3-, reacts with hydrogen sulfite ion, HSO3- (aq), in acid solution to
give iodide ion, I- (aq). The iodide ion then reacts with iodate ion to give molecular iodine, I 2 (aq).
Molecular iodine reacts instantly with starch in solution to form a complex that is blue-black in color. The
colored product allows you to determine the time required to reach this point in the reaction. Because this
reaction is predictable and easily observed it is often called the “iodine-clock reaction.”
Pre-lab Questions
1. What are the dependent and independent variables in this investigation? What variables, if any,
are held constant?
2. What is the intermediate product in this reaction?
3. How would it affect your results if I2 (aq) reacted with starch at a slower rate than the reactions to
produce I2 (aq)?
4. What is the purpose of the mixing pipette?
5. What are the hazards associated with using NaHSO3 solution, and what precautions should you
take?
Purpose
How does changing the concentration of a reactant affect the rate of chemical reactions?
Materials
Chemical splash goggles
Laboratory apron
Marking pen
Well plate
Sheet of white paper
Micropipette, adapted for mixing
4 micropipettes, containing the following
solutions:
Potassium iodate (KIO3), 0.02 M
Distilled water
Sodium hydrogen sulfite (NaHSO3),
0.02 M
Starch solution
Distilled water
Stopwatch or clock with second hand
Safety
Wear your goggles and lab apron at all times during the investigation. Handle the sodium hydrogen sulfite
in acid solution with care; it is corrosive. Avoid spills and contact with your skin. If you spill any, wash it
off with large amounts of water and tell your teacher.
Procedure
1. Put on your goggles and lab apron. Using the marking pen, label the wells from left to right along
the top of the well plate 1, 2, 3, 4, 5, 6. Down the left side, label the rows of wells A, B, C, D.
Place the well plate on a sheet of white paper.
2. Use the pipette labeled IO3- to add two drops of KIO3 solution to well A1, four drops to well A2,
six drops to well A3, eight drops to well A4, ten drops to well A5, and twelve drops to well A6.
Make sure to hold the pipette vertically to achieve consistent drop size.
3. Use the pipette labeled H2O to add ten drops of distilled water to well A1, eight drops to well A2,
six drops to well A3, four drops to well A4, and two drops to well A5. No water should be added
to well A6.
4.
5.
6.
7.
8.
9.
Use the pipette labeled HSO3- to add twelve drops of hydrogen sulfite solution to wells B1, B2,
B3, B4, B5, and B6. Then use the pipette labeled starch to add one drop of starch solution to each
of the wells in row B. CAUTION: Handle the sodium hydrogen sulfite in acid solution with
extreme care. Do not allow any of the solutions to contact your skin. If you spill any of them,
wash off immediately with plenty of water and notify your teacher.
Use the pipette labeled Mix to draw up all of the hydrogen sulfite solution in well B1. Then, as
rapidly and carefully as possible, transfer the contents of the pipette into well A1. The moment
the two solutions have come into contact, record the start time in the Data Table. Stir the solutions
by gently swirling the well plate on the paper. Once you have mixed the solutions thoroughly, it is
not necessary to continue moving the plate. Record the finish time in seconds when a dark blue
color appears. Then clean the Mix pipette by flushing it three times with distilled water.
Repeat step 5 by mixing solutions A2 and B2, A3 and B3, A4 and B4, A5 and B5, and A6 and B6.
Record the start and finish times in the Data Table. Also record the number of drops of HSO 3(aq) and IO3- (aq) in the combined solutions.
Calculate the elapsed reaction time for each mixture by subtracting the start time from the finish
time. Enter the results in the data table.
If time permits, repeat Steps 2-7 in a second trial.
Wash the chemicals down the sink with plenty of water. Clean up your work area and wash your
hands before leaving the laboratory.
Observations
DATA TABLE
A1/B1
A2/B2
A3/B3
A4/B4
A5/B5
A6/B6
Drops IO3(aq)
Drops HSO3(aq)
Start time
Finish time
Elapsed time
(s)
Analysis and Conclusions Questions
1. Make a graph of the elapsed time versus the quantity of iodate ion.
2. How does an increase in reactant concentration affect time?
3. How does an increase in reactant concentration affect the rate of reaction?
4. How is the reaction rate related to reaction time?
Applications Questions
1. Suppose you held the iodate ion concentration constant while varying the concentration of hydrogen
sulfite ion. How would you expect the reaction rate to depend on the concentration of hydrogen sulfite ion?
2. What effect do you think changing the temperature would have on your results?
3. Design an experiment to find an answer to the prediction you made in Question 2.