A Study of Reaction Rates
We shall study the effects of concentration and temperature upon the average rate of a chemical reaction. The
reaction to be used is one of a group known as "clock- reactions'. Present in the reaction mixture are iodate ions
(I03-); iodide ions (I-); hydrogensulfite ions (HSO3-); hydrogen ions (H+ or H30+), starch and water.
The initial reaction is: IO3- + 3 HSO3- = I- + 3 SO4 2- + 3 H+ All are aqueous, (aq), dissolved in water.
This reaction of the iodate ions with the hydrogen sulfite ions is the one whose rate we will study. What makes
this a "clock reaction" is that a second reaction can occur with the iodate. When all the hydrogensulfite ions are
used up, then the iodate can react with the iodide ions: IO3- + 5 I- + 6 H+ = 3 H2O + 3 I2.
The product of this second, very fast reaction is iodine molecules, I2, which react with the starch in the mixture
to produce an intense deep blue color. This blue color appears only when the first reaction has gone to
completion by using up all the hydrogensulfite ions.
Part 1. The Effect of Concentration Changes on Reaction Rate
We will vary the concentration of the iodate ion, keeping all other concentrations and factors constant. We can
do this because “Solution A” is just 0.0200 M KIO3. By using a smaller sample of Solution A and adding
deionized water to keep the same total volume, the final concentrations of all other materials, supplied by a
constant volume of 'Solution B", will stay constant. [Solution B contains, per liter, 0.20 g Na2S205, sodium
meta-bisulfite, 4.0 g of soluble starch, and approximately 5 cm3 of 1 M H2SO4. The meta-bisulfite ion reacts
with the water and acid to produce the hydrogensulfite ions used in the reaction. The amount of acid is changed
to control the time required for the reaction.]
Different lab groups will prepare several different dilutions of Solution A, and determine the time required for
the reaction to use all the (constant amount of) HSO3- ion. Class data will be collected and used to determine the
effect of the concentration of iodate ion on the reaction rate.
Procedure
A. a) Use a clean pipet to measure 10.0 cm3 of solution A into a clean dry test tube.
b) Use a different clean pipet to measure 10.0 cm3 of solution B into a second clean dry test tube.
c) While watching a clock- with a second hand, pour solutions A and B together into a clean dry small
container. Stir or swirl them quickly together.
d) Record the time in seconds from the instant the two solutions touch until the first appearance of the blue
color.
B. Prepare different dilutions of Solution A, as assigned by the teacher for your group. Each dilution is prepared
by pipetting 9.0 or 8.0 or 7.0. etc, cm3 of Solution A into a clean dry test tube, and then pipetting in 1.0 or 2.0 or
3.0. etc. cm3 of deionized water; the total volume must stay the same as before, 10.0 cm3. This diluted solution
is then mixed, as before, with 10.0 cm3 of Solution B, and the reaction timed. Record the time for each of your
reactions, and add your data to the class set. Caution! It is vital that NO contamination of the large class
supplies of Solutions A and B occur. NOTHING is to be put into the containers! No pipets, no medicine
droppers, no left-over solutions- NOTHING! Nor are the tops to be put back on the wrong containers! You may
pour a small amount, less than 60 cm3, of each solution into a small clean, dry container to take back to your
lab bench. You may then pipet from this smaller container. Label the containers, and also the pipets so they
don't get mixed up; you will need three-for Solution A, Solution B, and deionized water. CLEANLINESS is
important. There are a number of common substances that can CATALYZE these reactions, and cause a drastic
change in the rate from what is expected. If you get a near-instantaneous reaction when A and B are mixedthrow everything out and CLEAN!
Calculations and Questions for Part 1 1. The concentration of K103 in Solution A is 0.0200 M. Calculate the
initial concentration of IO3- in each of the A + B mixtures done by the class.
2. Why is it important to keep the total volume of A + water + B at 20.0 cm3?
3. Plot a graph of concentration of IO3- vs. time required for the reaction. Use graph paper. Use all the class
data. Circle or otherwise indicate which data points were from your group. Show time on the vertical axis, from
zero seconds up to the maximum needed. Horizontal axis should show the concentration of IO3- ion in the final
A + B + water mixture, from zero molar up to the maximum used. Adjust the scales on your axes so that most of
the graph paper is used. Draw the best smooth curve you can through (or close to) the plotted points.
4. What generalizations can you make about the effect of varying concentration on the time required for a
reaction?
5. You measured the time required for the reaction of a constant amount of hydrogensulfite ion. How is this
time related to the rate of the chemical reaction, measured in, say, moles per second?
6. What generalization can you make about the effect of concentration on the rate of a reaction? Does your data
justify a quantitative statement about the relation between IO3- ion concentration and the rate of this reaction?
That is, can you specify the order of the reaction for IO3-? If so, what is the order?
Part 2. The Effect of Temperature on Reaction Time
We will repeat the reaction of Part 1, but at temperatures other than room temperature. Each lab group will be
responsible for acquiring data at two different temperatures, over a range from about 5C to 35C.
Procedure
a. Sign your group up for two temperatures from the allowable range.
b. Fill a medium beaker about 1/2 full of tap water. Put in a thermometer. Adjust the water temperature to the
first of the two temperatures you signed up for. Use cold tap water and/or ice cubes for low temps, hot water or
heat over your burner for high temp. A temperature within 2of the assigned one is acceptable: record the
actual temperature.
c. Dilute 15.0 cm3 of stock Solution A to one-half its original concentration with deionized water; i.e., mix it
with 15.0 cm3 of water to make 30.0 cm3 of diluted solution.
d. Pipet 10.0 cm3 of the diluted Solution A into a clean dry test tube. Pipet (with a different pipet) 10.0 cm3 of
Solution B into a different dry clean test tube.
e. Put both test tubes into your beaker partly filled with water of the temperature you are to use. Keep the test
tubes immersed for several minutes, monitoring the temperature in the beaker with a thermometer. (Don't put
the thermometer in the actual Solution A or B -it may contaminate it and you'll lose some solution that sticks to
the thermometer.)
f. When you feel that both solutions are the right temperature, mix them quickly, as in Part 1, while watching
the clock. Record the temperature and the time required for the reaction. It will help if, after you have mixed A
and B, the container with the mixture is put back into the beaker of water, to keep the mixture at the same
temperature.
g. Adjust the water in the beaker to the next of your temperatures, and repeat the reaction at the new
temperature. Don't forget to rinse your test tubes and reaction container between uses.
h. If time permits, repeat the experiments at each temperature.
i. Record all your time-temperature data on the class data sheet.
Calculations & Questions for Part 2
1. Plot a graph of the whole class time-temp. data. Indicate on the graph which points are your data. Show
temperature from OC to 5OC on the horizontal axis, time from zero seconds to the maximum necessary, on
the vertical axis. Draw a smooth curve through (near) the points.
2. What generalization does the graph give about time for a reaction vs. temperature?
3. What seems to be the relation between reaction temperature and reaction rate?
4. Based upon your graph, predict the time required for the reaction to occur at OC? At 5OC?
Write-up: Teacher-initialled prepsheet. Class data, with your individual data indicated. Two graphs, on graph
paper (one on each side of one sheet is OK), again with your data indicated. Answers to questions.