Kinetics
INTRODUCTION TO REACTION RATES: THE REACTION OF SUGAR AND YEAST
Goals:
During the experiment, students will:
1. define the term reaction rate;
2. use Logger Pro and a dissolved oxygen probe to measure and graph the dissolved oxygen concentrations during
a chemical reaction between sugar and yeast; and
3. determine the average reaction rate of a chemical reaction.
Prelab:
1. Why does water naturally contain dissolved oxygen?
2.
What factors affect the concentration of oxygen dissolved in water or water solutions?
3.
Think back to your Biology days...what is the reaction that occurs when yeast reacts with sucrose (table sugar)?
4.
Predict what will happen to the concentration of dissolved oxygen in the sugar water solution after the yeast is
added.
Safety: Always wear safety goggles on the MCL.
Procedure: Run 1:
Note: See “Helpful Hints…” beginning on page 4
1. Pour 150 mL of distilled water into a 250 mL Erlenmeyer flask using the volume marks on the flask to estimate
the water's volume. Carefully drop a stir bar into the flask of water. Place the flask on the hot plate and warm
the water to between 40-45°C using the 4 or 5 setting on the hot plate.
2.
While the water is warming, weigh 3.0 grams of sugar. Add the sugar to the warm water and turn on the stir
mechanism to help dissolve the sugar. Record the measured mass of sugar used under Run 1 in the data table.
3.
Weigh 2.0 grams of yeast and set aside for later use. Record the measured mass of yeast used under Run 1 in
the data table.
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Reaction of Sugar and Yeast
Kinetics
4.
When the sugar has dissolved, reduce the stirring motion to a speed of 3 and move the flask so that the stir bar is
off to the side of the bottom of the flask rather than in the center. When the temperature of the solution has
reached approximately 40 °C turn off the heat. Record the measured temperature under Run 1 in the data table.
Data Table
Run 2
Run 1
Run 3
Run 4
Volume H2O (mL)
Mass Sugar (g)
Mass Yeast (g)
Water Temp (°C)
Start Time (sec)
Start [O2] (mg/mL)
End Time (sec)
End [O2] (mg/mL)
Remove the dissolved oxygen probe from the temporary storage flask and place it in the 250 mL Erlenmeyer
flask that contains the sugar water. Start measuring dissolved oxygen (DO) concentrations by clicking the
Collect button on the computer screen.
6.
Collect DO concentrations for 200 seconds. After 200 seconds quickly and carefully remove the DO probe and
add the yeast to the sugar water. Replace the DO probe in the solution and carefully stir the yeast and solution
with the probe for just a moment.
7.
Allow the computer to continue to collect DO concentrations until the 400 seconds are up.
8.
Sketch the resulting graph to the right. Label
the parts of the graph that represent the time
period before the yeast was added, when the
yeast was added, and after the yeast was added.
9.
In the data table, record the dissolved oxygen
concentration and the time at both the start and
the end of the reaction between the sugar and
yeast. (The column for Run 1 should now be
completely filled in).
Dissolved Oxygen
Concentration (mg/L)
5.
Time (sec)
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Reaction of Sugar and Yeast
Kinetics
Additional Runs:
1. Read back over the procedure for Run 1 and think about how the reaction rate might be changed by altering one
the variables of the experiment.
Discuss the proposed change with your instructor. How you think the reaction rate will change because of the
variable change?
3.
Repeat the procedure for Run 1 but alter the experiment to include the variable change discussed with the
instructor. Fill in the Run 2 column of the data table as you proceed.
4.
Sketch the graph that resulted from your procedure change in Run 2.
Dissolved Oxygen
Concentration (mg/L)
2.
Time (sec)
5.
As time permits, make additional runs changing only one parameter at a time. Record your data in the data
table and sketch your graphs below.
Run 4 Sketch
Dissolved Oxygen
Concentration (mg/L)
Dissolved Oxygen
Concentration (mg/L)
Run 3 Sketch
Time (sec)
Time (sec)
Clean-up: The MCL staff will clean up.
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Reaction of Sugar and Yeast
Kinetics
Postlab:
1. Chemists define reaction rates as how quickly reactants are used up or how fast products of a reaction are
formed. Common notations used for this are -∆[reactant]/∆time or ∆[product]/∆time. Use your data to
determine the average reaction rate for the reaction between the sugar and yeast for each of your runs.
2.
How do the rates compare with the original reaction rate? Explain the differences.
Helpful Hints for Sugar-Yeast Reaction
Weighing: Place the boat on the balance: it will register a weight. Press the “ON” button, and the balance will read
zero. You have “tared” the balance, so that you will only measure the weight of the sugar or the yeast. (“Taring”
subtracts the weight of the boat out of the balance). The weight of the sugar should be ~3 grams: (it does not have to
be exactly 3.000 grams). Record the exact weight that you get. For example, if you end up with 2.934 grams, record
that number. Weigh out the yeast on a piece of weighing paper, which has been folded once to form a crease. The
crease greatly facilitates the pouring of the weighed yeast into the flask.
Water Temperature: Turn the hot plate heat dial to “3”. When the temperature of the water rises to ~ 35oC, turn
the hot plate heat dial to “OFF”. The final temperature of the water should not rise beyond 40oC. (There is residual
heat in the hot plate: keep an eye on the temperature). A temperature of >45oC will kill the yeast, which will result
in no reaction.
Balances: BE CAREFUL WITH THE SUGAR!! Use your scoopula. Do NOT get sugar on the balance. It is very
sticky and can adversely affect the balances’ accuracy.
The DO probe is in a storage Erlenmeyer flask. When the temperature of the water in the experimental flask is
between 30oC and 35oC, pour in the weighed sugar and allow it to completely dissolve. Then, put the probe in that
flask and click on “Collect”. Allow the program to collect and record data for 200 seconds, at which time, remove
the probe and quickly pour in the weighed yeast. Immediately replace the probe and let the program collect and
record data for another 200 seconds after which, the program will stop.
Data: Highlight the graph window, click on “data”, and then “store latest run”. The graph will appear faded. A
new table window will open up on the right-hand side of the screen. Double click on “concentration” under “latest
run”. Change to a different color so you can keep your runs separate.
Run 2: Repeat all the steps from run 1, except this time change one variable, such as weight of sugar or yeast. You
can also change temperature, but it must be kept under 40oC. Of course you can lower it, but to no less than ~20oC,
or the reaction will proceed too slowly. You can do additional runs if time permits.
Printing: Highlight the graph window. This is very important! Click on “Print”: a box will appear that allows you to
enter your names and the experiment description. Click “okay”: another box will appear that will allow you to make
a copy for each member of your team. Enter the appropriate number and then click “okay”. The printouts will appear
at the printer in the gooseneck.
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Reaction of Sugar and Yeast