USING COLORIMETRY TO DETERMINE A RATE EQUATION
(DATA ANALYSIS ONLY)
Introduction
In this experiment we will be following the acid-catalysed iodination of propanone in order to determine the order of
the reaction with respect to propanone (CH3COCH3), iodine (I2) and the acid catalyst, so that we can determine the
values of x, y and z in the rate equation:
Rate = k[CH3COCH3]x[I2]y[H+]z
The overall equation for the reaction is as follows:
𝐶𝐻3 𝐶𝑂𝐶𝐻3 (𝑎𝑞)
colourless
+
𝐼2 (𝑎𝑞)
(yellow/brown)
𝐻 + 𝑐𝑎𝑡.
→
𝐶𝐻3 𝐶𝑂𝐶𝐻2 𝐼 (𝑎𝑞)
colourless
+
𝐻𝐼 (𝑎𝑞)
colourless
As you can see the reaction mixture will start off yellow/brown and gradually fade to colourless which means that
we can follow the reaction by colorimetry.
Colorimeters work by passing a beam of light of a particular colour through a sample and measuring either the
proportion of the light that passes through it (transmittance) or the proportion that gets absorbed (absorbance); in
this experiment we will use absorbance. A solution of a given concentration of iodine will always have the same
absorbance/transmittance (provided it is in the same container) so this gives us a way to monitor the concentration
without having to measure it with fiddly titrations.
When conducting experiments to determine the rate equation we need
to find a way to measure the ‘initial rate’ of the reaction - i.e. the rate of
the reaction at the very beginning - when we can be certain of the exact
concentration of each of the reactants, before they have had time to
change.
To do this we need to produce a graph showing the first 120 seconds or
so of the reaction so that we can draw a line of best fit. If we draw a
tangent to our line of best fit at time 0, the gradient of this line will give
us the initial rate of the reaction.
In order to know the concentration of iodine that corresponds to a particular absorbance we first need to produce a
calibration curve. We will measure the absorbance of iodine solutions of 10 different concentrations, graph it and
draw a line of best fit. We can work backwards using the line of best fit to turn absorbance back into concentration.
Task
You will be given a spreadsheet called, ‘Iodination of Propanone Data’ and will have to analyse it fully in order to
determine the rate equation. The task uses real world data, collected using the apparatus available in our lab, and so
won’t produce the nice round, clear cut numbers you have seen in examples.
All analytical approaches you use (and the experimental technique used) are things that you may be required to do
for an Extended Essay, or in an Internal Assessment.
Four experiments were completed, and only one run was completed for each one. The initial concentrations used
were:
 Sulphuric acid (aq): 1.00 mol dm-3
 Propanone (aq): 1.00 mol dm-3
 Iodine (aq): 0.0200 mol dm-3
The experiments contained the following proportions of each solution:
Volume (cm3)
Experiment
Sulphuric
Propanone
Iodine
Acid
1
0.75
1.50
0.75
2
0.75
0.75
0.75
3
0.75
0.75
1.50
4
1.50
0.75
0.75
Distilled
Water
0.00
0.75
0.00
0.00
You will be supplied with the data collected to produce the calibration curve, and the data collected by the
datalogger during the four experiments
Constructing the Calibration Curve
The calibration curve was produced by filling cuvettes
with 0.0200 mol dm-3 aqueous iodine solution and
distilled water in the proportions shown below, and
recording the absorbance in a colorimeter using light
with a wavelength of 430 nm (purple). You will need to
complete this section on the ‘CalibrationCurve’ page of
the spreadsheet.
Solution
Number
1
2
3
4
5
6
7
8
9
10
Volume of
0.0200 M iodine
solution
(cm3)
5.00
4.50
4.00
3.50
3.00
2.50
2.00
1.50
1.00
0.50
Volume of
distilled water
(cm3)
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
1. Using the initial concentration of iodine, and the
proportions given above, determine the
concentration of each iodine solution.
2. Construct an appropriate graph with absorbance on
the x-axis and concentration of iodine on the y-axis
 Note: Normally the independent variable
(i.e. concentration) should be on the x-axis, but we want to be able to produce a line of best fit with an
equation in terms of x, such that we calculate a concentration from a given absorbance.
3. Add a line of best-fit to your graph, choosing whichever you think is best, and include the formula and R2 value
for the line.
Calculating the Initial Rates of Reaction
On the ‘Data’ page you will see four empty columns labelled ‘Iodine Concentration’, you will need to use your
calibration curve to calculate the iodine concentration from each point. Once this has been done, we will need to
graph concentration vs time, and use the line produced to determine the rate.
4. Calculate the concentration of iodine for Experiment 1 at t = 0.0 by writing a formula with the same structure as
the equation of your calibration curve, using absorbance as your ‘x’ value.
5. Highlight all the cells in the concentration column (except the header), and press ‘Ctrl+D’ to copy the formula
into all these cells.
6. Highlight all the cells in the concentration column for Experiment 1, Press ‘Ctrl+C’ to copy, then highlight the first
concentration cell in Experiment 2, and press ‘Ctrl+V’ to copy the formulas into these cells. Repeat this for
Experiments 3 and 4.
7. Do the following for each experiment separately:
a. Produce an appropriate graph with time on the x-axis and concentration on the y-axis.
b. Add the most suitable line of best fit, including equation and R2 value.
c. Determine the initial rate and record it in the relevant column of the green table on the ‘Analysis’ page:
i. If your line of best fit is a straight line, this is simply the negative of the gradient
ii. If your graph is curved, you will need to differentiate the equation for the line and solve for x = 0.
Determining the Rate Equation
Now you have all you need to determine the rate equation. This should all be done on the ‘Analysis’ page.
8. Use the volumes of each solution and their initial concentrations to determine the concentration of each
reactant at the beginning of each reaction.
9. Using the reasoning you have seen in lessons, deduce the order of the reaction with respect to each reactant.
Note: the numbers involved do not work perfectly, as is the nature of real-life data, so you will have to use your
judgement in reaching answers.
10. Using your answers to question 9, determine the overall rate equation for the reaction.
11. Using your answer to question 10, calculate the rate constant for each run of the experiment.
Analysis and Evaluation
1. How do the values of the rate constant compare to each other? Are they close enough that you can reasonably
call them constants?
2. Are the shapes of your concentration time graphs consistent with the stated order of the reaction?
3. What improvements do you think could be made to the experiment to improve the overall accuracy and
reliability?
4. Why do you think the experiment uses purple light rather than red light in the colorimeter?