Core Practical 13a: Following the rate of the iodine-propanone reaction by a

titrimetric method
Brief

Propanone can be iodinated by a reaction with aqueous iodine, catalysed by the presence of a strong acid. This
forms a solution of colourless iodopropanone and hydrogen iodide as products. The reaction can be expressed as
follows:

CH3COCH3(aq) + I2(aq) → CH3COCH2I(aq) + HI(aq)

The order of the reaction with respect to the concentration of iodine can be determined by creating a mixture of the
reactants, in which the propanone is present in a large excess. A strong acid must also be added in order to catalyse
the reaction and allow it to occur at an observable rate. Samples of the reaction can then be withdrawn at regular
intervals, and quenched by the addition of sodium hydrogencarbonate. This slows the reaction enough to effectively
stop it completely. The quenched sample can then be titrated against sodium thiosulfate using a starch indicator.
The titre is proportional to the moles of iodine, and hence is also proportional to the concentration of iodine in the
sample. If the titre is plotted against the time at which the sample was quenched, a graph is produced that can
indicate the order of reaction with respect to iodine.

Aims
1. To fully risk assess a procedure enabling safe completion of practical work.
2. To carry out a titrimetric analysis of order with respect to one reagent.
3. To process data and interpret quantitative data on reaction rate using data-handling software.

CPAC to be evidenced in planning and writeup

 2d – selects appropriate equipment and measurement strategies in order to ensure suitably accurate results.
 3a – Identifies hazards and assesses risks associated with these hazards, making safety adjustments as
necessary, when carrying out experimental techniques and procedures in the lab or field.
 4b – Obtains accurate, precise and sufficient data for experimental and investigative procedures and records
this methodically, using appropriate units and conventions.
 5a – Use appropriate software and/or tools to process data, carry out research or report findings.
 5b – Sources of information are cited, demonstrating that research has taken place, supporting planning and
conclusions.

Necessary prepared sections:

Other sections not included here will be provided to you on the following pages.

Before experiment

1. Introduction
2. Risk assessment form
3. Equipment list
4. Results table

After experiment

5. Results processing
6. Analysis
7. Bibliography
The following provides some guidance for each section.
Introduction
Use your research to write a brief introduction in which you cover the following points. Ensure you correctly cite all
of your sources of information.
1. Describe what would be observed about the reaction mixture as the reaction progresses.
2. Why is sodium hydrogencarbonate able to “quench” the reaction as described? Write an ionic (with state
symbols) equation for the reaction which occurs when it is added to the reaction mixture.
3. How can you tell when a suitable excess of sodium hydrogencarbonate has been added?
4. Why is a strong alkali, such as sodium hydroxide, not used to quench the mixture? (consider the other
reactions of a methyl ketone such as propanone)
5. Write an equation for the reaction that occurs when the neutralised sample is titrated against sodium
thiosulfate solution.
6. What colour change will be seen over the course of the titration, with the presence of starch indicator?
7. If you are using the method described to determine the order with respect to iodine, why is it important that
the other reagents are present in a very large excess?
8. Sketch the shape of three possible graphs that could be produced when the concentration of iodine is
plotted against the time at which the sample was quenched. Describe what each of these possible graph
shapes would indicate about the order of the reaction with respect to iodine.

Chemicals
 50cm3 of 1.0 moldm-3 aqueous propanone solution.  20cm3 1% starch solution
 50cm3 of 1.0 moldm-3 sulfuric acid.  Sodium hydrogencarbonate
 50cm3 of 0.02 moldm-3 aqueous iodine solution
(in 0.02 moldm-3 KI solution)
 0.01 moldm-3 sodium thiosulfate (VI) solution

Method
1. Mix 25cm3 of 1.0moldm−3 aqueous propanone with 25cm3 of 1.0moldm−3 sulfuric acid in a suitable vessel.
2. Start the stop clock the moment you add 50cm3 of 0.02moldm−3 iodine solution. Swirl the vessel to mix well.
3. Withdraw a 10cm3 sample of the mixture and transfer it to a suitable vessel.
4. Stop the reaction by adding a small measure of sodium hydrogencarbonate. Note the exact time at which
the sodium hydrogencarbonate is added.
5. Titrate the remaining iodine present in the sample with 0.01moldm−3 sodium thiosulfate(VI) solution, using
starch indicator. Record your results in a suitable table.
6. Continue to withdraw 10cm3 samples at suitable time intervals (approximately every 3 minutes) and treat
them similarly. Always note the exact time at which the sodium hydrogencarbonate is added.

Equipment

Use the method provided to help fill out the table below with any equipment required in the experiment. When
specifying the numbers, think about how many will be needed in order to avoid cross contamination. Give a brief
justification of any choices. Extend the table if necessary.

Step(s) Required Number Size/volume Absolute error Justification of chosen option

equipment required (if applicable) (if applicable)
Risk assessment form
Before beginning the procedure, you must complete a risk assessment as part of your research and planning.

Risk assessments are most conveniently made on a standard form. See the example below. The procedure is straightforward
when you realize that the intention is to protect you from any risks.
The steps in making an assessment are:
 Write down the procedures you will be using (chemicals used or made, quantities, concentrations, techniques; any non-
chemical hazards).
 Use reference sources to identify any hazardous chemicals you are planning to use or make. The appropriate warning
symbol should be on reagent bottles and in suppliers’ catalogues.
 Record the nature of the hazards involved and the way you might be exposed to the hazard. There are standard
reference sources with this information such as the ‘Student Safety Sheets’ published by CLEAPSS.
 Decide what protective or control measure to take so that you can carry out your practical work healthily and in safety.
 Find out how to dispose of any hazardous residues from your practical work.
The protective measures you need to take will depend on your laboratory as well as your experiment. Hazards are usually
assessed for procedures in a well-lit, well-ventilated and uncrowded laboratory. Where conditions are different, additional
protective measures may be necessary.
Have your risk assessment approved by your teacher before beginning the procedure.

Title of Experiment: Following the rate of the iodine-propanone reaction by a titrimetric method
Brief outline of key procedures


Hazardous Nature of hazards (e.g. toxic, Quantities and concentrations being Control measures
substances being flammable) used or made (precautions)
used or made

- - EXTEND AS NECESSARY - -
Any non-chemical hazards and precautions to be taken Signed (pupil)
………………………………
Signed (teacher)
………………………………
Disposal of residues Date:
………………………………

Results table
Consider the raw data that you will need to record over the course of the experiment, and construct a fully labelled
table that can contain and suitable organise it. Remember to include relevant units in the headings of the table.

Carrying out your experiment

Carry out your experiment(s) and record your results. You may find that you have to modify your original plan to
respond to practical issues that may arise. Note down any such modifications in full and why they were necessary.
Results processing
The majority of this section is to be completed with the use of a suitable data-processing program.
1. Transfer your raw data into a labelled spreadsheet.
2. Use a suitable formula function to produce a table converting the titre into:
a. Number of moles of sodium thiosulfate (vi) in each titre.
b. Number of moles of iodine in each quenched sample.
c. Concentration of iodine in each quenched sample (moldm-3).
Record the formulae used, and print out a copy of the appropriately headed table of processed data.
3. Use the program to plot a scatter graph of concentration of I2 against time of quenching, remembering to
include appropriate axis labels and units.
4. Print out a copy of the scatter graph and carefully add a line of best fit to your data points.

Analysis
1. Consider the shape of the graph of iodine concentration against time of quenching. What does this tell you
about the order of reaction with respect to iodine, and what are the key features that tell you this?
2. Give a brief explanation as to why the concentration of iodine changed in this way over the course of the
reaction, with reference to the order you have deduced with respect to iodine.
3. Comment on the accuracy of your data. Is there any clear indications of errors? How have these impacted
your conclusions?

Extension opportunities
What changes would need to me made to the method in order to determine the order of reaction with respect to
propanone, instead of with respect to iodine? What problems would need to be overcome to produce a method that
could collect accurate results in such an experiment?

Records
As evidence for the Practical Endorsement, you should have recorded evidence of all your measurements and any
relevant observations in a suitable format. You should have a record of your planning activity, with all sources clearly
cited. Any modifications you made to your original planned procedure should be recorded too. All work should be
clearly dated.

In addition, in preparation for the assessment of practical work in the written examinations and to help you develop
your understanding of the underlying chemical theory, you should complete the questions in the Analysis and
Extension opportunities sections. For calculations, you should show full workings and give final answers to the
appropriate number of significant figures.