10.1

Question 1

10.1 Rates – Iodine Cloc•k To use appropriate measurements to monitor the rate of reaction using an initial rate method.
• To use appropriate software to process data.

Answer 1

Peroxodisulfate(VI) ions, S2O82–, and iodide ions, I–, react together in solution to form sulfate ions, SO42–, and iodine, I2.
S2O82–(aq) + 2I–(aq)  2SO42–(aq) + I2(aq)
The reaction can be carried out in the presence of a fixed amount of aqueous thiosulfate ions, S2O32–(aq), which reduces the iodine back to iodide.
2S2O32–(aq) + I2(aq)  S4O62–(aq) + 2I–(aq)
When all the S¬2O32– ions have been used up, the iodine will react with starch solution, producing a blue-black colour.

You will time how long it takes from the start of each experiment for the blue colour to appear. The initial rate of disappearance of I–(aq) can be determined from this time.

You are going to plan and carry out an experiment to find the order of this reaction with respect to I–(aq) ions and also to find the rate constant for the reaction.

Question 2

Aqueous potassium iodide, KI(aq), 1.00 mol dm–3

Answer 2

Currently no hazard classification at this concentration

Question 3

Aqueous dipotassium peroxodisulfate(VI), K2S2O8(aq), 0.0400 mol dm–3

Answer 3

DANGER
May cause respiratory irritation
May cause allergy or asthma symptoms or breathing difficulties if inhaled.

Question 4

Aqueous sodium thiosulfate, Na2S2O3(aq), 0.0100 mol dm–3

Answer 4

Currently no hazard classification at this concentration

Question 5

Procedure

Answer 5

1. Measure out 5.00 cm3 KI(aq), 2.00 cm3 S2O32–(aq) and 1.00 cm3 starch solution into a suitable container. Stir the solution with a glass rod to mix the contents.
2. Measure out 2.00 cm3 of K2S2O8(aq).
   • Add the 2.0 cm3 of K2S2O8(aq) to the mixture containing KI(aq), 2.00 cm3 S2O32–(aq) and 1.00 cm3 starch and immediately start the stop clock.
   • Mix the mixture thoroughly with a glass rod.
   • Stop the clock when the blue-back colour is just visible.
   • Record the time, t, and the temperature.
3. Carry out further experiments in the same way but varying the concentration of KI(aq).
   The total volume used in each experiment must be the same.
   For each experiment,
   • the initial rate of reaction is equal to 2  10–3t mol dm–3 s–1
   • [I–(aq)] = volume of KI (in cm3)10 mol dm–3
   • Your results will need to include the initial rate and [I–(aq)] for each experiment

Question 6

Analysis of your results

Answer 6

1. Enter your data into a spreadsheet and use the software to plot a graph that you can use to find the relationship between the rate of reaction and the concentration of iodide ions. Alternatively, plot the graph on graph paper.
   If using a spreadsheet, you could input just your raw experimental results of volume KI and time t and set up columns using formulae that automatically calculate the rate and concentration of iodide ions for you.
2. Use the graph to determine the order of the reaction with respect to I–(aq) ions.
3. Determine the gradient of your graph.
4. The rate equation for this reaction is shown below.
   rate = k [I–(aq)] [S2O82–(aq)]
   • Work out the concentration of S2O82–(aq) that you used in each of your experiments.
   • Rearrange the rate equation so that you can find the rate constant, with units, for the reaction. You will need both the concentration of S2O82–(aq) ions and the gradient of your graph.

Question 7

1. Look back at the procedure and introduction to the task.
   (a) The initial rate of reaction is shown as 2x10–3/t mol dm–3 s–1
   Show that this value is the rate of disappearance of I– ions in each experiment.

Answer 7

For each experiment, 2.00 cm3 of 0.0100 mol dm–3 Na2S2O32–(aq) has been used.
a. n(S2O32–(aq)) used = 2.00  10–5 mol.
This is equivalent to 2.00  10–5 mol I– being used up.
Total volume is 10 cm3 so [I–(aq) being used up is 2.00  10–3 mol and rate = 2x10–3/t

Question 8

(b) The concentration of iodide ions, [I–(aq)], is shown as volume of KI (in cm3)10 mol dm–3
Show that this value is correct.

Answer 8

a. For each experiment, a different volume of 1.00 mol dm–3 KI(aq) has been used. This is diluted to a final volume of 10.
Therefore [I–(aq)] = volume of KI (in cm3/)10 mol dm–3

Question 9

2. What evidence is there that the reaction takes place in more than one step?

Answer 9

2. The rate equation includes I–(aq) and S2O82–(aq) in the ratio 1 : 1. The stoichiometry from the overall equation shows I–(aq) and S2O82–(aq) in the ratio 2 : 1. So another step must be involved to account for the other I– ion.

Question 10

3. Propose a possible two or three step mechanism for this reaction assuming that the first step is the rate-determining step.

Answer 10

2. Various mechanisms can be proposed provided that each step adds up to give the overall equation and the first step uses I–(aq) and S2O82–(aq) in the ratio 1 : 1. A suggested mechanism is shown below (although the intermediate is unlikely)

Step 1 S2O82–(aq) + I–(aq)  SO42–(aq) + ISO4–(aq)
Step 2 ISO4–(aq) + I–(aq)  SO42–(aq)
Overall S2O82–(aq) + 2I–(aq)  SO42–(aq) + I2(aq)

Question 11

Equipment

Answer 11

• Boiling tubes
• Test tubes
• Beakers (assorted sizes)
• Measuring cylinders (assorted sizes)
• Glass rod
• Wash bottle containing distilled or de-ionised water 

• Burettes or graduated pipettes (1 cm3, 2 cm3 and 5 cm3)
• –10 – 110 oC thermometer
• Stop watch or stop clock