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Module 1: Practical chemistry for exams›PAG 10.1 - Rates – iodine clock

PAG 10.1 - Rates – iodine clock

An overview of PAG 10.1 - Rates – iodine clock
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In OCR Chemistry Required Practical 10 (PAG 10.1), the rate of reaction for the iodine-clock reaction is measured using the initial rate method.

Peroxodisulfate(VI) ions, and iodide ions, react together in solution to form sulfate ions, and iodine,

The reaction is carried out in the presence of a fixed amount of aqueous thiosulfate ions, which reduces the iodine back to iodide and prevents the colour change.

When all the ions have been used up, the iodine will react with starch solution, producing a very rapid blue-black colour.

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In OCR PAG 10.1: Rates – Iodine Clock, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

  • Aqueous dipotassium peroxodisulfate may cause respiratory irritation – ensure the laboratory is well ventilated.
  • Dispose of your chemicals as directed by your teacher.
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Experimental method for OCR Chemistry Required Practical 10 (PAG 10.1)

The image is a step-by-step guide for an experimental method labeled 'OCR PAG 10.1'. It includes five steps with illustrations and text. Step 1: Prepare five burettes with solutions: distilled water, 0.040 mol dm⁻³ Na2S2O8 (aq), 1% starch indicator solution, 1.00 mol dm⁻³ KI (aq), and 0.010 mol dm⁻³ Na2S2O3 (aq). Illustrations show burettes labeled KI, Starch, Water, S2O3²⁻, and S2O8²⁻. Step 2: Collect solutions for reactions according to a table. A table labeled 'Volume in cm³ of solution' includes columns for Exp, KI, Water, S2O3²⁻, Starch, and rows with volumes for experiments 1 to 5. Step 3: Collect S2O8²⁻ samples using small beakers to collect five 2.0 cm³ volumes. Step 4: Prepare a results table. The table labeled 'Prepare a results table' shows columns for Exp, KI cm³, Water cm³, [I⁻] in 10 cm³, Time (s), and rows for experiments 1 to 5. Step 5: Tip the S2O8²⁻ solution into one of the reaction beakers from step 1, record the time for the color change to dark blue. Illustrations show a beaker and a stopwatch reading 145 seconds. Repeat the reaction for other mixtures with a final table showing results for experiments 1 to 5, similar to the previous table.
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Clean, dry glassware should always be used in any experiment where concentration is important.

An illustration showing a graduated beaker with a blue stirring rod, a round-bottom flask, and a conical flask.
Do

Ensure glassware is clean and dry before use to prevent dilution or unwanted reactions.

Four laboratory glassware pieces, including two conical flasks, one round-bottom flask, and one beaker, all featuring orange stains.
Don't

Use wet or contaminated glassware – this can affect concentrations and skew results. Rinsing with distilled water directly prior to use is not appropriate here.

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Accurately measuring time is essential in rate investigations.

A round yellow stopwatch displaying the time 00:00:20 on a green digital screen.
Do

Start timing immediately after mixing reagents to ensure an accurate measurement of the reaction rate.

A round yellow stopwatch displaying the word 'ERROR' in large green letters on its screen.
Don't

Stop the timer to take measurements. The reaction hasn’t stopped, so neither should the timer.

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The fixed thiosulfate concentration means the blue-black colour appears after a fixed amount of iodine has formed.

Use initial rate = fixed change in ⁻ ÷ time, then plot initial rate against ⁻ to determine the order.

A line graph showing the relationship between iodine concentration and initial rate. The x-axis is labeled 'Iodine concentration (mol dm⁻³)' and the y-axis is labeled 'Initial rate (mol dm⁻³ s⁻¹)'. The graph consists of a red line with several upward-sloping segments. No specific numeric values or plotted markers are visible on the axes. The line indicates a general increase in initial rate as iodine concentration increases.

Calculate the rate constant using the relationship:

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