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Practical chemistry for exams›CP 13b - Use a clock reaction to determine a rate equation

CP 13b - Use a clock reaction to determine a rate equation

An overview of CP 13b - Use a clock reaction to determine a rate equation
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In Edexcel Core practical 13b: Investigating a ‘clock reaction’ (Harcourt–Esson, iodine clock) you will use a clock reaction to determine the rate equation for a chemical reaction between potassium iodide and sodium peroxodisulfate .

This will be achieved by measuring the time taken for a visible colour change to occur (from colourless to blue) and varying the concentrations of reactants.

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In Edexcel core practical 13b: Use a clock reaction to determine a rate equation, normal lab practice such as wearing goggles is adequate when carrying out the practical steps.

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The image illustrates the experimental method for Edexcel Core Practical 13 Part 2. It is divided into five steps along with two tables. Step 1: A diagram shows measuring 10.0 cm³ of potassium iodide solution into a small beaker placed on a white tile. Step 2: Add 5.0 cm³ of sodium thiosulfate solution to the potassium iodide, and add 10 drops of starch solution, which acts as an indicator. A measuring cylinder is shown with liquid being poured into a beaker, followed by an arrow to another beaker. A stopwatch reads 00:00.5 seconds. Step 3: Measure 10.0 cm³ of sodium peroxodisulfate solution and pour it into the beaker. Start the stopwatch immediately. Stop when a blue color appears and record the time. A stopwatch shows 00:00.3 seconds and another shows 00:30.5 seconds. Step 4: Repeat the steps using different volumes of sodium peroxodisulfate and potassium iodide solutions as per the table. Ensure the total volume is 25.0 cm³ by adding distilled water. A table lists mixtures (a) to (l) with volumes for S2O8²⁻, I⁻, S2O3²⁻, and H2O. Step 5: Record all results in a table for analysis. A second table shows mixtures (a) to (l) with concentrations for S2O8²⁻ and I⁻, time in seconds, and 1/t (rate) in s⁻¹.
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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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From the results collected, the table below can be calculated.

A table with five columns and four rows. The columns are labeled: 'Mixture', 'Vol. S2O8^2- (mol dm^-3)', 'Conc. I^- (mol dm^-3)', 'Time (s)', and '1/t (∞rate) (s)'. The table is empty with no data entered in the rows.

After completing the table above, plot a graph of rate against concentration, where concentration of iodide ions has changed. Using the shape of the graph, deduce the order of the reaction with respect to iodide ions.

Another graph can be plotted of rate against concentration, this time using the values where peroxodisulfate ion concentration has been changed. Using the shape of the graph, deduce the order of the reaction with respect to peroxodisulfate ions.

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