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Module 1: Practical chemistry for exams›PAG 10.2 - Rates – thiosulfate and acid

PAG 10.2 - Rates – thiosulfate and acid

An overview of PAG 10.2 - Rates – thiosulfate and acid

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In OCR Chemistry Required Practical 10 (PAG 10.2), rate of reaction is measured using the initial rate method. The reaction between aqueous thiosulfate ions, and aqueous acid, is often called the disappearing cross reaction.

The rate of this reaction can be followed by observing the time taken for precipitation of sulfur. We can vary the concentration of both reactants to investigate how they affect this.

In OCR PAG 10.2: Rates – Thiosulfate and acid, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

Dispose of chemicals as directed by your teacher.
Sulfur dioxide is produced and can cause respiratory issues – ensure the laboratory is well ventilated.

Experimental method for OCR Chemistry Required Practical 10 – Part 1 (PAG 10.2) – The effect of changing the concentration of thiosulfate ions.

The image shows an experimental method labeled 'OCR PAG 10.2 PART 1' with a step-by-step procedure. Step 1: Three burettes are shown, labeled '0.150 mol dm⁻³ S₂O₃²⁻ (aq)', 'Distilled H₂O', and '2.0 mol dm⁻³ HCl (aq)'. Instructions: 'Prepare three burettes for the reaction. Fill up each burette to the 0.00 cm³ mark with the stated solutions. Measuring cylinders can be used as a less precise method.' Step 2: Five reaction beakers are labeled 1 to 5. A table shows 'Volume S₂O₃²⁻ (cm³)' and 'Volume H₂O (cm³)' for each beaker, with values: 1 (50, 0), 2 (40, 10), 3 (30, 20), 4 (20, 30), and 5 (10, 40). Note: 'Do not yet add any HCl to the beakers.' Step 3: Five test tubes are shown. Instructions: 'Using the hydrochloric acid burette, dispense 5.0 cm³ of the acid into five test tubes.' Step 4: A beaker is placed over a cross on paper. Instructions: 'Draw a cross in marker pen on a piece of white paper. Place the beaker over the cross.' Step 5: A results table shows 'Exp', 'S₂O₃²⁻ (cm³)', 'Water (cm³)', 'HCl (cm³)', and 'Time (s)' with values for experiments 1 to 5. Step 6: Instructions: 'Tip the HCl into the beaker to begin the reaction. Swirl to mix, and start the stop-clock immediately. Observe until the cross disappears and stop the clock.' A stopwatch shows '027 s'. Step 7: Instructions: 'Repeat for the other concentrations. Record the times for each experiment.' A duplicate results table is shown. © Medify.

Experimental method for OCR Chemistry Required Practical 10 – Part 1 (PAG 10.2) – The effect of changing the concentration of ions.

The image is titled 'Experimental Method OCR PAG 10.2 Part 2'. It is divided into two sections labeled 1 and 2. Section 1 is titled 'Prepare reaction beakers and test tubes' and contains instructions: 'Prepare beakers and test tubes as before, but this time: Keep the volume of thiosulfate solution constant at 10 cm³ in each test tube. Make up the different acid concentrations in the beakers with mixtures of HCl and water from the burettes. Keep the total volume the same for each reaction as in experiment 1.' Next to the text is a table with columns labeled 'Exp', 'S₂O₃²⁻(aq) (cm³)', 'Water (cm³)', 'HCl (cm³)', and 'Time (s)'. The rows are: 1 with values 10.0, 40.0, 5.00; 2 with values 10.0, 35.0, 10.00; 3 with values 10.0, 30.0, 15.00; 4 with values 10.0, 25.0, 20.00. Section 2 is titled 'Carry out the reactions' and contains instructions: 'Perform each experiment as before with the cross on the paper. Record the time taken for the cross to disappear.' There is an illustration showing a beaker labeled '1' with a cross beneath it, a stopwatch reading '000 s', and a second stopwatch reading '027 s'.

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.

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.

Calculate the initial rate for each experiment using:

Plot two graphs of initial rate against and and use the shape of each graph to determine the reaction order with respect to each reactant.

A graph with the x-axis labeled '[S2O3^2-] (mol dm^-3)' and the y-axis labeled 'Initial rate 1/t (s^-1)'. The graph shows a red line with a positive slope, indicating an increase. There are no visible numeric axis values or plotted markers on the graph. The line starts near the origin and moves upwards to the right.

Use these orders to propose the rate equation.

For the graph, calculate the gradient of the best-fit line to obtain a value proportional to the rate constant.