Measuring the Rate of a Reaction (Oxford AQA International A Level Chemistry)

Required Practical 7: Measuring the Rate of a Reaction

Required practical 7 is split into:

• Part A: Measuring the rate of reaction by an initial rate method

• Part B: Measuring the rate of reaction by a continuous monitoring method

Part A: Measuring the rate of reaction by an initial rate method

Objective

Use an 'iodine clock' experiment to investigate the reaction of iodide(V) ions with hydrogen peroxide in acidic solution and determine the order of the reaction with respect to iodide ions

Apparatus

• 0.25 mol dm^–3 dilute sulfuric acid, H₂SO₄ (aq)

• 0.10 mol dm^–3 potassium iodide solution, KI (aq)

• A burette of 0.05 mol dm^–3 sodium thiosulfate solution, Na₂S₂O₃ (aq)

• 0.10 mol dm^–3 hydrogen peroxide solution, H₂O₂ (aq)

• Starch solution

• 50 cm³ burettes with funnel, stand and clamp

• White tile

• Pipette

• Measuring cylinders

• 100 and 250 cm³ beakers

• Stirrer

• Timer

• Distilled / deionised water

Method

Experiment 1

1. Rinse and fill the burette with potassium iodide solution

2. Place the following in a clean, dry 250cm³ beaker:

   • 25 cm³ of sulfuric acid, using a 50 cm³ measuring cylinder

   • 20 cm³ of distilled / deionised water, using a 25 cm³ measuring cylinder

   • 1 cm³ of starch solution, using the pipette

   • 5.0 cm³ of potassium iodide solution

   • 5.0 cm³ of sodium thiosulfate solution

   • The sodium thiosulfate solution must be added last

3. Stir the mixture in the 250 cm³ beaker

4. Transfer 10.0 cm³ of hydrogen peroxide solution to the 250 cm³ beaker

   • Immediately start the timer

5. Stir the mixture

6. Stop the timer when the mixture in the 250 cm³ beaker turns blue-black

   • This experiment could take several minutes

7. Record the time to an appropriate precision in a suitable results table

8. Empty the 250 cm³ beaker

9. Rinse the 250 cm³ beaker with distilled / deionised water

10. Dry the 250 cm³ beaker

Experiments 2–5

• Repeat steps (2) to (10) in four further experiments using the following volumes:

Diagram

Practical Tip

• Hydrogen peroxide is typically found in 'volume' concentrations, based on the volume of oxygen given of when it decomposes:

2H₂O₂ (aq)  → O₂ (g) +  2H₂O (l)

• For example in school laboratories, a suitable concentration of hydrogen peroxide may be listed as 3% or '10 vol'

  • '10 vol' means that when 1 cm³ of hydrogen peroxide decomposes it generates 10 cm³ of oxygen

  • '10 vol' or 3% hydrogen peroxide has a concentration of 0.979 mol dm³

Results

• Record your results for each test carefully in a suitable table like the one below: 

Evaluation

• Calculate the concentration of KI (aq)

  • 1 dm³ of KI (aq) = 0.1 mol dm^-3

  • 5 cm³ of KI (aq) = 0.1 / 200 = 0.0005 mol dm^-3

  • 10 cm³ of KI (aq) = 0.1 / 100 = 0.0010 mol dm^-3

• Convert the time to rate

  • Rate = 1 / time

• Plot a graph of the results

  • x-axis = concentration of KI (aq) / mol dm^-3

  • y-axis = rate of reaction / s^-1

• Use the graph to deduce the order with respect to iodide ions

Part B: Measuring the rate of reaction by a continuous monitoring method

Objective

Use a continuous monitoring method to investigate the reaction of magnesium and hydrochloric acid

Apparatus

• 6 cm strips of magnesium ribbon

• 0.8 mol dm^–3 hydrochloric acid

• 50 cm³ measuring cylinder

• 100 cm³ conical flask

• Gas collection equipment:

  • Stand, boss and clamp

  • Gas syringe with bung and delivery tube
    OR
    Water trough with inverted 100 cm³ measuring cylinder, bung and delivery tube

• Timer

• Distilled / deionised water

Method

Experiment 1

1. Transfer 50 cm³ of 0.8 mol dm^–3 hydrochloric acid into the conical flask

2. Set up the gas collection equipment (as shown in the diagrams below)

3. Add one 6 cm strip of magnesium ribbon to the conical flask

4. Immediately place the bung firmly into the top of the flask and start the timer

5. Record the volume of hydrogen gas collected every 15 seconds for 2.5 minutes

Experiment 2

1. Mix 25 cm³ of 0.8 mol dm^–3 hydrochloric acid with 25 cm³ of distilled / deionised water

   1. This produces 50 cm³ of 0.4 mol dm^–3 hydrochloric acid

2. Transfer the 50 cm³ of 0.4 mol dm^–3 hydrochloric acid into the conical flask

3. Set up the gas collection equipment (as shown in the diagrams below)

4. Add one 6 cm strip of magnesium ribbon to the conical flask

5. Immediately place the bung firmly into the top of the flask and start the timer

6. Record the volume of hydrogen gas collected every 15 seconds for 2.5 minutes

Diagram

Gas syringe method

Water trough method

Practical Tip

• If the magnesium does not look new and shiny, you may need to clean the surface with a bit of sandpaper

Results

• Record your results for each test carefully in a suitable results table like the one below: 

Evaluation

• Plot a graph of the results

  • x-axis = time / seconds

  • y-axis = volume of gas / cm³

• Add one smooth curve of best fit for the 0.8 mol dm^-3 results

  • Draw the tangent starting at t = 0 s

  • Calculate the gradient of the tangent to get the initial rate of reaction

• Add one smooth curve of best fit for the 0.4 mol dm^-3 results

  • Draw the tangent starting at t = 0 s

  • Calculate the gradient of the tangent to get the initial rate of reaction