Physical Chemistry Practicals (AQA A-Level Chemistry): Revision Notes

4.1.4 Required Practical 3 - Effect of Temperature on Reaction Rate

Aim

To investigate how changing the temperature affects the rate of reaction between sodium thiosulfate and hydrochloric acid by observing the time taken for a cross to disappear under a reaction mixture as a precipitate of sulphur forms.

Equipment

Chemicals

• 1 mol dm$⁻³$ hydrochloric acid ($HCl$) (about 10 cm³ per trial)
• 0.05 mol dm$⁻³$ sodium thiosulfate ($Na₂S₂O₃$) (10.0 cm³ per trial)
• Sodium carbonate solution (for neutralising acid spills)

Apparatus

• Plastic container for thermal insulation
• Measuring cylinders (10 cm³)
• Thermometer (0.1°C precision)
• Stopwatch
• White tile or paper with a black cross
• Hot water supply (kettle or hot tap)
• Cold water supply (for adjustments)
• Safety lid (optional, with holes for tubes)

Risk Assessment

• Hydrochloric acid is an irritant:
  • Wear safety goggles, gloves, and a lab coat.
• Sulfur dioxide ($SO₂$), released during the reaction, is toxic:
  • Perform the experiment in a well-ventilated area or fume cupboard.
  • Use a lid with holes to minimise $SO₂$ escape.
• Dispose of waste solutions using appropriate neutralization with sodium carbonate.

Method

1. Prepare the setup:

• Use a plastic container with a black cross marked on white paper underneath.
• Place about 10 cm³ of 1 mol dm$⁻³$ $HCl$ in a glass tube.
• Do not place it over the cross.

2. Sodium thiosulfate preparation:

• Measure 10.0 cm³ of 0.05 mol dm$⁻³$ sodium thiosulfate in another tube.
• Place this tube over the black cross.
• Insert a thermometer into the sodium thiosulfate solution.

3. Recording initial temperature:

• Measure and record the starting temperature.
• Add 1 cm³ of $HCl$ to the sodium thiosulfate solution, immediately start the stopwatch, and observe from above.

4. Timing:

• Record the time taken for the cross to disappear due to sulphur precipitate formation.
• Note the final temperature of the solution.

5. Repeat at different temperatures:

• Adjust the water temperature in the plastic container using a hot water supply.
• Allow the tube with sodium thiosulfate to acclimate to the new temperature for about 3 minutes.
• Repeat steps 3 and 4 for at least five different temperatures.

6. Dispose of reaction mixture:

• Use a sodium carbonate solution to neutralise any remaining acid and minimise environmental hazards.

Results

• Record temperature readings and the time taken for the cross to disappear for each trial.
• Plot a graph of temperature vs. time to observe trends.

Errors and Improvements

• Errors:
  • Heat loss to surroundings can affect temperature control.
  • Measurement errors due to subjective judgement of when the cross disappears.
  • The assumption is that the density and specific heat capacity of solutions are equivalent to water.
• Improvements:
  • Use a thermometer with higher precision or an electronic temperature sensor for accuracy.
  • Ensure temperature uniformity by stirring gently but consistently.
  • Conduct in a temperature-controlled environment to reduce heat exchange.
  • Employ a light sensor to objectively determine when the solution reaches the endpoint.