Hydrochloric Acid & Sodium Thiosulfate (WJEC GCSE Chemistry: Combined Science)

Introduction

• Sodium thiosulfate reacts with hydrochloric acid to form a solid precipitate of sulfur

• As the solid sulfur precipitate forms, the solution becomes cloudy
• Therefore, the rate at which this precipitate forms can be used as a way to measure the rate of the reaction
• This experiment will look at how changing temperature affects the rate of reaction

Apparatus

• 10cm³ measuring cylinder
• 25cm³ measuring cylinder
• 250cm³ conical flask
• White paper with cross marked on it
• Stopwatch
• 1 mol / dm³ hydrochloric acid
• Thermometer
• 40g / dm³ sodium thiosulfate solution at 5°C
• 40g / dm³ sodium thiosulfate solution in a waterbath at 60°C

Diagram

Observing the sodium thiosulfate reaction

Method

1. Measure 25 cm³ of hot sodium thiosulfate into the 25 cm³ measuring cylinder
2. Transfer the hot sodium thiosulfate into the conical flask
3. Record the temperature of the hot sodium thiosulfate solution
4. Measure out 5cm³ of the hydrochloric acid into the 10 cm³ measuring cylinder
5. Place the conical flask onto the cross
6. Add the hydrochloric acid to the conical flask
7. Swirl the conical flask to mix the contents, at the same time as starting the stopwatch
8. Observe the cross from above the mixture and stop the stopwatch as soon as the cross disappears
9. Record the time taken for the cross to disappear
10. Repeat steps 1 to 9 using different temperatures of sodium thiosulfate solution

Making different temperatures of sodium thiosulfate solution table

Practical Tip

• To avoid the reaction starting before the chemicals are mixed in steps 6 and 7:
  • Make sure that you use the 25 cm³ measuring cylinder for the sodium thiosulfate and the 10 cm³ measuring cylinder for the hydrochloric acid
  • Wash / rinse the conical flask thoroughly between experiments 

Analysis of results 

Results

• You will be required to create a suitable results table for this experiment 

An example results table 

Evaluation

• Plot a line graph of the results, using appropriate scales on each axis
  • The independent variable, temperature, is on the x-axis
  • The dependent variable, the time for the cross to disappear, is on the y-axis

Conclusion

• Describe what the graph shows
  • e.g. as the temperature increases, the time taken for the cross to disappear decreases
• If it is appropriate, give a more detailed description 
  • e.g. as the temperature increases, the time taken for the cross to disappear decreases until 50 ^oC when the time for the cross to disappear remains at 5 seconds
• Use particle theory to explain the description