Investigating Rate of Reactions (Oxford AQA IGCSE Combined Science Double Award)

Required Practical: Investigating Rate of Reaction

Objective

To investigate how the concentration of a solution affects the rate of a chemical reaction.

Apparatus

• 40 g/dm³ sodium thiosulfate solution, Na₂CO₃ (aq)

• 2.0 mol/dm³ dilute hydrochloric acid, HCl (aq)

• 10 cm³ measuring cylinder

• 100 cm³ measuring cylinder

• 100 cm³ conical flask

• White paper with cross marked on it

• Stopwatch

Method

1. Using the 100 cm³ measuring cylinder, measure 50 cm³ of sodium thiosulfate solution into the conical flask

2. Place the white paper with cross marked on it underneath the conical flask

3. Using the 10 cm³ measuring cylinder, measure 10 cm³ of dilute HCl

4. Add the hydrochloric acid into the conical flask and immediately start the stopwatch

5. Look down at the cross and stop the timer when the cross can no longer be seen

6. Record the time, in seconds, in the results table

7. Repeat steps 1 - 6 using the different concentrations of sodium thiosulfate solution described in the table:
   

   Sodium thiosulfate (cm3)	Water (cm3)	Concentration (g / dm3)
   50	0	40
   40	10	32
   30	20	24
   20	30	16
   10	40	8

8. Repeat each experiment three times

Diagram

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 

Results

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

Evaluation

• Calculate the mean time for each of the thiosulfate concentrations and record it in the final column

  • Do not include anomalous values in your calculations

• Plot a line graph of the results

  • x-axis = sodium thiosulfate concentration in g/dm³

  • y-axis = mean time taken for the cross to disappear in seconds

• Draw a smooth curved line of best fit

• 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

• Other questions:

  • Identify the control variables

  • Were your results repeatable?

  • Is the investigation reproducible?