Investigating Rate of Reactions (Oxford AQA IGCSE Combined Science Double Award)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
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
Using the 100 cm3 measuring cylinder, measure 50 cm3 of sodium thiosulfate solution into the conical flask
Place the white paper with cross marked on it underneath the conical flask
Using the 10 cm3 measuring cylinder, measure 10 cm3 of dilute HCl
Add the hydrochloric acid into the conical flask and immediately start the stopwatch
Look down at the cross and stop the timer when the cross can no longer be seen
Record the time, in seconds, in the results table
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
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:
Sodium | Time taken for the cross to disappear (seconds) | |||
|---|---|---|---|---|
Experiment 1 | Experiment 2 | Experiment 3 | Mean | |
8 |
|
|
|
|
16 |
|
|
|
|
24 |
|
|
|
|
32 |
|
|
|
|
40 |
|
|
|
|
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?
Worked Example
A student studied the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid. They obtained the following results.
Sodium | Time taken for the cross to disappear (seconds) | |||
Experiment 1 | Experiment 2 | Experiment 3 | Mean | |
8 | 93 | 95 | 94 |
|
16 | 53 | 43 | 53 |
|
24 | 28 | 30 | 31 |
|
32 | 15 | 14 | 13 |
|
40 | 7 | 7 | 6 |
|
a) Calculate the mean time taken for the cross to disappear.
b) Plot a graph of concentration of thiosulfate against the mean time taken for the cross to disappear.
c) State what conclusion can be drawn about how concentration affects the rate of this reaction. Explain your conclusion using particle theory.
Answers:
a) The values for the mean time taken for the cross to disappear are:
Sodium | Time taken for the cross to disappear (seconds) | |||
Expt. 1 | Expt. 2 | Expt. 3 | Mean | |
8 | 93 | 95 | 94 | 94 |
16 | 53 |
| 53 | 53 |
24 | 28 | 30 | 31 | 29.7 |
32 | 15 | 14 | 13 | 14 |
40 | 7 | 7 | 6 | 6.7 |
The result for experiment 2, 16 g / dm3 of sodium thiosulfate should not be included in the mean calculation
b) The graph of the results is:
c) What conclusion can be drawn about how concentration affects the rate of this reaction?
Statement:
As the concentration increases, the time taken for the cross to disappear decreases
OR
The higher the concentration, the faster the reaction
Explanation:
This is because increasing the concentration means that there are more particles in the same volume
This leads to more frequent, successful collisions and a faster rate of reaction
Last updated:
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?
