9.1

Question 1

9.1 Rates - Decomposition of hydrogen peroxide• Measurement of rate of reaction by continuous monitoring method
• Measurement of time
• Use of appropriate software to process data.

Answer 1

Hydrogen peroxide, H2O2, decomposes into water and oxygen:

2H2O2(aq)  2H2O(l) + O2(g)


The rate of this decomposition can be increased by adding manganese(IV) oxide, MnO2, which behaves as a catalyst.
You will investigate the rate of the decomposition of H2O2.

You can follow the progress of the reaction by measuring the volume of oxygen produced over time.

Question 2

H2O2(aq)

Answer 2

Currently no hazard classification at this concentration

Question 3

MnO2

Answer 3

Harmful if swallowed or inhaled

Question 4

Procedure

Answer 4

1. Invert a water filled burette into a trough to act as a suitable method of collecting gas. You will be required to clamp your burette and could use a test tube rack to support your boiling tube.
2. You will be measuring time and the burette reading. You will need to work out the volume, V, of O2 collected and the concentration of H2O2. You can work out the volume of O2 easily but the concentration of H2O2 involves a more complex calculation that will need to be repeated at each reading. You can either carry this out using calculator or you can use a spreadsheet to calculate the concentration of H2O2. You will need a table with the four columns shown below
   time/s burette reading/cm3 volume of O2 collected/cm3 concentration of H2O2/mol dm–3
3. Fill in the ‘time’ column from 0 s to 240 s, decide on an appropriate interval to take these readings.
4. Accurately measure 20.0 cm3 of H2O2 and add to the boiling tube.
5. Weigh out between 0.05 g of MnO2.
6. There is a lot to do at this stage. Working with someone, organise how you are going to mix the chemicals and take results efficiently
7. Add the MnO2 to the boiling tube.
8. Quickly insert the bung and shake the boiling tube to mix the contents.
9. Take a burette reading and start the stop watch.10. Record the burette readings in your table at every interval up to 240 seconds.
10. Decide if you need to repeat your experiment.

Question 5

Analysis and extension opportunities

Answer 5

The concentration of the hydrogen peroxide in the boiling tube decreases as it decomposes producing oxygen.
The concentration of hydrogen peroxide, [H2O2(aq)] can be calculated using the expression below. V is the volume, in cm3, of O2(g) collected at each time.
[H2O2(aq)] = (40 – V)/240 mol dm–3
1. Work out the volume of O2 collected after each time measurement and calculate the concentration of H2O2. It is easiest to get the spreadsheet for this repeated calculation.
2. Plot a graph of the concentration of [H2O2(aq)] against time t. You can do this either on graph paper or within the spreadsheet.
3. Determine two successive values for the half-life of the reaction.
4. Do your half-life values suggest that the reaction is first order with respect to hydrogen peroxide?
5. Using tangents, calculate the rate of reaction at t = 0 s and when the [H2O2(aq)] has decreased to half the original value.
6. Compare the rates of reaction and the [H2O2(aq)]. What do these results suggest about the order of reaction with respect to H2O2.
7. Using your results, suggest the rate equation for the decomposition of H2O2 using MnO2 as a catalyst.

Question 6

Equipment

Answer 6

o a side arm boiling tube would be the most suitable and then the boiling tube can be placed in a test tube rack, and narrow tubing can be used as a delivery tube into the burette. The bung should fit the boiling tube and the delivery tube that it carries should be long enough to enter the open end of an inverted burette which is held under the surface of water in the trough.
• Test-tube rack or clamp stand with boss and clamp to support boiling tube
• Burette (50 cm3)
• Measuring cylinder (25 cm3)
• Clamp stand with boss and clamp
• Suitable trough for collection of gas over water – a small plastic washing-up bowl or similar
• Stop clock or stop watch
• Each student will also need access to the following:
• Top-pan balance weighing to 0.01 g

Question 7

notes

Answer 7

The initial production of oxygen is quite fast and students have to be ready to record the volume of gas as soon as they have put the bung in the boiling tube after adding the manganese(IV) oxide.
The values of the half-lives obtained may produce varying results and conclusions which can depend on the quantity of catalyst used
Measuring the gradient of tangents gives values for the rate of reaction at different hydrogen peroxide concentrations.