Mandatory Experiment 6.1

Mandatory Experiment 6.1

Mandatory Experiment 6.1

Monitoring the rate of production of oxygen from hydrogen peroxide using manganese dioxide as a catalyst

Student Material

Theory

Hydrogen peroxide decomposes into water and oxygen as follows:

H2O2(l) → H2O(l) + 1/2 O2(g)

This occurs much too slowly to be monitored. However, manganese dioxide acts as a suitable catalyst, and the reaction occurs at a measurable rate.

Method 1 – using an inverted graduated cylinder

Chemicals and Apparatus

Hydrogen peroxide (20 volumes)

Powdered manganese(IV) oxide n

100 cm3 graduated cylinder

Beehive shelf

Large trough

Conical flask with suitable stopper and delivery tube

Stop-clock

Small test tube

Thread

Teat pipette

Procedure

NB: Wear your safety glasses.

  1. Measure out 5 cm3 of hydrogen peroxide and dilute to 50 cm3 with water. Place it in the conical flask.
  1. Weigh about 0.5 g manganese(IV) oxide into the small test tube, and use the thread and stopper to suspend the test tube in the conical flask. Avoid contact between the manganese(IV) oxide and the hydrogen peroxide.
  1. Place sufficient water in the trough to allow the graduated cylinder to be filled with water and inverted over the beehive shelf. Using a teat pipette, inject air into the graduated cylinder until the water level is at the 10 cm3 mark.
  1. Arrange the delivery tube for the oxygen produced to be collected in the graduated cylinder by displacement of water.

  1. Loosen the stopper momentarily to allow the thread to fall into the flask and shake vigorously, thus bringing the manganese(IV) oxide into contact with the hydrogen peroxide. The stop-clock should be started as this contact is made. Record the total volume of gas in the graduated cylinder every 30 seconds. Readings should be taken at eye level.
  1. Present the results in the following table:

Time (min) / Total volume of gas (cm3) / Total volume of oxygen(cm3)
  1. Draw a graph of total volume of oxygen against time, putting time on the horizontal axis.

Method 2 – using a gas syringe

Chemicals and Apparatus

Hydrogen peroxide (20 volumes)

Powdered manganese(IV) oxide n

100 cm3 gas syringe

Conical flask with single-holed stopper and suitable delivery tube

Stop-clock

Small test tube

Thread

Graduated cylinder

Procedure

NB: Wear your safety glasses.

  1. Using the graduated cylinder, measure 8 cm3 of hydrogen peroxide and dilute to 50 cm3 with water. Pour this solution into the conical flask.
  1. Weigh about 0.5 g manganese(IV) oxide into the small test tube, and use the thread and stopper to suspend the test tube in the conical flask, avoiding contact between the hydrogen peroxide and the manganese(IV) oxide.
  1. Connect the delivery tube to the gas syringe, ensuring that the syringe is set to the zero reading.

  1. Loosen the stopper momentarily to allow the thread to fall into the flask, and shake vigorously, thus bringing the manganese(IV) oxide into contact with the hydrogen peroxide. The stop-clock should be started as this contact is made.
  1. Record the total volume of oxygen in the syringe every 60 seconds.
  1. Present the results in the following table:

Time (min) / Total volume of oxygen(cm3)

Questions relating to the experiments

  1. Why is the slope of the graph steepest in the early stages of the reaction?
  1. At what stage is the reaction complete?
  1. What would be the effect on the graph of doubling the amount of manganese(IV) oxide?
  1. Would doubling the manganese(IV) oxide create a practical difficulty? Explain your answer.
  1. What would be the effect on the graph of doubling the concentration of hydrogen peroxide?
  1. Would doubling the concentration of hydrogen peroxide create a practical difficulty? Explain your answer.

Teacher Material

  • As hydrogen peroxide decomposes in storage unless an inhibitor has been added, apparent concentrations can be unreliable. Consequently, the quantities suggested should be tested beforehand to ensure that the rate can be measured satisfactorily, and that the volume of gas produced does not exceed the capacity of the collecting vessel.
  • Hydrogen peroxide may be purchased in 130 cm3 bottles in most pharmacies. This may make it easier to have good quality hydrogen peroxide available for this experiment.
  • The concentration of hydrogen peroxide suggested for the gas syringe method is greater than that for the collection over water, as a greater pressure of gas seems to be needed to produce readings in the gas syringe.
  • As temperature is a factor affecting the rate of reaction, it is advisable not to use cold tap water in diluting the hydrogen peroxide. Deionised or distilled water at room temperature should be used instead.

Quantities per working group

5 cm3 of 20 volumes hydrogen peroxide for Method 1, and 8 cm3 of 20 volumes hydrogen peroxide for Method 2.

0.5 g of manganese(IV) oxide in each case.

Safety considerations
  • Safety glasses must be worn.
  • The use of gloves is recommended.

Chemical hazard notes

Hydrogen peroxide : Burns skin and eyes. Avoid skin contact if possible. Causes serious internal damage if swallowed.

Manganese(IV) oxide :Inhalation of dust is harmful. It is irritating to eyes and skin.

Disposal of wastes

Filter the manganese(IV) oxide – this should then be mixed with sand and placed in a refuse bin. Dilute the filtrate with excess hot water and flush to foul water drain.

Specimen Results – Method 1

Time (s) / Total volume of gas (cm3) / Total volume of oxygen(cm3)
0 / 10 / 0
60 / 31 / 21
120 / 42 / 32
180 / 50 / 40
240 / 55 / 45
300 / 60 / 50
360 / 65 / 55
420 / 68 / 58
480 / 70 / 60
540 / 73 / 63
600 / 75 / 65
660 / 78 / 68
720 / 80 / 70
780 / 81.5 / 71.5
840 / 83 / 73
900 / 85 / 75
960 / 86.5 / 76.5
1020 / 88 / 78
1080 / 89 / 79
1140 / 90 / 80
1200 / 91.5 / 81.5
1260 / 93 / 83
1320 / 94 / 84
1380 / 95 / 85
1440 / 95 / 85
1500 / 95 / 85
1560 / 95 / 85
1620 / 95 / 85
1680 / 95 / 85

Specimen Results – Method 2

Time (sec) / Total volume of oxygen(cm3)
0 / 0
60 / 25
120 / 36
180 / 45
240 / 53
300 / 60
360 / 66
420 / 70
480 / 75
540 / 78
600 / 82
660 / 85
720 / 87
780 / 89
840 / 91
900 / 92
960 / 92
1020 / 92
Suggested Answers to Student Questions
  1. Why is the slope of the graph steepest in the early stages of the reaction?

Since rate is proportional to concentration, the greatest rate, indicated by the

steepest slope, is evident in the early stages when the concentration of hydrogen

peroxide is at a maximum.

  1. At what stage is the reaction complete?

When the graph becomes horizontal.

  1. What would be the effect on the graph of doubling the amount of

manganese(IV) oxide?

The increased surface area of catalyst would speed up the reaction, giving a

steeper slope and an earlier completion. The volume of oxygen produced would

be unchanged.

  1. Would doubling the manganese(IV) oxide create a practical difficulty? Explain

your answer.

Yes. The production of oxygen could become too quick for accurate monitoring.

  1. What would be the effect on the graph of doubling the concentration of

hydrogen peroxide?

Increasing the concentration of a reactant would speed up the rate, as indicated

by a steeper slope. Doubling the concentration would produce double the final

volume of oxygen.

  1. Would doubling the concentration of hydrogen peroxide create a practical

difficulty? Explain your answer.

Yes. The capacity of the collection vessel could be exceeded.

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