Practical

Investigate the effects of various catalysts on the decomposition of hydrogen peroxide.

It is important in this practical to use appropriate apparatus to make and record a range of measurements accurately, including time, mass and volume. This includes the safe use of apparatus, and monitoring chemical changes.

This outlines one way to carry out the practical. Eye protection must be worn.

Aims

To investigate the effect of different catalysts on the rate of a reaction.

Hydrogen peroxide solution decomposes slowly at room temperature to water and oxygen:

hydrogen peroxide → oxygen + water

2H2O2(aq) → O2(g) + 2H2O(l)

The volume of oxygen given off can be measured using a gas syringe, or by collecting in an inverted measuring cylinder. The effectiveness of the catalyst can be judged from the initial rate of the oxygen production.

Method

A delivery tube takes gas from a conical flask containing hydrogen peroxide solution and a chosen catalyst into a clamped measuring cylinder placed in a trough of water.
  1. Using a measuring cylinder, add 50 cm3 of hydrogen peroxide solution to a conical flask.
  2. Fill the measuring cylinder with water, seal the top with your finger, and invert it into the water trough, as shown in the diagram.
  3. Loosely connect the bung into the conical flask and make sure the delivery tube connects to the inverted measuring cylinder.
  4. Measure 0.5 g of a catalyst.
  5. Add 0.5 g of a catalyst to the flask, put the bung back into the flask and start the stopwatch.
  6. Record the volume of gas given off every 10 seconds. Continue timing until no more oxygen appears to be given off.
  7. Clean the apparatus as directed by your teacher.
  8. Repeat steps 1 to 7 for another two catalysts.

Results

Record the results in a table. This table gives some example results of the volume of gas produced every ten seconds.

Catalyst10 s20 s30 s40 s50 s60 s
Manganese dioxide588192100100100
Iron344854575960
Copper(II) oxide132025283031

Analysis

  • Compare the results for the three catalysts and reach a conclusion. If necessary, plot a graph to show:
    • volume of gas (cm3) on the vertical axis
    • time (s) on the horizontal axis
    • a curve of best fit
Question

Which is the best catalyst for the decomposition of hydrogen peroxide? Explain your answer.

Manganese dioxide is the best catalyst because it produces the highest volume of oxygen more quickly. A graph of volume of oxygen against time has the steepest gradient for this catalyst.

Evaluation

Question

Suggest a reason why the same mass of catalyst is used in each experiment.

The surface area to volume ratio of the catalyst may affect the rate. It cannot be precisely controlled, but the same mass of powdered catalyst will have roughly the same surface area to volume ratio.

Hazards, risks and precautions

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardHarmPrecaution
Hydrogen peroxide is corrosiveHydrogen peroxide could get onto the skin when adding to test tubeWash hands immediately if any hydrogen peroxide gets onto them
Powdered catalystsMany powdered catalysts can be irritants if inhaledAvoid spilling the powdered catalysts
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