Experiment: Effect of a catalyst on the rate of reaction

A catalyst is the fourth factor the Form 4 Rate of Reaction chapter asks you to investigate, and it is the one that surprises students most, because a small amount of solid can transform a slow reaction into a fast one without being used up. The decomposition of hydrogen peroxide into water and oxygen is the standard demonstration, using manganese(IV) oxide as the catalyst. This guide sets out the aim, method, observations and the Paper 3 skills the practical assessment rewards.

Aim

To investigate the effect of a catalyst, manganese(IV) oxide, on the rate of decomposition of hydrogen peroxide solution, measured by how quickly oxygen gas is produced.

Apparatus and materials

• Conical flask
• Gas syringe, or a measuring cylinder and trough for collecting gas over water
• Delivery tube with a stopper to fit the conical flask
• Stopwatch
• Measuring cylinder (for example, 50 cm³)
• Electronic balance
• Spatula
• Hydrogen peroxide solution of one fixed concentration
• Manganese(IV) oxide powder (the catalyst)
• Wooden splint (to test for oxygen)

Procedure

1. Measure a fixed volume of hydrogen peroxide solution into the conical flask using the measuring cylinder.
2. Without adding any catalyst, fit the stopper and delivery tube connected to the gas syringe and start the stopwatch, recording the volume of oxygen collected at regular time intervals.
3. Wash out the flask and measure the same fixed volume of the same hydrogen peroxide solution into it.
4. Weigh a fixed mass of manganese(IV) oxide powder and add it to the solution, immediately fit the stopper and delivery tube, and start the stopwatch at once.
5. Record the volume of oxygen collected at the same regular time intervals until the reaction stops.
6. To confirm the gas is oxygen, hold a glowing wooden splint at the mouth of the tube; it relights.
7. Plot the volume of oxygen against time for both runs on the same axes, and compare the steepness of the curves.
8. Optionally, filter, dry and reweigh the manganese(IV) oxide to show its mass is unchanged.

Expected observations

Without the catalyst, only a little gas is produced slowly, so the gas syringe plunger moves out very gradually. With the manganese(IV) oxide added, there is rapid effervescence, the mixture may feel warm, and oxygen is collected quickly, so the curve of gas volume against time is much steeper at the start. A glowing splint relights, confirming the gas is oxygen. When the manganese(IV) oxide is recovered, dried and reweighed, its mass is the same as before, showing it was not used up.

Inference and conclusion

The much steeper curve with the catalyst shows a much faster rate of reaction. Manganese(IV) oxide is a catalyst: it speeds up the decomposition of hydrogen peroxide by providing an alternative reaction path with a lower activation energy, so a larger fraction of collisions have enough energy to react. Because its mass is unchanged at the end, it is not consumed by the reaction. The conclusion is that a catalyst increases the rate of reaction without itself being used up, and the final volume of oxygen depends only on the amount of hydrogen peroxide that decomposes.

Science process skills (Paper 3 style)

Stating a hypothesis. Adding manganese(IV) oxide increases the rate of decomposition of hydrogen peroxide. The hypothesis links the manipulated variable to the responding variable.

Identifying variables. The manipulated variable is the presence or absence of the catalyst. The responding variable is the rate of reaction, measured as the volume of oxygen collected in a fixed time. The controlled variables are the volume and concentration of the hydrogen peroxide and the temperature.

Tabulating data. Draw a table with a column for time and separate columns for the volume of oxygen collected with and without the catalyst, all to the same precision as the gas syringe allows.

Plotting a graph. Plot volume of oxygen (y-axis) against time (x-axis) for both runs on the same axes. The steeper curve, with the catalyst, shows the faster reaction; both curves level off at the same final volume.

Making an operational definition. A catalyst is defined operationally as a substance that increases the rate of the reaction, shown by faster gas collection, but whose mass is unchanged at the end.

Making an inference. The catalyst works by lowering the activation energy, so more collisions are effective and the rate rises, while the mass staying the same shows it is not used up.

Safety precautions

• Wear safety goggles, because hydrogen peroxide can irritate the eyes and skin.
• Add the manganese(IV) oxide carefully and fit the stopper without forcing it, because the reaction can be vigorous and may spray the mixture.
• Keep the glowing splint away from the reaction until testing, and keep flames away from the oxygen collected, because oxygen supports and speeds up burning.
• Wash any hydrogen peroxide splashes off the skin at once with plenty of water, because it can bleach and irritate.

Common errors

• Different volumes or concentrations of hydrogen peroxide. These change the rate independently of the catalyst; use the same solution and volume in both runs.
• A delay in fitting the stopper for the catalyst run. The reaction is fast, so oxygen escapes if the stopper is late; have the apparatus ready and connect it the instant the catalyst goes in.
• Testing the gas with a lit splint instead of a glowing one. Oxygen relights a glowing splint; use a glowing, not flaming, splint for the correct test.
• Assuming the catalyst is a reactant. It does not appear in the equation and its mass is unchanged; describe it as a catalyst, not a reactant.
• Reading the gas volume with parallax. Read the syringe at eye level so the volume is correct.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we use this experiment to nail the definition of a catalyst that the exam wants: it speeds up the reaction by providing a path of lower activation energy and is not used up, so its mass is unchanged. We rehearse the oxygen test, a glowing splint relights, and the two-curve graph that shows the catalyst run rising faster to the same final volume. Because the catalyst factor is examined across SPM Chemistry and this is a classic Paper 3 practical (Paper 3 is a practical test assessing science process skills), a clear method and a precise definition here protect marks that are easy to lose.