1.4.1 Rate of Decomposition of Hydrogen Peroxide

PAG 9.1: Rate of Reaction - Decomposition of Hydrogen Peroxide

At room temperature, hydrogen peroxide solution decomposes slowly to form water and oxygen

2H₂O₂ (aq) → O₂ (g) + 2H₂O (l)

The rate of this decomposition reaction can be monitored by measuring the volume of oxygen gas released over time
- This experiment can be performed using:
  - Downward displacement of water - bubbling the oxygen into an upturned measuring cylinder / burette that is placed in a trough of water

1-4-1-a-gas-syringe

- In both cases, the volume of gas produced may need to be considered so that it doesn't exceed the volume of the measuring cylinder / burette / gas syringe
Since this reaction is slow, a catalyst is often added to speed up the reaction
- Common catalysts for this reaction include iron, liver and the enzyme catalase
- However, the most common and OCR exam board specified catalyst for this reaction is powdered manganese(IV) oxide, MnO₂ (s)

Method:

Add 50 cm³ of hydrogen peroxide solution into a conical flask
Fill a fresh measuring cylinder with water, seal the top with your finger, and invert it into the water trough
Loosely connect the bung into the conical flask and make sure the delivery tube connects to the upturned measuring cylinder
Measure 0.5 g of manganese(IV) oxide
Add 0.5 g of manganese(IV) oxide into the conical flask, replace the bung and start the stopwatch
Record the volume of gas released every 10 seconds. Continue timing until no more oxygen appears to be given off
- It may be necessary to perform a rough initial experiment to determine the potential maximum volume of oxygen released and adjust the size of the upturned measuring cylinder accordingly

Specimen results

Rate of oxygen production in the MnO₂ catalysed decomposition of hydrogen peroxide table

Time (s)	10	20	30	40	50	60
Volume of oxygen produced (cm³)	43	71	86	89	90	90

Graphing the results

Analysis

The curve of best fit is drawn on the graph
A tangent can then drawn starting from (0,0) to find the initial rate of reaction
The gradient of the tangent is calculated
- This is the rate of reaction

1-4-1-hydrogen-peroxide-decomposition-graph-2

In the example above, the rate of reaction is:
- Gradient $= \frac{Δ y}{Δ x} = \frac{75}{18} =$ 4.17 cm³ s^-1

Extend your tangent to be as long as possible
This is because shorter tangent lines have the greatest potential for error
If your tangent line is longer then you are less likely to be outside the examiner's accepted range of answers

The example above is the OCR specified practical for PAG 9.1
One simple variation of this practical could be using a variety of catalysts and comparing rates
Another variation, although it has never yet been seen in exams and most teaching classrooms, is that this practical could potentially be performed (or examined) by measuring the mass lost from the conical flask
- This could potentially bring in other aspects of practicals:
  - Would the rate be too high / low to get meaningful results?
  - Would powdered catalysts be lost up the sides / out of the top of the conical flask? Therefore, affecting the results
  - Would the volume of oxygen produced cause a high enough mass loss?
  - Does the concentration of hydrogen peroxide need to be considered / calculated to ensure that enough mass is lost for meaningful results?