Rate of Decomposition of Hydrogen Peroxide (OCR A Level Chemistry A): Revision Note

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 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
    

    

  • A gas syringe
    

    

• In both cases, the theoretical volume of gas produced should be considered

  • This allows an appropriate volume measuring cylinder / gas syringe to be selected

• Since this reaction is slow, a catalyst is often added to speed it up 

  • Common catalysts for this reaction include iron, liver and the enzyme catalase

  • The most common and OCR-specified catalyst is powdered manganese(IV) oxide, MnO₂ (s)

 Method

• Add 50 cm³ of hydrogen peroxide solution into a conical flask

• Fill a clean measuring cylinder with water

  • Seal the top with your finger and invert it into a water trough

• Loosely insert the bung into the conical flask and ensure 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

  • You may need to perform a rough trial run to estimate the total gas volume and choose an appropriately sized measuring cylinder

Specimen results

• Here is a set of typical results for this experiment

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

Graphing the results

Analysis

• Draw a curve of best fit on your graph

• Draw a tangent starting from (0,0) to find the initial rate of reaction

• Calculate the gradient of the tangent

  • This is the rate of reaction

• In the example above, the rate of reaction is:

  • Gradient 4.17 cm³ s^-1 

Experimental variations

• The example above is the OCR specified practical for PAG 9.1

• You could vary this practical by testing different catalysts and comparing reaction rates

• Another variation, although rare in classrooms or exams, involves measuring the mass lost from the conical flask as oxygen escapes

  • This brings additional considerations:

    • Will the rate be too fast or too slow to collect useful data?

    • Could powdered catalysts be lost from the flask, affecting results?

    • Will the volume of oxygen released result in a measurable mass loss?

    • Should you calculate the concentration of hydrogen peroxide to ensure enough gas is produced?

Practical skills reminder

• This practical develops essential skills in measuring rates of reaction by gas collection

• It also supports:

  • The use of appropriate apparatus to record time and volume (e.g. stopwatches, gas syringes, measuring cylinders)

  • Drawing and analysing a graph to determine the initial rate from a tangent

  • Understanding how to control variables and improve accuracy by selecting suitable equipment and extending tangent lines