Catalytic Decomposition of Hydrogen Peroxide: Experimental Determination of Reaction Order

Experimental Overview

The catalytic decomposition of hydrogen peroxide to water and oxygen can be effected by a range of catalysts such as platinum, catalase (an enzyme found in liver) or manganese dioxide.

2H₂O₂ à 2H₂O + O₂

In this experiment the MnO₂ is prepared in situ by the reaction of H₂O₂ with KMnO₄.

H₂O₂ + 2KMnO₄ à 2MnO₂ + 2KOH + 2O₂

The aim of the experiment is to determine the order of the reaction w.r.t. hydrogen peroxide. The concentration of the catalyst, MnO₂, remains constant and the concentration of peroxide will decrease with time. The rate of decomposition will be monitored by removing a number of samples from the reaction vessel over the course of the reaction and determining the concentration of peroxide by titration against KMnO₄

Reagents:

You will be supplied with the following solutions:

v 5 cm³ of 100 vol H₂O₂ made up to 250 cm³ with water in a volumetric flask (100 vol means that 100 vol of O₂ will be released for every 1 vol of H₂O₂).

v A standard solution of 0.1 M KMnO₄.

Each pedestal is required to prepare:

ü A buffer made by dissolving sodium hydrogen carbonate (2.07 g) in 0.1 M NaOH (10 cm³) before diluting to 250 cm³ in a volumetric flask.

ü A standard solution of 0.01 M KMnO₄ prepared by diluting the supplied 0.1 M solution in a volumetric flask.

Procedure:

Rinse the burette with DI water and 0.01 M KMnO₄ solution. Place the 0.01 M KMnO₄ solution in the burette; this will be used as the catalyst and also in the titration to determine the concentration of the solution of hydrogen peroxide.

Give a detailed method for correctly filling the burette. (3 marks)

Make up the reaction mixture - 150 cm³ of distilled water, 50 cm³ of the buffer solution (using a measuring cylinder) and 25 cm³ of the peroxide solution (using a pipette) in a conical flask and plug the neck of the flask with cotton wool. Record the temperature of the solution in the flask.

Why is it necessary to record the temperature? (1 mark)

Why is it necessary to plug the neck of the conical flask with cotton wool? (1 mark)

Before starting the run, check the concentration of H₂O₂ present in the reaction mixture. There is no need to standardise the permanganate provided you dilute the stock solution accurately. Add a 10 cm³ sample of the reaction mixture to a flask containing 20 cm³ of dilute (1M) sulfuric acid and titrate with the standard KMnO₄ solution to a faint pink colour.

Why is it necessary to determine the concentration of H₂O₂ in the reaction mixture? (1 mark)

Explain how you would transfer the 10 cm³ sample of the reaction mixture. (2 marks)

When you are ready to start the kinetic run, add 1.5 cm³ of the permanganate solution
(0.01 M) to the reaction mixture and mix thoroughly. Now take 10 cm³ samples from the mixture at fixed intervals (initially 3 mins and then 5 mins) until the reaction is nearly complete (probably about half an hour).  Deliver the samples into a flask containing dilute sulfuric acid (20 cm³) and titrate to a faint pink colour with the standard KMnO₄ solution. Shake the flask before sampling in order to relieve the supersaturation of the gas.

What gas is present? (1 mark)

What is the role of the dilute sulphuric acid? (1 mark)

Laboratory results sheet:

Data analysis (34 marks):

1. Use your results to calculate the concentration of H₂O₂ during reaction. (10 marks)

2. Using Excel:

a) Draw a suitable table indicating your results; (3 marks)

b) Plot a graph of [H₂O₂] against time; (5 marks)

c) Manipulate your results in a suitable manner to determine the order of the reaction and the rate constant for the reaction. (10 marks)

3. Research the expected order and rate constant for this reaction. Compare your result to your value and explain why the differences have arisen. (6 marks)

Extension questions (21 marks):

1. The reaction of nitric oxide with hydrogen at 1200°C is:

2NO_(g) + 2H_2(g) à N_2(g) + 2H₂O_(g)

A series of experiments were conducted to determine the initial rate of the reaction at a range of concentrations.

All working out MUST be shown to gain marks.

a) Determine the order of the reaction with respect to both reactant. (4 marks)

b) Write the rate equation and determine the rate constant k (to include units). (3 marks)

2. The dosage frequency for paracetamol is 4-6 hours as the half-life of paracetamol in plasma is 4 hours. The order of the reaction with respect to paracetamol is 1^st order. Determine the rate constant (3 marks) and the percentage of paracetamol which would remain in the blood plasma 12 hours after one dose (3 marks).

3. The hydrolysis of primary alkyl halides is first order w.r.t both reactant. Use the experimental data below to determine the activation energy for the reaction given:

CH₃CH₂Br+ ^-OH à CH₃CH₂OH + Br^-

You may use Excel. (8 marks)