Practical investigations
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In my investigation, I am going to use potassium iodide to determine the concentration, in mol/dm3, of hydrogen peroxide in the '20 volume' solution by a titration method and I am also asked to show that the decomposition is first order with respect to hydrogen peroxide. How do I go about doing this?
Ulex replies
You must do your own planning – that’s part of the assessment of your investigation. All I can do is to point you in the right direction.
1. First of all, work out the rough answer from the definition of ‘20-volume’. It means 1 cm3 of 20 volume hydrogen peroxide solution produces 20 cm3 of oxygen on complete decomposition into water and oxygen. Write the equation for this decomposition; work out the number of moles of molecules in 20 cm3 of oxygen; use the equation to tell you how many moles of hydrogen peroxide were decomposed; multiply by 1000 to give you a concentration in mol dm-3.
To find the accurate concentration, take a measured volume of the solution (pipette), add a spatula measure of solid potassium iodide and some dilute sulphuric acid. Leave for an hour or so for the reaction to complete.
Titrate with a suitable concentration of sodium thiosulphate solution and use the result to calculate the concentration.
2. There is a very neat and clever method for verifying the order of the reaction. It is called the Harcourt-Esson experiment. I am sure you will find it using Google.
See Nuffield Chemistry Special Study Chemical Engineering on the React website. Both these procedures are in it.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 23 May 2007
