You are required to plan and carry out a practical investigation in the laboratory. The investigation must be written up as a laboratory report and a paper copy of the written report submitted. An electronic copy must be submitted via Blackboard on Moodle. The experimental work may be carried out either individually or in pairs but each student is required to produce and submit an individual report.
Deadline: The report must be handed in by 2.00pm Monday 21st June 2021 (Blackboard).
INVESTIGATING THE RATE OF A CHEMICAL REACTION
Peroxodisulphate (VI) ions and iodide ions react together in solution to form sulphate (VI) ions and iodine.
Both reactants and the sulphate ions are colourless, so the progress of the reaction can be measured by following the colour of the iodine produced. The iodine can be detected even more clearly by placing some starch in the reaction mixture; iodine forms an intense blue-black complex with starch.
It is possible to make the amount of time taken to produce a particular amount of iodine easier to measure by adding some thiosulphate ions to the mixture. Thiosulphate ions turn iodine back to iodide ions.
No starch-iodine colour can appear until all the thiosulphate has been used up. The blue colour, therefore, only appears after a particular amount of iodine has been produced. This method of studying reaction rates is called the ‘clock method’. The reaction you will use here is an example of an ‘iodine clock’.
burettes or pipettes
1 mol dm-3 potassium iodide solution
0.04 mol dm-3 potassium peroxodisulphate solution
0.01 mol dm-3 sodium thiosulphate solution
1% starch solution
0.1 mol dm-3 copper(II) sulphate solution
Wear Eye Protection
Make up the reaction mixture 1 from the table and measure how long it takes for the blue starch-iodine colour to first appear.
Mixture Volume of KI soln / cm3 Volume of water / cm3 Volume of Na2S2O3 soln / cm3 Volume of starch soln / cm3 Volume of K2S2O8 soln / cm3 1 5 0 2 1 2 2 2 1 2 3 2 1 2
The potassium iodide, sodium thiosulphate, starch and water can be placed straight into a test tube. The potassium peroxodisulphate(VI) solution should be measured into a separate contain. The potassium peroxodisulphate(VI) solution is poured into the mixture in the test tube and simultaneously the timing is started. Stir the reaction mixture thoroughly to ensure complete mixing.
Record the time taken for the blue colour to appear in a suitable table.
Plan and carry out a series of experiments to investigate how the rate of this reaction depends on the concentration of the potassium iodide.
Carry out a second series of experiments to investigate how the rate of this reaction is affected by the addition of differing (very small) amounts of copper (II) sulphate solution to the potassium peroxodisulphate(VI) before it is added to the reaction mixture.