When peroxodisulfate (VI) ions and iodide ions react together in solution they form sulfate (VI) ions and iodide. This reaction is shown below: S2O82-aq+ 2I-aq SO42-aq+ I2(aq)
The reactants and the sulfate (VI) ions are colourless however the Iodine is a yellow/brown colour. This allows you to measure the progress of the reaction through the colour change when the iodine is produced. In order to determine the order of the reaction we need to measure the initial rate of the reaction by timing how long the reaction takes to produce a small, fixed amount of iodine. This can be measured clearly and effectively by using the iodine clock method. When you add thiosulfate (VI) ions to the starting reaction mixture the thiosulfate (VI) ions turn iodine back into iodide ions. The equation for this reaction is shown below:
2S2O32-aq+ I2aq S4O62-aq+ 2I-(aq)
The products of this reaction are both colourless so no Iodine colour will appear until all the thiosulfate (VI) ions have been used up. The appearance of the iodine can be made much clearer by adding some starch to the reaction mixture as iodine forms an intense black/blue colour in its presence. This means that the mixture will suddenly experience a colour change from colourless to intense blue/black. By measuring the time taken for this colour change to occur you know how long it took to use up all the thiosulfate (VI) ions and so how long it took to produce the iodine that reacted with it. This is known as the iodine clock method and can be used to find the order of a reaction and determine the effect of Iodide ion concentration on rate of reaction.
First of all I measured out 5 different volumes of 1.00 mol dm⁻³aqueous potassium iodide solution and put these into 5 different boiling tubes. This was the only reactant that differed in concentration throughout the investigation. All other reactant concentrations were kept constant and water was added to the mixtures in order to maintain a constant volume of solution throughout. Next I added the aqueous 0.0100 mol dm⁻³ sodium thiosulfate (VI) solution to each boiling tube. I added 2.00 cm³ of this each time. I then added 1.00cm³ of starch solution to the mixture in each boiling tube. In order to keep the volume of the mixture in each boiling tube constant I added enough water to each mixture to bring the volume up to 10cm³. I measured the temperature of the solution in each boiling tube with a thermometer to ensure that each reaction occurred at the same temperature as difference in temperature could have an effect on the rate of reaction. After these mixtures were complete I could then add the potassium peroxodisulfate (VI) solution to each mixture containing varying KI concentrations. I added 2.00cm³ of potassium peroxodisulfate (VI) solution to the first mixture and started timing immediately, using a stopwatch, until the colour change was observed. I used the thermometer to continuously stir the solution. I then repeated this process with the remaining for mixtures and recorded the time taken each time. The table below shows the quantities used in each mixture:
Mixture| Volume of KI (aq)/cm³| Volume of water/cm³| Volume of Na₂S₂O₃ (aq)/cm³| Volume of starch solution/cm³| Volume of K₂S₂O₈(aq)/cm³| 1| 5.00| 0.00| 2.00| 1.00| 2.00|
2| 4.00| 1.00| 2.00| 1.00| 2.00|
3| 3.00| 2.00| 2.00| 1.00| 2.00|
4| 2.00| 2.00| 2.00| 1.00| 2.00|
5| 1.00| 4.00| 2.00| 1.00| 2.00|
To measure out all the solutions I used a 5cm³ burette ensuring the burette was rinsed thoroughly between different reactants to avoid contamination. I measured the solution so that the meniscus was touching the graduation at eye level, ensuring the tip was full, and kept this constant throughout. I also ensured that I touched the side of the boiling tube with the end of the burette each time to ensure the last...