By the use of an Iodine clock reaction I hope to obtain the length of time taken for Iodine ions (in potassium iodide) to react fully with Peroxodisulphate ions (in potassium Peroxodisulphate). I will do three sets of experiments changing first the concentration of iodide ions, then the concentration of Peroxodisulphate ions and finally the temperature of the solution in which the reaction is taking place. From these results, I hope to draw conclusions as to the effects of these changes to the environment of the reaction on the rate and also determine the order of the reaction and the activation enthalpy. Background information
The rate of a reaction is determined by a number of factors. These include: pressure, temperature, concentration of reactants, surface area of reactants, presence of a catalyst and radiation. The effect of these factors can be explained using collision theory. Reactions occur when the reactant particles collide, provided the colliding particles have enough energy for the reaction to take place. As the molecules approach their electron clouds repel. This requires energy – the minimum amount of which is called the activation enthalpy’ – and comes from translational, vibrational, and rotational energy of each molecule.
If there is enough energy available, this repulsion is overcome and the molecules get close enough for attractions between the molecules to cause a rearrangement of bonds and therefore an effective’ reaction has taken place. The more collisions of particles with kinetic energy over the activation enthalpy that occur, the faster the overall reaction. During this investigation I am focusing on the effect of temperature and concentration while aiming to maintain other rate determining factors at a constant level in order to ensure reliable results.
The concentrations of the solutions were suggested in a similar experiment found on a website (listed in references) and when considering the moles involoved in the reactions the concentrations are appropriate. I have chosen to make up 250cm3 of each solution because this amount is manageable whilst allowing me numerous repeats for each experiment (which will be necersery until I obtain results within 8%) and also allows for trials, errors and mistakes where I may have to start again. Through calculations I have determined that when making 250cm3 of solution the smallest amount of solid to be added is 0. g of sodium thiosulphate.
The scales used will allow me to measure this amount accurately so I do not feel the need to make up a larger volume of solution. The amount of each solution used in each set of concentration-focused experiments was determined through trial and error (although again I had come across suggestions from similar experiments on websites and in books). The chosen quantities allow use of a small amount of solution each time (to save wasting) yet still maintain a high level of accuracy. 1cm3 starch solution gives a very obvious dark blue whilst not effecting the occuring reactions.
In a preliminary experiment I found that 1-5 cm3 of the reactant I was varying (the remaining amount made up with water so that there was always the same volume in each individual experiment) meant that at lowest concentration the length of time taken for the solution to change colour was not too long to effect what I am able to achieve in the time allocated for my project (roughly 500 seconds for I- ions and 350seconds for S2O82-ions), and at highest concentration the colour change took long enough to record (roughly 40seconds for I- ions and 60 seconds for S2O82- ions).
The choice of using mixture 3 in the set of temperature-focused experiments was decided upon after the concentration experiments had been done. Form my results, I was able to see that a mixture of 3cm3 KI, 2cm3 distilled water (and the rest as normal) took on average 82. 3 seconds to change colour at around 20C. For every 10K increase I expected the rate to half, and so at 50C I expected a result of just over 10seconds which is long enough to record accurately. Method I will use burettes to measure out the quanitites as this means the quanities are accurate 0. 05cm 3 compared to a measuring cyclinder which can only measure quantities 0. m3.
I have chosen to record three results for each different concentration/temperature and repeat until these three results are within 8% of each other. This means that at my highest concentraion and therefore rate, there will be no more than 3 seconds between results, and at my lowest concentration there will be no more than 25 sconds between results. Although this seems like a large variant through preliminary experiments I have discovered that it is extremely difficult to obtain results that are within 20seconds at the lower concentrations, and the amount of time I have would not allow me to continue repeating until this occurs.
A margin of 8%, therefore, I do not feel is inaccurate enough to supply me with unreliable results. The turning point (at which I stop the stopwatch) will be at the very first hint of blue colour. Through practice experimerents I could see that if not mixed fully the blue colour begins to form each at the surface or at the bottom, yet this is not the correct turning point as if shaken the colour disappears. Therefore, it is important to shake vigorously at the beginning and in longer experiments again at around 2 minute intervals so that the reacants do not settle.
Although text books have informed me that the reactions are only very very slightly exothermic and the overall tempertaure of the solution will not change during the experiements, it is important to record the temperatures of every experiment and take into consideration how this may affect the results. Also, it is important to record the start and end tempertaure of the solution during the temperature-focused experiments because higher temperature lose their heat more quickly and if the temperature drops during the time of the reactions this information has to be involved in finding the average temperature.
