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The Iodine Clock Investigation

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The Iodine Clock Investigation PLANNING Introduction This is an investigation into the rate of a reaction and the factors that contribute to how fast a reaction will take place. Through the recording and analysis of raw data, this investigation also allows us to apply generally accepted scientific rules and to test them against results gained from accurate experimental procedures. Aim The aim of this experiment is to investigate the rate at which iodine is formed when the concentration and temperature of the reactants are varied, and to attempt to find the order and activation energy. The Chemistry 'THE IODINE CLOCK' - This is the experiment that will be used to investigate reaction rates, and it is a reaction between acidified hydrogen peroxide and potassium iodide: 2H+(aq) + 2I� (aq) + H[-1]2O2 (l) ( I2 (aq) + 2H2O2 (aq) Iodide ions are firstly oxidised by the hydrogen peroxide, as shown in the above equation. The iodine that is then produced reacts immediately reacts with thiosulphate ions as follows: I2 (aq) + 2Na2S2O3 (aq) ( 2NaI (aq) + Na2S406 (aq) As soon as all of the thiosulphate ions have reacted with the iodine, the excess iodine molecules react with the 2% starch solution that is present in the reaction. This can be seen as an instant change in colour, from a colourless solution, to a deep purple coloured solution. This change in colour denotes the completion of the reaction. Factors affecting the rate of reactions: All chemical reactions occur at a definite rate under particular conditions. In order to increase the rate at which reactions occur, the frequency at which reacting molecules collide must be increased. This may be achieved in a number of ways: 1. By increasing the concentrations of reacting species. 2. By increasing the temperature. 3. By increasing the pressure (only really significant in reactions involving gases). 4. By the use of a suitable catalyst. 5. In the case of solids, by reducing particle size and thus increasing the effective surface area. ...read more.


Protective measures When carrying out this investigation and handling these chemicals, certain laboratory safety procedures must be carried out at all times: * Labs coat must be worn * Goggles must be worn * Work areas must be cleared of any unnecessary objects (e.g. bags; books; stools) The procedures mentioned previously, are all areas where chemicals are being used, and there is certain amount of risk. Therefore when conducting these procedures, extra care and vigilance must be taken; acknowledging these possible risks should result in safe laboratory work. Units of Concentration The units of concentration that will be used to measure out H2SO4, Na2S2O3 and KI are mol dm-3 or 'M'. In the case of H2O2 however, the concentration will not be measured in mol dm-3, but in 'Vol.'. Hydrogen peroxide is sold commercially in '20 Vol.' and '10 Vol.' solutions. A 20 Vol. solution of hydrogen peroxide is one that liberates 20 times its own volume of oxygen when heated. 2H2O2 ? 2H2O + O2 2 x 34g. 22.41 at standard conditions If the solution is a 2 Vol. solution of H2O2, (2 x 34g) of H2O2 must be contained in a volume of solution, which is 1/2 of the volume of oxygen it produces. Therefore: A 2Vol. solution contains (34 x 2) x 2 g of hydrogen peroxide per litre; 22.4 that is 6.07g per litre of water. Therefore, if it was required to calculate the concentration of H2O2 in mol dm-3 for the use in an equation for example, this could now be done. For 2 Vol. solution of H2O2, the concentration in terms of mol dm-3 is: Moles = Mass/g Molar mass/g mol-1 So: Moles = 6.07 = 0.18 34 If: Concentration/mol dm-3 = Moles Volume/ dm-3 Then: Concentration/mol dm-3 = 0.18 = 0.18 1 Main Investigation - Implementing 1) An Investigation Into The Rate At Which Iodine Is Formed When The Concentration Of Reactants is Varied Aim The first part of this investigation is to investigate how varying concentration of H2O2 and KI affects the rate of the reaction. ...read more.


Therefore there is a possibility that volumes were not always measured to the accuracy capable, this may have been due to bad technique, or possibly the fact that there was limited time and a certain amount of pressure to complete all of the practical work. Modifications that could be made to perhaps increase the accuracy of the volumes measured could include using more accurate burettes; spending more time on both practising the technique, and conducting the actual experiment. Deciphering exactly when the reaction was complete was not always as clear as expected, and this may account for any possible errors. For a number of the experiments conducted, the colour change that indicates when the reaction is complete, was not always as instantaneous as previously described. On these occasions, the colour change was relatively slow, and this made it difficult to determine when exactly the whole of the solution had changed colour. Therefore there may have been some variation in actually deciding the end point of the reaction, which may have lead to errors and inaccuracies in the results. I am not sure why there was variation in the times it took for the actual colour change to occur, and therefore modifications to the method cannot be made, however this opens up the possibility for further investigation. When the ways in which temperature affected the rate of reaction was investigated, it was decided to use a thermostatically controlled water bath after modifying the original procedure. Although this was much more accurate than using a bunsen to heat the water, the accuracy of the water bath to maintain the desired temperature is questionable. The water bath that was being used seemed to be temperamental and not always particularly accurate, this may have lead to errors in the results. Therefore a modification that might be made could be to perhaps use a more sophisticated and reliable water bath. Thus, as shown above there numerous possibilities where error may have occurred. These areas of error must therefore be used to explain why the results appeared as they did, making it difficult draw definite conclusions. ...read more.

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