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Investigating the rate of reaction between peroxydisulphate(VI) ions and iodide ions

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Introduction

In this investigation I will be looking into: * The effect of changing the concentration of reactants upon the rate of reaction. I can calculate the order of the reaction with respect to the variable reactant (potassium iodide ions), and then I can use the known orders of other reactants to obtain the rate equation. From the rate equation I can then work out the overall order of reaction. * The effect of changing temperature upon the rate of reaction. I can find the rate of reaction for different temperatures and substitute these into the rate equation, allowing me to work out a value of k for each temperature. These k can be used to draw graphs from which I can calculate the activation enthalpy. Section 1 - Theory The iodine clock reaction is a two stage reaction. In the first (R1), peroxydisulphate (VI) ions react with iodide ions to form sulphate (V) ions and iodine: Both reactants and the sulphate ions are colourless. To measure the initial rate of this reaction, the colour of the iodine produced can be followed. The colour change can be detected more clearly by adding starch to the mixture. This causes a blue-black solution to form in the presence of iodine. A way to measure the initial rate of this reaction is to time how long it takes for the reaction to produce a small, fixed amount of iodine. The time taken can be enhanced by adding thiosulphate ions to the reaction mixture at the beginning. These thiosulphate ions turn iodine back to iodide ions: This means that no starch-iodine will appear until all the thiosulphate has been used up. What will be seen is a colourless solution that after a certain amount of time turns a vivid blue/back colour. By measuring the length of time taken for this colour to appear, you will know how long it took to produce the equivalent amount of iodine Rate Rate of reaction is concerned with how quickly a reaction reaches a certain point. ...read more.

Middle

Start temperatures must be controlled to ensure accurate data Test tubes 35 (approx) Contains reactants Simple way to transfer chemicals. Easily poured. Boiling tubes 35 (approx) Contains reactants, and is where each reaction takes place Glass is colourless, and so allows me to observe colour change easily. Clamp stands and Clamps 5 Holds burettes upright Secures the burettes and makes sure they stay level. Funnels 5 Help to refill burettes A safe way to avoid spillages Stop clock 1 Time the reaction A digital stop clock is accurate to 0.01 seconds. White tile 1 Provides a neutral background colour to compare the solution mixture to The colour chance will be easier to judge if there is a constant colour to compare it to throughout the entire investigation Table 3.2 - A table to show the chemicals used in this investigation Chemical Formula Concentration Potassium iodide solution KI 1.00 mol dm-3 Potassium peroxydisulphate (VI) solution K2S2O8 0.00400 mol dm-3 Sodium thiosulphate solution Na2S2O3 0.0100 mol dm-3 Freshly made starch solution N/A N/A Table 3.3. A table to show the different concentrations that make up each mixture Mixture Test Tube Boiling tube Volume (cm3) of KI(aq) Volume (cm3) of water Volume (cm3) of Na2S2O3(aq) Volume (cm3) of starch solution Volume (cm3) of K2S2O8(aq) 1 5 0 2 1 2 2 4 1 2 1 2 3 3 2 2 1 2 4 2 3 2 1 2 5 1 4 2 1 2 Preliminary work I felt that it was necessary to undertake some preliminary work before starting this investigation. From this, I aimed to: * Find out a suitable concentration to use for the reactions which involve temperature changes. * Find a suitable range of temperatures to use * Get a better understanding of the technique. Table 3a shows the results of this preliminary. Table 3.4. A table to show the results of the preliminary experiment Mixture Time 1 Time 2 Average 1 51:91 48:17 50.04 2 69:15 66:85 68:00 3 99:49 97:12 98:31 4 181:14 174:24 177:96 5 389:68 386:24 387.97 The rate of reaction roughly doubles by a rise in 10. ...read more.

Conclusion

Accuracy of method In this investigation, there was inevitably a time delay between mixing the reactants together and starting the stop clock. When the reaction was longer, small time delays were not as significant. However, for the reactions at high temperatures, time recordings were much shorter, and so this delay will have been much more significant. I do not think this affects the validity of my results, as each time delay would be generally quite similar to each other, as they were each carried out by me. In this investigation, I was judging the colour change by eye. For the lower concentrations, there was more of a gradual change in colour, and it was difficult to judge exactly when to stop the timer. The temperature of the lab varied throughout the day. Sometimes, the room would be slightly below 20�C in the morning, and then towards the afternoon would rise slightly above 20�C. Being slightly colder isn't a problem as such, as waterbath can easily warm the water, however it cannot cool the water, and so if the environment were above 20�C, the waterbath would slowly warm too. The biggest relative significance comes from the time delay between mixing the reactants and starting the stop clock. As the time delay can vary between each experiment, the significance of the error can increase. Suggested improvements If it were possible, a better idea would be to shine a light through the solution and into a detector, which would measure the amount of light passed through the solution. This would ensure that I could stop the timer is at the same point in the reaction much more accurately, without having to judge it by eye each time. To improve on this method, I would ask for assistance in starting the stop clock as I mix the reactants. In theory, this would eliminate any time delays, however in practice human error still plays a part, and so this would still not be 100% accurate. ?? ?? ?? ?? 1 ...read more.

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