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An experiment to investigate the factors controlling the rate of the sodium thiosulphate / acid reaction.

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Introduction

An experiment to investigate the factors controlling the rate of the sodium thiosulphate / acid reaction. Planning I am investigating the factors controlling the rate of thiosulphate / acid reaction. The reaction that will be taking place will follow the rate at which sulphur is formed in the reaction of sodium thiosulphate with dilute hydrochloric acid. In the experiment the sulphur will appear as an extremely fine precipitate. This will slowly be followed by a milky appearance in the reaction mixture in the conical flask. As the amount of the sulphur in the mixture increases, the precipitate will become milkier. Gradually the liquid will become more and more opaque. To investigate the rate of the reaction, we will use the cross idea. A cross will be placed under the conical flask and when it cannot be seen anymore, because the liquid is opaque, I will know that the reaction is completed. In the experiment I will change the thiosulphate concentration but I will keep the temperature and concentration of hydrochloric acid constant at all times. The equation for the experiment is as simple as the experiment: Na2S2O3(aq) + 2HCL(aq) S(s) + S02(g) + 2NaCl(aq) + H20(I) There are five factors that will affect the rate of a given reaction, however, only two of these needs to be considered in this experiment. These five factors are temperature, concentration, surface area, pressure and catalysts. Only the effect of a change in concentration and temperature will change the rate. The surface area would only be a factor in an experiment when a reagent is a solid and so the reaction had taken place between a gas/solid or a solution/solid and the two reagents are both liquids. Pressure, as a factor in an experiment would only occur if a gas were present. As the reagents used are solutions, gas pressure will not make any difference. Finally a catalyst (a substance which increases the rate of chemical reaction, but is not used up in the process) ...read more.

Middle

> Once each and every one of the liquids has been measured we must place them in a conical flask. Under the conical flask there should be a cross. > One must place a thermometer in the flask so one can make sure that the temperature is kept constant. > Once everything is ready start the clock. > Once the solution has turned opaque you can start again. > Note that for the main experiment one should have at least five different results (in my case six) and repeat them twice. The second variable, which will remain constant during the experiment, is the effect of temperature. I am not going to carry out this experiment but I will explain an experiment to test the effect of temperature on the rate of reaction of thiosulphate. The aim of this experiment is to find how big an effect temperature has on the rate of reaction of thiosulphate with dilute hydrochloric acid. Method > Apparatus: Thermometer, waterproof pen, conical flask, burette, safety glasses, clamp and a stopwatch. > One must always wear safety glasses, as there are dangerous chemicals in the experiment we are doing. > I believe that a concentration of thiosulphate, which is sensible to use for this experiment, is 25cm3. If I chose 50cm3 as a volume to use then I would find that there would be not much change in the times. As a volume 25cm3 is not too big and not too small. > We measured each liquid in a burette. A burette is the most accurate form of measurement that I can use. > After every part of the experiment one should wash the conical flasks out because if one doesn't then there will be bits of the solution still on the sides of the conical flask, which could easily influence the experiment. One could even put a brush in the flask and try to dry it out. ...read more.

Conclusion

If we want to go further into our investigation we can look into how temperature effects the rate of reaction. Method > Apparatus: Thermometer, waterproof pen, conical flask, burette, safety glasses, clamp and a stopwatch. > One must always wear safety glasses, as there are dangerous chemicals in the experiment we are doing. > I believe that a concentration of thiosulphate, which is sensible to use for this experiment, is 25cm3. If I chose 50cm3 as a volume to use then I would find that there would be not much change in the times. As a volume 25cm3 is not too big and not too small. > We measured each liquid in a burette. A burette is the most accurate form of measurement that I can use. > After every part of the experiment one should wash the conical flasks out because if one doesn't then there will be bits of the solution still on the sides of the conical flask, which could easily influence the experiment. One could even put a brush in the flask and try to dry it out. > Before doing anything I will mark the bottom of the conical flask with a waterproof pen. This is because once I can't see the mark anymore then the reaction has finished and the solution is opaque. > When the liquids have been measured one should find the conical flask with the mark on it and clamp it with a clamp to the water bath. Water Bath Conical Flask > Pour all the liquids into the conical flask and make sure that the HCL goes in last as the HCL starts the reaction. > Just in case one should place a thermometer into the water bath just to monitor the temperature. > Finally one may start the clock and note down the results in a neat table. > Note that one must do this experiment at various temperatures. So for example for this experiment I am going to use the temperatures 10 degrees, 20 degrees, room temperature, 30 degrees, and 40 degrees. I will repeat the experiment twice more. ...read more.

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