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detremining the rate equation

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The determination of a rate equation Aim The aim of this experiment is to plan an experimental procedure leading to a graphical method to determine how the concentration or the volume of the components affects the rate of the reaction whose equation is given below: 2HCl(aq) + Na2S2O3(aq) --> 2NaCl(aq) + SO2(g) + S(s) + H2O(l) Introduction In this experiment in order for the reaction to take place the reactant particles must collide, and only some of those colliding particles lead to a chemical change. The rate of reaction is defined as the amount of in moles of a reactant which is used up in a given time, in this case it's the amount of sulphur formed in a given time. To work out the rate equation I will conduct two methods. In order to relate the rate of reaction with the concentration of the components involved I will have to obtain results to help me determine the rate of expression of the reaction. There are two methods used to determine the rate expression. This is done by measuring the concentration over time. The concentration has to be measured throughout the reaction. From the results obtained graphs are drawn and from that the order of the reaction is determined. The reason I'm not using this method is I don't have the right equipment to do that. The concentrations will be constantly changing and therefore making it harder to keep track of and record. Therefore I'm going to use the other method. The method I will use to determine the order of the reaction is initial-rate method. This method is used when the initial concentrations of the reactants that are going to be mixed together in the reaction flask, from this the initial rate of the reaction can be found. From my experiment I will obtain the time taken for the reaction to take place, from that I will work out the rate of the reaction and plot graphs of rate against concentration. ...read more.


* Fill up all the burettes with their solution. * Using the burettes and the reading from table 1 ( for method 1) fill the right amount of Na2S2O3 water in the test tube * Using the burette which contain the distilled water fill the right amount into the same test tube(as above) * Now measure 5cm3 of HCl into the conical flask and place it on top of the white tile which is marked with the black cross. * Get ready to start timing before adding the reactants together * Add the Na2S2O3 and the water you already measure in the test tube into the conical flask and start the stop watch * Make sure you are looking from above to see the black cross and try to avoid inhaling the gas being released. And for the other concentrations make sure you are looking from the same point and try to stop the stop watch when the cross disappears. * Stop the stop watch when the black cross has completely disappeared. * Record the results in the table n seconds * Wash the flask and thoroughly to avoid any contaminations for the next concentrations. * Now repeat the experiment in exactly the same way for the other concentrations. * After obtaining the results for method 1 and repeating the experiment at least two times and working out the average from that. This will make your results more reliable. * Now carry out the same procedures for the data on table 2 but now changing the concentration of HCl instead of Na2S2O3. Table 1 Volume of Na2S2O3/ cm3 Volume of H2O/cm3 Moles Na2S2O3/ mol Volume of HCl/ cm3 Temperature 25 0 0.40 5 23 20 5 0.32 5 23 15 10 0.24 5 23 10 15 0.16 5 23 05 20 0.08 5 23 Table 2 Volume of HCl/ cm3 Volume of H2O/cm3 Moles HCl/ mol Volume of Na2S2O3l/ cm3 Temperature 25 0 2.0 5 23 20 5 1.6 5 23 15 10 1.2 ...read more.


It was hard to decide when to start the stopwatch and when to stop it. I t was very hard to judge whether the cross disappeared or not. As the solution started to get cloudy and the cross was about to disappeared there was at least a 5 second gap where the solution will be at the same intensity and this lead to the most significant error. Where you would be unsure whether the reaction has come to completion or not. In order to draw the graph the rate had to be worked out, dividing one by the time taken and if the timing error was around 5second this will be a major different in the value obtained and this will lead in anomalous results. So in the future to avoid this significant error I would use a colorimeter. I can also use a light sensor which more suitable for this type of experiment. This will measure the amount of light passing through the solution. The light sensor will have a fix point when to stop the timing. It will stop timing when the there is no light passing through the solution. And this will result in a more accurate and reliable results. If a calorimeter was used it would measure the amount of light adsorbed by the solution. An appropriate filter would be used allow only certain colour though. As the solution gets cloudy and starts to change colour the amount of light absorbed by the solution will also increase. When all the light is absorbed by the solution the timing should stop. To make this more accurate I will link the colorimeter to a data logger which is in turn linked to a computer where accurate timing will be recorded. I can also stir the solution to help me obtain more reliable results. This will enable all the particles to interact with each other. To get the most out of this method will have use a machine that will stir all the solutions at the same speed. ...read more.

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