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The determination of the rate equation between

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The determination of the rate equation between the reaction of HCl and Na2S2O3 Aim To determine the rate equation between the reaction of known concentrations of HCl and Na2S2O3. Scientific Background To carry out this task we need to find the initial rate of reactions of different concentrations of the substances. We are supplied with the following chemicals, all of known concentration: 0.4 mol dm-3 sodium thiosulphate (Na2S2O3) 2.0 mol dm-3 hydrochloric acid (HCl) Deionised water (H2O) 2HCl(aq) + Na2S2O3(aq) --> 2NaCl(aq) + SO2(g) + S(s) + H2O(l) Many conditions affect the initial rates of reaction including the concentrations of the substances, temperature, total volume and pressure for example. This is all described in collision theory where for molecules to react, they must collide and have the required energy for the reaction. All of the factors will affect the probability of the molecules colliding. Concentration: this is effectively the amount of the molecules in a certain volume. If there is a low concentration, and therefore are few of the molecules in a certain volume, there will be few of them to collide so the rate of reaction will differ due to its concentration. Temperature: this determines how fast the molecules are moving. If they are moving faster then they will collide more which will increase the chances of them making a successful collision and reaction. It is also the energy the molecule has and if its energy is enough then it will be able to react otherwise there will be no reaction. In a mixture of molecules, there may be some which do not have the required energy to react and some which do. The proportion of these molecules can be shown by the Maxwell-Boltzmann distribution curve. As shown on the graph, all the molecules in the shaded area have the required energy (activation energy) for the reaction and will react if it makes a successful collision. ...read more.


To then find the concentration it is Amount of moles divided by Total volume which is 75cm3. Final Concentration = Initial Concentration x Initial Volume / 0.075dm3. E.g. for the first value of HCl on the table: Final Concentration = (2.0 x (2.5/1000)) / 0.075 = 0.005 / 0.075 = 0.067 moldm-3 H20 vol / cm3 HCl vol /cm3 Na2S2O3 vol / cm3 Total vol / cm3 [HCl] / moldm-3 [Na2S2O3] / moldm-3 Time / s Rate / s-1 47.50 2.50 25.00 75.00 0.067 0.13 119.8 0.0083 45.00 5.00 25.00 75.00 0.13 0.13 93.2 0.011 40.00 10.00 25.00 75.00 0.27 0.13 82.5 0.012 25.00 25.00 25.00 75.00 0.67 0.13 64.4 0.016 20.00 5.00 50.00 75.00 0.13 0.27 42.6 0.023 15.00 10.00 50.00 75.00 0.27 0.27 39.1 0.026 Preliminary Analysis From this set of results I have determined a suitable range of volumes of the solutions. 2.50cm3 of HCl is too low but 5.00cm3 and above should be suitable for HCl. For Na2S2O3 50.00cm3 is not too fast and its lower values are not too slow so a large range can be taken for this solution. From these results we can see that as the concentration of Na2S2O3 doubles, the rate doubles which would suggest it is to the first order. However as the concentration of HCl increases there is only a slight increase to the rate which is not even close to doubling or quadrupling as predicted. A major problem did arise from the method, when adding the Na2S2O3 to the conical flask it was difficult to start the timer at exactly the same time and there was a slight delay for to timer to be engaged. A solution to this would be to get help from somebody else to start the timer for you. Final Apparatus Burette with stands and specialised clamps. (x3) These are required to accurately measure out the chemicals being used in the experiment. ...read more.


The results can also be affected by ambient temperature due to temperature and pressure having an effect on reaction rate. This can be avoided by using a controlled environment or by ensuring that these conditions are the same by measuring them before the experiment takes place. Doing everything on the same day will make it easier to keep these conditions the same. Burette with stands and specialised clamps. (x3) No suggested improvement other than more precision from better manufacturing however this will not make much of an improvement. Funnel (x3) No suggested improvement. 100ml Beaker (x3) No suggested improvement. Conical Flask No suggested improvement. Tile with Mark Becoming Obscured This caused a large problem in terms of when the mark became obscured. This was down to the opinion of the individual and the point of it becoming obscured could vary due to light levels in the room for example. A major improvement on this would be to use a colorimeter connected to a data logging system to get a much more accurate and precise set of results. Timer and Mixing Chemicals This also caused a large problem as human reaction time and the difficulty to start the timer and mix the chemicals at the same time. Using a data logging system would be a much better improvement. The chemicals could be mixed together easier if they were placed in separate containers and were then put into the conical flask after a valve had been opened for example. This could start the timer on the data logging machine and could be stopped when the colorimeter reported that a certain threshold had been reached. As in all experiments it would be better to take many more repeat readings and have a larger range of readings to see the full extent of the concentration vs. rate graphs. I believe that this set of results was adequate to come to the conclusion of the orders of the reactants however the HCl vs. Rate graph was overcomplicated with the autocatalysis affecting it and may have been interpreted incorrectly. ...read more.

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