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Investigating Rates of Reactions

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Chemistry Experiment Coursework Investigating Rates Of Reactions Objective: My aim is to devise an experiment to measure the effect of changing one variable on the rate of reaction between magnesium ribbon and dilute hydrochloric acid; Mg(s) + 2HCl (aq) � MgCl2 (aq) + H2 (g) I will be conducting several experiments with different values of a chosen variable and will be comparing/calculating gradients using graphs and results, to see the effect on changing one variable to a reaction and justifying the outcome of the experiment, by thoroughly evaluating and analyzing the results and ultimately concluding to see if my experiment was a success or not. i) PLAN: First of all...what is "the rate of a reaction"? The rate of a reaction is the change in concentration of reactants or products in unit time. We can see how the rate of a reaction changes under different conditions by measuring the rate at which the reactants are used. 1) The different possible ways of measuring the rate of the reaction: i) Measuring the volume of gas as time goes on using a gas syringe; * As the magnesium reacts with the HCl, hydrogen is produced, and we are able to measure the amount of hydrogen using the gas syringe every x seconds for x minutes/seconds either until the set time period has been achieved or once all of the magnesium has been able to react with the HCl. There are a very little number of errors that are feasible using this method and I found this method extremely user-friendly and this reflects upon the results I received (shown later). The only minor point that I can state is that there will be some gas loss, as we are only able to place the rubber bung after we have placed the magnesium in the acid, so by the time we have secured the conical flask the reaction would have already started. ...read more.


I halved the time period and therefore found more precise results as I thought considering we were only interested in the initial reaction it was better to measure every 5 seconds. * I ensured that I folded the magnesium ribbon in half twice instead of dropping it in as a whole because i found that it would plunge into the mixture and one half would still be out because it was quite long, but by folding it I made sure that every part of the magnesium was reacting at the same time. * I am going to decide to lengthen the overall time period of the experiment to 1 and a half minutes. Problems: I found that when using the burettes, even though there was a 0.1M concentration difference between exp.1 (0.8M concentration) and exp.2 (0.9M concentration) the rate of reaction seems to have been similar (i.e. after 10 seconds both reactions produced 5cm� of gas. However if we take exp.2 (0.9M concentration) and compare with exp.3 (1.0M concentration) we find that after 10 seconds have elapsed, exp.3 had produced 10cm� of gas considering it is only 0.1M more concentrated. But before when I was using measuring cylinders to measure the amount of water and HCl this problem did not happen, I think there is an error here and this could have possibly been caused by measuring mistakes, I might have measured the wrong amount of HCl or water. But other than that and the anomalous run for 1.0M concentration, no other problems were found in my preliminary results. iii) RESULTS: Experiment 1 40cm� 2.0M HCl, 60cm� water 0.8M concentration Time (s) Run 1 (cm�) Run 2 (cm�) Run 3 (cm�) 0 0 0 0 5 3 2 3 10 5 4 6 15 8 6 8 20 10 9 10 25 13 14 13 30 15 17 16 35 17 19 18 40 19 20 20 45 20 21 21 50 21 22 22 55 22 23 23 60 23 23 23 Experiment 2 45cm� 2.0M HCl, 55cm� water 0.9M concentration Time (s) ...read more.


To me, an anomalous run is satisfactory and can be excluded and we can safely say that the experiment was reliable however an anomalous experiment is just extreme. It is as if saying that a fifth of my experiment was anomalous (as one of my five experiments was done improperly), which we can obviously say is simply not reliable as they have not shown to be completely consistent. Evidence of this is proven in the table where we see a very slight increase in rate as we increase from 40% to 45% acid concentration. However when we increase from 50% to 55%, and then from 55% to 60% we get a significant consistent increase. Also I have taken percentage variation calculations; Exp. 1: (after 10 secs) 5cm�/s � 2cm�/s % variation...2/5 x 100 = 40% Exp. 2: (after 10 secs) 5cm�/s � 0cm�/s % variation...0/5 x 100 = 0% Exp. 3: (after 10 secs) 9.333...cm�/s � 0.666...cm�/s % variation...0.666/9.333 x 100 = 7.14% Exp. 4: (after 10 secs) 11.333...cm�/s � 0.666...cm�/s % variation...0.666/11.333 x 100 = 5.88% Exp. 5: (after 10 secs) 13.333...cm�/s � 0.666...cm�/s % variation...0.666/13.333 x 100 = 5% As we can see we find some consistency in the final three experiments but we simply get very awkward data from the first two. We can further point out this evidence by looking at the "rate against concentration" graph, where I have circled one average rate which did not follow the graph trend; this is showing inconsistency as it does not match with what the rest of the data has shown us. So as I have said above, I am not happy with safely concluding this experiment due to the fact that I did not get the results that I was expecting and all my evidence showed inconsistency which I have to acknowledge. By looking at the graph we see we don't get the directly proportional linear line that we were aiming for and the fact that it was not a fair test simply made it difficult for me to conclude. 01.03.06 01.03.06 ...read more.

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