Columns 2, 3, 4 and 5 show the volume of gas (in cm3) produced when different concentrations of sulphuric acid are used in the reaction with magnesium. 0. 5M ethanoic acid 1 M ethanoic acid 1. 5 M ethanoic acid 2 M ethanoic acid Concluding and Evaluating:
From the graphs I have produced using my results, I can find the initial rate of each reaction. I will do this by working out the value of volume of gas produced / time for the initial gradient of each graph. Knowing the rate of reaction at different concentration, I can find the order of the reaction by plotting concentration against rate. A straight line would show that the reaction was first order, and a straight line when concentration2 is plotted against rate shows that the reaction is second order. Hydrochloric acid: 0. 5M: Initial gradient286 Initial rate = 0. 286 1M: Initial gradient = 65/80 = 0. 813.
Initial rateInitial rate = 3. 64 We can now see from these graphs that a straight line is obtained from the graph of rate against concentration2, therefore this reaction between hydrochloric acid and magnesium is second order with respect to hydrochloric acid. Sulphuric acid: Initial gradient = 70/47 = 1. 49 Initial rate = 1. 49 Initial gradient = 100/32 = 3. 13 Initial rate = 3.
13 Initial gradient = 120/31 = 3. 87 Initial rate = 3. 87 Initial gradient = 100/18 = 5. 56 Initial rate = 5. 56 We can now see from these graphs that a straight line is obtained from the graph of rate against concentration2, therefore this reaction between sulphuric acid and magnesium is second order with respect to sulphuric acid. Ethanoic acid: Initial gradient = 18/100 = 0. 180 Initial rate = 0. 180 Initial gradient = 15/78 = 0. 192 Initial rate = 0. 192 Initial gradient = 15/83 = 0. 181 Initial rate = 0. 181 Initial gradient = 20/94 = 0. 213.
Initial rate = 0. 213 I have removed one of the points which would have been on the graphs involving ethanoic acid, because it greatly distorted the graph and was clearly an error. Having only three points on the graph makes it very difficult to make a conclusion about the order of this reaction. However, I believe that this reaction has exactly the same rate determining step as the hydrochloric and sulphuric acid reactions, for reasons which are explained below, and so this reaction will also be second order with respect to ethanoic acid. Mechanism:
Now that I know that the reaction between hydrochloric acid and magnesium is second order with respect to the hydrochloric acid, I can make some conclusions about the rate determining step. There must be two moles of H+ ions in this step, and looking at the ionic equation for the reaction, 2H+(aq) + Mg(s) ? Mg2+(aq) + H2(g) it is clear that this must be the rate determining step, and as this step takes the reaction to completion, it must also be the only step in the reaction. The same theory applies to the reactions of sulphuric acid and ethanoic acid with magnesium, as they all have the same rate determining step.
Techniques: During my experiments, I noted a few operations which may have contributed to error, and which I would address if I were to perform this investigation again. After adding the acid to the magnesium inside the conical flask, there is a small interval before the stopper is put in when the gas can escape, below is a diagram of a better setup which would bypass this problem. The acid is poured into the separating funnel with the lower stopper shut, then the stopper can be opened to release the acid onto the magnesium, this provides a far more accurate and efficient way of adding the acid.
As the reaction takes place, a small amount of acid spray is thrown up, and this can get in between the syringe and its containing cylinder, causing friction. Twisting the syringe can help to prevent the syringe from becoming stuck, but it still does not run smoothly, which could be affecting the results. Taking the syringe out and cleaning it with a dry cloth would help to minimize this effect. I would also use more different concentrations, so that when it came to plotting the graphs of rate against concentration, there would be many more points and so the graphs would be more accurate.