I will use 10cm of magnesium ribbon for each separate reaction, and I will rub each piece with sandpaper beforehand, in order to remove magnesium oxide which will have formed on the surface. This amount is an excess, because I do not want the concentration of the magnesium to change significantly during the reaction. Although this would still be a problem with the amount of magnesium I am using if I was letting the reaction carry on for a longer time, but since I am only interested in the initial rate of each reaction, the change in concentration of the magnesium will be negligible.
I will measure the volume of gas produced over time for each acid at each concentration. In this way I will be able to calculate the rate of each reaction, and hence determine the order of each reaction. I will use 10 seconds intervals for my measurements.
To carry out this procedure I will use the apparatus as shown in the diagram below.
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Equations
For hydrochloric acid:
Stoichiometric 2HCl(aq) + Mg(s) → MgCl2 (aq) + H2(g)
Ionic 2H+(aq) + Mg(s) → Mg2+(aq) + H2(g)
For sulphuric acid:
Stoichiometric H2SO4(aq) + Mg(s) → MgSO4(aq) + H2(g)
Ionic 2H+(aq) + Mg(s) → Mg2+(aq) + H2(g)
For ethanoic acid:
Stoichiometric 2C2H5OH(aq) + Mg(s) → Mg(C2H5O)2(aq) + H2(g)
Ionic 2H+(aq) + Mg(s) → Mg2+(aq) + H2(g)
I predict that as the concentration of each acid is increased, the rate of the reaction will also increase. This is because when there is a higher concentration of acid, there will be more collisions per second than at lower concentrations, and so the reaction will occur at a faster rate. I also predict that the order of reaction with respect to hydrochloric acid, and also ethanoic acid will be first order. This is because they both have one proton to donate per mole of acid. Although both of the hydrogen ions on the sulphuric acid can dissociate, which originally led me to believe that the order of reaction with respect to sulphuric acid will be second order, the ionic equation is the same as for hydrochloric or ethanoic acid, and so I believe that the rate with respect to sulphuric acid will also be first order.
Implementing:
All of the experimental work involving hydrochloric and sulphuric acid went very smoothly, and my results appeared to be sound. However, I did repeat several of the reactions in order to make sure that my results were repeatable. The results of these repeats were almost exactly identical to the originals, and so I felt no need to change them. I also repeated all of the experiments involving ethanoic acid, as I was unsure whether or not my results were accurate. The graphs of 1.5M and 2M clearly showed an increase in rate over time. However, repeating the experiments simply backed up my original ones.
Results of the reaction between hydrochloric acid and magnesium:
Columns 2, 3, 4 and 5 show the volume of gas (in cm3) produced when different concentrations of hydrochloric acid are used in the reaction with magnesium.
0.5M hydrochloric acid
1M hydrochloric acid
1.5M hydrochloric acid
2M hydrochloric acid
Results of the reaction between sulphuric acid and magnesium:
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 sulphuric acid
1M sulphuric acid
1.5M sulphuric acid
2M sulphuric acid
Results of the reaction between ethanoic acid and magnesium:
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 gradient = 30/105 = 0.286
Initial rate = 0.286
1M: Initial gradient = 65/80 = 0.813
Initial rate = 0.813
1.5M: Initial gradient = 120/46 = 2.61
Initial rate = 2.61
2M: Initial gradient = 120/33 = 3.64
Initial 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.
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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.