Reactions may take place at very different speeds - Sample Essay
We need to understand what factors determine the rate of a reaction, and hence how to change the rate of a reaction. We then have the ability to control reactions more, and in industrial reaction the rate can be altered to suit the economics of certain manufacturing processes. Knowledge of how the rate of a reaction is determined is very important in providing evidence of that particular reaction mechanism. In this investigation, I am going to investigate how the order of reaction with respect to the acid varies when magnesium is reacted with acids of different types.
I am also going to determine how the concentration affects the order and rate of the reaction. As a preliminary experiment, I took sulphuric, hydrochloric and ethanoic acid, and reacted them separately with magnesium powder. I used a conical flask which was connected by a rubber tube to a glass syringe, so that the amount of gas produced could be measured. This was simply a test to see roughly the quantity of gas produced by each, so that I could decide on appropriate concentrations for the acids, and also suitable time intervals at which to take the measurements.
However, I found that when using sulphuric and hydrochloric acid, the reaction took such a short time before the whole 100cm3 syringe was filled, that it was impossible to accurately measure the rate. I decided to use instead of magnesium powder, magnesium ribbon, which has a much lower surface area, and so the reaction would be slower. I could have lowered the concentrations of the acids, but since I was using only 2M solutions and below already, the acids would have to be scaled down to such a low concentration to produce measurable results, that it would have become very inaccurate.
I have decided to use hydrochloric acid, which is a strong mono-basic acid, sulphuric acid, which is a strong di-basic acid, and ethanoic acid, which is a weak mono-basic acid. This provides a wide variety of acids for comparison. Each acid will be reacted with the magnesium at 0. 5M, 1M, 1. 5M and 2M concentrations. 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. 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: time/s 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.