A rate of reaction can be increased by either increasing the total number of collisions which take place or by increasing the number of effective collisions.
Safety:
While doing these experiments we must consider the safety factors. I must wear safety goggles at all times because the salt solution may get in our eyes and be irritating. I must wear a lab coat in case some of the salt solution gets on my skin. I must not eat or drink in the lab as this is unhygienic.
Preliminary Method:
Apparatus:
A raw potato
5 plastic cups
White tile
Cutting machine
Forceps
Diagram:
Preliminary experiments
We completed our preliminary results and found that we had to make a change to our original plan. We did an experiment with four centimetres of magnesium and three molar of hydrochloric acid. The reaction was over within ten seconds and was far too quick for us to measure the amount of gas collected. We did the same experiment but instead of four centimetres of magnesium ribbon, we used six. This was also too quick to measure. We then did eight centimetre of magnesium ribbon and this was a measurable speed so we decided to use this.
We then did the experiment for 8 cm magnesium ribbon and 1 molar hydrochloric acid. We needed to try the highest and the lowest concentration of hydrochloric acid and if they worked then the ones in between would work and we wouldn’t have to test them.
For the experiment for three molar we had to measure the volume of gas every three seconds because the reaction was over very quickly and if we had measured the gas every ten or twenty seconds then we wouldn’t have enough points to plot a graph. We tried to measure the experiment for one molar every 3 seconds but the reaction was way too slow and we had far too many points to be able to plot them on a graph.
At the end of our preliminary experiments we found that we could use a range of 1 to 3 molar and 8cm of magnesium ribbon.
Preliminary results:
These are the results for 3 molar hydrochloric acid and 8cm magnesium ribbon.
We can see from these results that although the reaction is relatively fast, it is not so fast that it is impossible to measure the rate of reaction.
These are the results for 1 molar hydrochloric acid and 8cm magnesium ribbon.
Method for real experiments:
We are using the same method for our actual experiments as we did for our preliminary experiments.. We will measure out a piece of magnesium ribbon which is eight centimetres long. We will then measure out the hydrochloric acid and water solution which will always have a volume of 30 cm . We will put the hydrochloric acid into the side armed flask. We will connect the calibrated gas syringe to the side armed flask. We will place the strip of magnesium ribbon into the side armed flask at the same time as putting in the bung and starting the stop clock. We will time how long the reaction takes until it is stopped and record the results.
The only things we are changing are we are making is to make the magnesium ribbon a zig zag shape because when we had it straight we found that it didn’t totally fit in the side armed flask so that it was all covered in hydrochloric acid. This meant that only one half of the magnesium was reacting. We couldn’t fold the ribbon in half because then the insides of the magnesium could not react. The other thing we have changed is the length of the magnesium ribbon, so instead of being 4cms, it is 8cms.
Fair test:
There are several factors that we have to consider to make the experiments a fair test. These are:
- Concentration of the hydrochloric acid
- Temperature
- Volume of the acid
- Length, width, depth of the magnesium
- Presence of a catalyst
- Curled or straight magnesium
The things that we must keep the same or change:
1. The concentration of the hydrochloric acid is the only thing that we change.
2. Temperature stays the same.
3. The volume of the acid stays the same.
4. The length and width and depth will all be the same, because we can measure the length and the width and depth will be the same because all our magnesium will be taken from the same roll of magnesium ribbon.
5. There will be no catalyst present.
6. We will have straight magnesium ribbon.
Observations:
We have drawn up the tables of the results we got from the experiments. We have also done graphs. The tables show the time against the volume of hydrogen, and the graphs show the same thing, with a best fit curve. We can see that when the curve on the graph goes up, the volume of the hydrogen is increasing, but the rate of reaction is decreasing.
Results:
Results table 1 (3 molar)
Results table 2 (2.5 molar)
Results table 3 (2 molar)
Results table 4 (1.5 molar)
Results table 5 (1 molar)
Analysis:
We did the experiment and we have made tables showing the time taken for the reaction, and the volume of hydrogen collected at each of these times. We also drew graphs showing the same thing and with a best fit curve. With the graphs it is possible to find the volume of hydrogen collected at any time, not just at the times which we measured. We can do this by going up to the curve at the time we want to find the volume of hydrogen for (e.g. 7 seconds) and then going across to find the volume of hydrogen at that time. We found the rate of reaction using the graphs. We drew a tangent to the graph right at the beginning as it is the initial rate that we are interested in. Then we joined the lines to make a triangle. Then using ‘rise over run’, we found the gradient, which tells us the rate of reaction.
E.g.:
Experiment 1: Rate of reaction = rise = 46.5 = 1.86
run 25
I then drew another graph on which I plotted the concentrations against the rate of reaction. I drew a best fit curve which went through all the point, except two of them. At 1.5 the point was just above the graph and at 2.5 the point was below the curve. I put this down to experimental error.
Points read off the graph:
My prediction was that if we doubled the concentration of the hydrochloric acid then the reaction rate would double. We are going to use a range of 3 to 1 molar so we have two doubles which are 1 and 2 and 1.5 and 3. This means that our results will be directly proportional. From looking at our results, and the table of concentration and rate of reaction, we can see that the rate of reaction did not double when the concentration doubled. This is because the formula for this experiment is:
Mg + 2Hcl → MgCl + H
There is one magnesium particle reacting with two hydrochloric acid particles. It is not possible for three particles to react at the same time, so the magnesium reacts with the first hydrochloric acid particle and that rate of reaction is doubled. Then the ones which have just reacted, reacts with the second hydrochloric acid particle, and that rate of reaction is also double, so it makes it quadruple.
We can see that in my results the first doubled concentration (one and two) quadrupled. This means that my prediction is partially correct because I said that the results would be directly proportional, and they are. The second doubled concentration (1.5 and 3) have not doubled, or quadrupled so they do not agree with what my prediction says. This means that there must be some experimental errors in these experiments, as the first one did partially agree with my prediction. I will talk about these experimental errors in my evaluation.
Evaluation:
We got these results by doing each experiment three times. We did each one three times because it is a lot more accurate to repeat each one, that to just do one experiment. This is because if we had only done one experiment and we had made a mistake which we didn’t know about, then the results would have been wrong. By repeating the experiments three times we can check to see that all three sets of results are relatively similar, as well as taking an average to plot the points on a graph. It would have been more accurate however, to repeat each experiments more times for example we could have repeated each one five times instead of three. There is some experimental error. There are some reasons for this. We used an 100cm measuring cylinder, and the error of this is always plus or minus 1cm . To make this more accurate we could have used a burette. This is more accurate and the error is only plus or minus 0.1cm .
We can see from the five graphs that we have drawn, that there are some anomalous results. We do not have very many anomalies which mean that the experiments were quite accurate. In graph 2 there is an anomaly at 9 seconds. It is just below the curve so it could have occurred because there was just slightly too little hydrochloric acid to make up the right concentration, in one of our three repeats of this experiment. This would mean that it brought the average down for this time. There is another anomaly in graph 3 which probably occurred for the same reason. There are however other reason why these anomalies may have occurred. The magnesium ribbon, although always the same length, may not be the same width and also may not have weighed the same. There are also two anomalies on the graph on which I plotted rate of reaction against concentration, at 1.5 molar and 2.5 molar on the x axis. These are dues to some sort of experimental error.
Another reason is that possibly the concentration of the hydrochloric acid was slightly different each of the three times we repeated each experiment. With the highest concentrations of hydrochloric acid, the reaction was over very fast, and the initial rate, which is what we are most interested in, was so fast that it was hard to measure the volume of hydrogen collected at exactly regular intervals. We might have been a fraction of a second out and that would have made a lot of difference to each individual result.
I think that it altogether the procedure used to carry out the investigation is quite accurate and suitable. There are thing which could have been done to improve it. There are many limitations to this experiment because it is difficult to be exact with such limited apparatus. It is difficult to put the magnesium into the hydrochloric acid at the same time as putting the bung in and starting the stop clock. This means that some of the magnesium may already have reacted with the hydrochloric acid before it was possible to put the bung into the side armed flask.
To find out more relevant information, we could have used a wider range of concentrations as then we could have seen more proof of our prediction being partially correct.
I think that altogether the results are quite good and accurate because I have so few anomalies, and this is proof that it is quite accurate, because we can see where the inaccuracies are (where there are anomalies) and there aren’t many.
