I will be changing the concentration of acid/water, whilst maintaining the same total volume. I will do this so that I can see how fast or slow the particles react with different levels of acid. And here are the volumes of acid/water to be used in step 4.
Volume of Acid (cm³) Volume of Water (cm³)
50 0
40 10
30 20
20 30
- 40
Is it a fair test?
It is very important that this experiment is a fair test, other wise my results would be totally wrong. To make it a fair test I will make sure that the only factor of this experiment that actually changes will be the concentration of the acid/water. I will try to keep the temperature at room temperature because at a high temperature the reaction would gain more energy therefore more collision leading to a quicker reaction time and if at a lower temperature there would be an opposite effect, less energy therefore less collisions leading to slower reaction time. I will also try to keep the same total Volume/weight of the substances I am using so that there will not be more of one and less of another each time, so that this does not effect the chance of collision. By keeping all of these elements the same, I will hopefully gain a set of results, which have not been altered by a change in anything other than concentration.
Preliminary work
In order to check that all our experiment and equipment worked, we did some preliminary work. This is basically doing a test run of our experiment. In our preliminary test our aim was to check that there is a reaction between magnesium and hydrochloric acid, and that gas is given off when they react. Also to check that we used the right amount of magnesium and right amount of acid/water together, so that the gas wouldn’t be given off to quickly so we couldn’t record accurately or too slowly so that it couldn’t be completed in the amount of time available. And finally to check that all our equipment works properly.
If our equipment doesn’t work properly, we will simply use the same type of instrument but a different one. Then if the reaction is too fast, we will use a smaller volume of acid/water and if the reaction is to slow, we will use a bigger volume of acid/water. And if there is no reaction, I will change the substances I have used.
Well there was no trouble with the equipment, or the amounts of magnesium and acid/water, or with the substances I used.
Here are the results for the 1 molar acid I used in preliminary work are shown in the table below:
Preliminary tests – 40cm³ acid/10cm³ water and 0.10g magnesium
Time (seconds) Volume of gas (cm³)
O 0
10 20
20 33
30 45
40 57
50 70
60 80
70 86
80 87
90 87
100 87
From the set of results in the table above, we can see that the choice of 1 molar acid was a good choice as the amount of gas released is given off over a sensible amount of time. And it doesn’t go past 100cm³, which means I used the right amount of magnesium and acid/water so the gas produced was enough to move the syringe without going past 100cm³. The amount of magnesium I used was 0.10g; this was because it is an easy amount to weigh. Now I know what measurements and masses I will use in my experiment. The amount I have chosen has been carefully considered, because if I had too much magnesium and not enough water the reaction would be to fast, and would not give us enough time to record our results. But if we had too much water and not enough magnesium, the reaction would be to slow and might not fill the gas syringe enough. Also if I do not have enough time, I can do repeats of this experiment which might help improve the accuracy of this experiment.
Prediction
I predict that in this experiment the more I increased the concentration of acid, the more the rate of reaction will increase. I predict this because there will be more particles to collide with the same size surface of the magnesium, as the concentration of acid is increased. I know that if the solution is made more concentrated it means there are more particles of reactant knocking about between the water molecules, which makes collisions between the important particles more likely.
Results
Results when 0.10g of magnesium was used:
Analysis
From my results you can clearly see that the rate of reaction increases as the concentration of acid is increased. You can clearly see this from the
results listed in graph 1. The gradient of the line becomes steeper as the concentration is increased. This because when the concentration of acid was high, the gas was being made much quicker. When there was 50cl of acid only and no water, the amount of gas was produced very quickly i.e. 100cm of hydrogen was produced in only 60 seconds. After that time no more hydrogen was produced. This explains why the line levels off at 60 seconds on the graph.
Graph 2 represents the rate of the reaction. I have used the time it took to collect 20cm³ of gas as this was near the start of the experiment when the concentration and temperature had not changed much.
I was unable to do this for the 10cm³ of acid experiment, as 20cm³ of gas was not collected. If this line is straight it shows rate and concentration are proportional but if not, then they depend on each other but are not proportional. As my graphs line is straight it shows rate and concentration are proportional. This means rate α 1 .
time
My prediction was therefore correct, cause in the experiment as the concentration was increased the speed at which gas was let off increased. This is because the concentration affects the rate of reaction because the more concentrated acid, the more successful and frequent the collisions will be, because particles are closer together.
More concentrated acid, so particles hit the magnesium more frequently.
Less concentrated acid, so particles not hit the magnesium as frequently.
Although I did not measure the temperature of the reaction, I know it was an exothermic reaction. I know this because I felt the side of the conical flask during the experiment, which had risen in temperature. So it was giving out heat, and exothermic reaction is when heat is given out in the reaction. The reaction being exothermic could have also lead to and increase in rate of reaction cause when particles get warmer they have more energy, so they move quicker and so the will be more successful collisions. Here is the diagram to show what happened as it was an exothermic reaction:
Evaluation
I believe that the experiment was successful but some of the results were unexpected/unreliable. The line on the graphs for 10ml acid/40ml water had anomalous point, but other than that all other results were reliable.
Other factor which could have given me unreliable results could have been that the gas syringes were wet causing them to jam and so not giving correct results or that the bung was not placed on the top of the conical flask fast enough which allowed gas to escape.
Most points were on the best fit lines showing definite trend suggesting accuracy. There were 2 points less accurate, not fitting the best fit lines and I have circled these odd points. The gas syringe sticking might have caused these, when points were below the line. The other for when above the line could have been caused, just by the amount of gas going up in the time it took me to look from the stop watch to the gas syringe.
To improve the experiment I would find a way of attaching and releasing the magnesium inside the conical flask above the acid (with a bung at the top of the conical flask) so that the magnesium could be dropped into the acid without any gas being lost.
I have thought of two ways in which I could do this. The first is to put the magnesium between the conical flask and rubber bung, then when ready to start. Quickly lift the rubber bung out and back in, quickly enough so I will not lose any gas.
The second is the put the conical flask on its side, and have the magnesium on the raised part and acid in the dip. Then when ready to start, just stand the conical flask up.
Additional work that could be carried out is to repeat the experiment using, hydrochloric acid of different mole content. The investigation could also be extended to investigate other factors affecting the rate of reaction such as catalysts, temperature or particle size of the magnesium.
The range of results seems fair and covers a good range as they spread evenly between the graph axes. But as the gap between 20ml and 30ml of acid and 30ml and 40ml of acid is quiet large, I think next time I might be able to go in between the results. This could be done by going up in fives instead of tens. But none below 10ml of acid, as the reaction would be too slow to be accurate enough. Also no higher than 50ml of acid, as the reaction might be too fast to record the results.
I would also next time do repeats as this could help my experiment to be more accurate and would make sure I had the right results. Also in my results, I had an anomaly which might not have been there if I repeated the experiment and noticed the mistake, which I could have then changed. I feel repeating the experiment five times would be a good idea, even if it took up a lot of time. Because I would know my results were totally correct and I would not have any anomalies.
If I had included more concentrations, I could have got more points on the ‘rate’ graph, which would show the shape more clearly. It might be better to put points between more experimental ones not extend the range, as they would give very fast or slow readings which are not easy to measure accurately.
