Before Reaction After Reaction
Variables:
Every experiment has a number of variables. In order to ensure that it is a reliable experiment, all variables must be kept constant, apart from the variable being used to investigate the rate of reaction. I am using concentration of acid to investigate the rate of reaction. Other variables that must be kept constant are mass of magnesium, surface area of magnesium, the volume and temperature of acid
To make certain that the results will be precise, I will keep the temperature of the reaction at room temperature, although I am aware that this is subject to change, but probably not enough to affect the results greatly. The mass and surface area of magnesium also needs to be controlled, I will make sure each piece of magnesium ribbon is 4 cm long and a very similar mass to each other as is possible, however it is difficult to measure and monitor the surface area. I can easily measure the volume of acid used, and maintain the same volume for each test, using accurate measuring cylinders.
Method:
- Put on safety glasses to protect eyes from harmful acids.
- Fill a washing up bowl half full of water, then fill a 100ml measuring cylinder full of water and submerge the top end in water, making sure no water escapes and no air gets in.
- Put a boiling tube in a test tube rack and put a delivery tube bung into it. Place the end of the rubber tube into the washing up bowl and put it under the measuring cylinder.
- Measure out 0.5M of Hydrochloric acid using a measuring cylinder, a pipette and a beaker(see chart below) and pour it into the boiling tube.
Step 5. Measure out 4cm of magnesium ribbon and drop it into the boiling tube, being careful that the bung is replaced and the stopwatch is started as soon as the bung is put on.
Step 6. Observe the measuring cylinder, and stop the stopwatch when the volume of Hydrogen gas reaches 30cm3. Record the time it took to reach 30cm3. This information shows haw fast each reaction is taking place.
Step 7. Repeat steps 1-6 another 2 times to get accurate results and ensure a greater reliability
Step 8. Repeat steps 1-7 but in step 3 replace 0.5M Hydrochloric acid with the following different molars:-1M, 1.5M, 1.75M, 2M, use the table to make the different molars.
Results:
Analysis of results:
The results shown above conclude that as the concentration of hydrochloric acid was increased, the rate of reaction also increases. However I predicted that the as the concentration was doubled, the rate of reaction would also double, this has not happened. I also predicted that when the concentration was doubled, the time taken for 30cm3 to be produced would be halved, again, this did not happen.
The results for the 1.5M acid had to be calculated because the magnesium strip was dissolved before 30cm3 of hydrogen gas had been produced. This leads me to believe that the line of best fit on the graph may have been affected by this anomaly, as the calculation I made did not take into account the fact that reactions are usually slower towards the end because there are less molecules to have a successful collision.
I think the main reasons for my results not matching my prediction is the fact that it is difficult to keep variables stable, the most complicated to keep constantly the same was the surface area of each piece of magnesium ribbon because magnesium has a light mass and a top pan balance would not give an accurate enough reading. This means that each strip of magnesium has a diverse number of molecules.
Therefore there are different chances of a successful collision for each piece which means that even when reacting with the same molar acid, the results of each time will be noticeably different. The results from 0.5 molar acid show this well as there is a difference of 1.79 minutes, this shows a great variation in the amount of molecules in each magnesium ribbon.
However, the evidence does show some support to the prediction, this is that I predicted that as the molar of the acid increased, the time taken to produce 30cm3 of hydrogen would decrease and the rate of reaction would increase.
Conclusion:
Due to the evidence I obtained, there were some significant flaws to my experiment, and the way I gathered my results. If you take the way that the points were plotted on the graph, it is obvious that something went wrong. Either the results obtained from 1M or 1.5M are inaccurate. And because of the problems experienced when performing the test for 1.5M I believe it is this result that is an anomaly.
In the experiment we found that when we placed the magnesium ribbon in the boiling tube sometimes part of the ribbon got stuck to the side and so only part of it was submerged, and so only part was dissolved, this meant that I had to repeat certain test additional times, wasting time and causing me to rush later tests. I learnt from this mistake and from then on I rolled the magnesium ribbon into a roll so it did not stick and fell better, this should have been mentioned in the plan and done from the beginning so that results were more accurate.
As mentioned in the results section, in one set of tests, 1.5M, the results had to be calculated because the magnesium had all been dissolved before 30cm3 of gas had been produced. One way that problems such as this could have been avoided is to have collected the results differently, e.g. recorded the time taken for the magnesium ribbon to dissolve each time. This would have ensured that each test was completed without running out of either chemical. Another way to ensure that the magnesium would not run out is if I had used a longer strip of magnesium, such as 6cm instead of 4cm and still used the method of recording time taken to produce 30cm3 of hydrogen.
One further problem that may have caused results to be inaccurate is the fact that the bung has to be put into the boiling tube after the acid has been added, this means that the reaction has already started before it is being timed, therefore the time does not include the gas that was produced before the bung was put in, this means that in tests with higher M acids, there may not be enough gas produced. Ways to overcome this problem are mentioned in the above paragraph.
In conclusion, there are many ways in which an experiment can go wrong, but if thought out properly, these can be overcome and a successful experiment with accurate results can be the outcome. If I did this same experiment again, I would use more magnesium and time how long it takes for the magnesium to be dissolved by the acid, rather than time how long it take to produce 30cm3 hydrogen gas. This means the calculated rate of reaction will be more accurate and a better set of results will be given. Also in an ideal environment I would have more time, so that I can repeat each test at least 5 times, to give a better average.
Laura Barnes
Equipment:
Boiling tube
Delivery tube
Washing up bowl
100ml measuring cylinder x2
Test tube rack
Stopwatch
2M hydrochloric acid
Magnesium ribbon
Safety glasses
Beaker
Pipette
