Method: To investigate how the rate of reaction changes with the concentration of the acid, I will use a stopwatch to record how long the reaction takes with different concentrations of hydrochloric acid. I will take it that when I can no longer see the magnesium ribbon that the reaction has finished.
To make it a fair test I must keep the following the same: volume of acid, weight of calcium carbonate, make sure the test tubes are washed thoroughly and make sure that the results are not affected by human error i.e. inaccurate measuring or observation of the time taken to react.
To make this experiment safe, I will wear goggles to protect my eyes. I will also stand up to ensure that, should acid be spilt, I will be able to move out of the way quickly.
- Set up 6 test tubes containing the following acids (15ml) in a test tube rack: 1, 1.5, 2, 2.5, 3, 3.5 and 4 molar.
- Add magnesium ribbon to each concentration of acid and start the stopwatch
- When the reaction stops, stop the stopwatch
- Record results in a table
Diagram:
Results:
Conclusion:
From the results table and the line of best-fit graph, you can see that the results did match my prediction in the form of a curved graph line. However, there were two anomalous results in the form of the 3.0 and 3.5 concentrations. After a repetition of these two (below) I deduced that something must have been wrong with the original test
This abnormality of results could have been anything from the following:
- Cross contamination of acid
- Human error (timing)
- Higher room/acid temperatures
Of these three, I suspect the first one was the most likely occurrence. In a classroom environment where acid is being shared between groups, there is a chance that acid could get cross contaminated.
Graph:
Line of best fit:
Evaluation:
I worked well during this investigation. My prediction came to be proved correct and I have shown that the concentration of acid affects the rate of reaction.
However, if I were to do this experiment again, I would make the following two changes:
- Conditions: I would use a fume cupboard so that I knew for sure that there was no way that my investigation could differ each time.
- Equipment: I would weigh the magnesium using the most accurate pair of scales so that I knew that the weight would be exactly the same each time.
Another factor that I could investigate in future, is the fact that when the reaction takes place, carbon dioxide is given off, there for the mass will get lighter. If I used a mass balance and put on it a flask of HCL and CaCO3, the flask would get lighter. Furthermore, if I were to connect the mass balance to a computer, I could record the changes and program the computer to produce a graph automatically.
The problem with doing the experiment in classroom environment is that there are many ways in which the experiment can be tampered with so that it is not accurate. Ideally all experiments would be done in a controlled laboratory using computers that can be programmed to do the same things exactly the same over and over again. Also computers can measure time into fractions of a second.
Taking all of this into account I cannot say that my conclusion can be scientifically proved. The conditions were not the same each time and, although I tried hard, experiments in a classroom can never be repeated exactly the same each time.