3. 10ml of 2M HCl, 10ml of water
4. 5ml of 2M HCl, 15ml of water
5. 20ml of water (this is the control)
Diagram
Results
These are the results we obtained for the experiment:
We also did another small experiment measuring the temperature rise during the reaction of the two chemicals. They were taken when experiment 2 was carried out:
Graph
Conclusion
Looking at the results obtained from the two trials of the experiment, I can deduce that the rate of reaction is affected by the concentration of the solution. When there is 20ml of 2M HCl acid with no water, the 2HCL + Mg → MgCl2 + H2 reaction is completed within around 36 seconds, but with just 5ml of HCl and 15ml of water, the reaction takes much longer, almost 700 seconds (~11minutes). This suggests that a lower concentration means a slower rate of reaction, and a higher concentration means a faster reaction. This is because there are more particles if there is a greater concentration of acid to ‘bump’ into the magnesium strip, causing it to chemically react and dissolve, and produce hydrogen gas. Adding water to the solution reduces the concentration, and this would explain the fact that more water with the solution gives a slower reaction.
Evaluation and Possible Sources of Error
Although we tried to keep the experiment as accurate and as fair as possible, some of the results were not very consistent. For example, for the two trials with 5ml of solution and 15ml of water, there was approximately 30 seconds difference between the two results. Some of these may be the reason why such mistakes happened:
The magnesium oxide surrounding the outer layer of the magnesium strip was not completely wiped off, and so the different strips of Mg ribbon did not have the same amounts of magnesium oxide on them.
The measurements are not exactly accurate (human error) - timing on stopwatch, length of ribbon, amount of acid/water. (Two of the five magnesium strips were 1mm shorter than the other three for Trial 1).
We may not have seen properly whether or not the fizzing had completely stopped or not, and could have stopped the timer when the reaction was not actually complete.
During one of the trials, we used a thermometer to record the temperature rise, as this reaction is an exothermic one. However, one time, the piece of magnesium stuck to the thermometer, and some time was wasted because then, the magnesium could not dissolve as it should have done. (This would explain the large difference in time for the solution with 15ml acid and 5ml water between the two trials).
As shown in the temperature experiment, the initial temperatures of all the solutions were different. Temperature is also a factor determining rate of reaction, so this could also explain inconsistent results.
What I would do differently if the experiment was repeated
If the experiment had to be done again, I would do the following differently:
Make the measurements as accurate as possible – make all the lengths of magnesium exactly the same, as well as volumes of acid and water.
Try and rub off all the magnesium oxide on the magnesium ribbon.
Try and make the initial temperatures of the solutions the same so that temperature is not a contributing factor of rates of reaction.
Make sure the thermometer does not get in the way of the reaction so that the results can be as accurate as possible.
Check and ensure that the reaction is definitely complete before stopping the timer.