The volume of the acid must also be kept constant so the magnesium gets the same amount of acid every time, so that it does not become a factor that changes my results as I am measuring the rate of reaction between hydrochloric acid and magnesium. I have considered these elements so they do not create anomalous results.
Preliminary work:
I practised a preliminary experiment and I found rudiments to my final experiment. Firstly I had to find a suitable length of magnesium ribbon which was to stay the same length through out the investigation. Secondly, the volume of acid, which is to react with the magnesium ribbon. The size of the ribbon that I finally have decided to use in my investigation is 11.5cm. This size of ribbon will give off a maximum of 100cm3 of hydrogen gas when it reacts with the 30ml of the strongest acid, 2 Mole.
The measurement of 11.5cm ribbon and the volume of acid were found out by the method of trial and error. The reason that I want the maximum amount of gas produced 100cm3 is because this is all my gas syringe can hold. Not all acid will reach the maximum limit of gas so the reading will be taken of the gas syringe when the reaction has stopped.
To find out which concentration is the fastest reactant, I will need to time the experiments. From the process of trial and error, I have decided to take a reading (volume of gas) of the reaction every 15 seconds. The reason for this particular figure is because some reactions will take place very quickly and others will take a long time. My 15 seconds interval balances out and works well with all four concentrations.
Measuring the speed of reaction
A reading will be taken every 15 seconds from the gas syringe until the reaction has stopped between the acid and magnesium.
Reliability and accuracy: To make my results reliable, I have repeated each experiment three times. This will give me an indication of any anomalous results. I intend to find the range of my results to find out their reliability. To make my results accurate, three people from the group measured the gas at exactly the same time in case any one of them miscounted, we all came up with same results.
In this experiment the range of measurements that I will be taking are going to be 100cm3 hydrogen gas produced and is timed for how long the gas took to reach its maximum potential volume from the different reaction. Each will be repeated three times for reliable results.
Apparatus:
- Gas syringe
- Tissue paper
- Beaker
- Four different concentrations of hydrochloric acid, 2M, 1.5M, 1M & 0.5 M
- Clamp stand
- Stopwatch
- Twelve 11.5cm magnesium ribbons
- Conical flask with side nozzle/opening
- Rubber pipe
- Rubber bun
- Measuring cylinder
- Ruler
- Safety goggles
- Thermometer
- Bench Mat
Diagram:
Method:
- Gather all apparatus
- Make a results table
- Throughout the experiment, a pair of goggles must be worn, as there is a lot of handling of acids.
- Assemble apparatus: Firstly clamp the gas syringe about half way. Make sure that there is some tissue paper between the clamp and the syringe as this stops the fragile gas syringe from cracking. A safety measure.
- Link the conical flask via the side opening/nozzle to the gas syringe with a rubber pipe. Push the plunger of the syringe to 0.
- Place the bench mat under the conical flask in case of any spills of acid.
- Now measure 30ml of the strongest acid, 2M, with a measuring cylinder and carefully pour it into the conical flask. Record the temperature of the acid to keep it constant, put in the results table.
- Get a piece of 11.5cm magnesium ribbon in one hand and a rubber bun to seal the conical flask to stop the gas escaping in another hand. Get another member of the group to standby with a stopwatch
- Drop in the magnesium ribbon and shout for the stopwatch to be started the moment the magnesium hits the acid. At the same time, quickly seal the conical flask with the rubber bun, allowing minimum hydrogen from escaping.
- The magnesium and the acid should be reacting.
- Take a reading off the gas syringe every 15 seconds until the reaction stops between the acid and the magnesium ribbon. The reading should be put in the results table.
- Repeat the whole experiment three times in total.
- Repeat stages 9-10 three times with acid of 1.5 M
- Repeat stages 9-10 three times with acid of 1 M
- Repeat stages 9-10 three times with acid of 0.5 M
- Make sure that all of the results table is filled in and the average of the results are worked out. Safely clear up all the apparatus
Safety aspects to consider:
- Always wear safety goggles when handling acids.
- Get more people to help set up the apparatus to avoid breakage of gas syringe, etc.
Observation:
Analysis:
From the results in the table and the graph a trend is clear that there is a steady increase in the rate of the reaction as the concentration of the acid is increased. 2M, was the fastest to react with the magnesium. Then were 1.5M, 1M and finally 0.5M.
Total reaction times:
2M: 30 seconds
1.5M: 45 seconds
1M: 75 seconds
0.5M: 300 seconds
Conclusion:
My results prove the following points. The stronger the acid, the quicker the reaction takes place. This is because by having a stronger acid, there are a lot more hydrochloric acid molecules present, which have a greater chance of colliding and therefore reacting with the magnesium. By adding more hydrochloric acid molecules we even increase the energy, as by adding more molecules of acid, we are adding more energy.
My prediction was correct when I mentioned that the higher the concentration of hydrochloric acid, the faster the reaction would take place. The scientific knowledge highlighted in my plan corresponds to the interpretation of the results from the experiment. The time taken by 2M acid took 30 seconds, which was the fastest, whereas 0.5M took 300 seconds, which was the slowest.
The only thing that did not go according to my prediction was that the concentration of the acid is proportional to time taken for reaction. If we double the concentration of 1M to 2M, the reaction time does not become half as quick, i.e. 1M = 75 seconds
If proportional should equal 2M = 37.5secsonds
Actual result 2M = 30secsonds
Gradients:
I have worked out the gradients of the graphs of the results.
Gradient = y
x
-
2M: 7.3
-
1.5M: 2.7
-
1M: 2.3
-
0.5M: 0.5
The gradients show that as the concentration of acid, mole, increases, the gradient increases. We can see by the results that that even though the higher the concentration, the higher the gradient but there are not at all proportional.
-
2M: 7.3 + 4.6
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1.5M: 2.7 +0.4
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1M: 2.3 The gradients are not proportional
-
0.5M: 0.5 + 1.8
Evaluation:
The experiment was quite fair. My results are accurate because I took as many measures as possible to make them accurate. Three people did the reading at one time for the amount of hydrogen gas produced. My results are reliable because everything was repeated three times and the results agreed with each other. The difference between each result was rather small, but could always be improved upon. I think the experiment was successful because the results proved the prediction. This means that the plan was devised suitably to at least a minimum satisfactory standard. There were a few problems during the practical. There was equipment failure, but this was remedied and we continued as normal. (It was the gas syringe that we experienced a problem with).
My results are accurate enough to make a firm conclusion.
The experiment was suitable but a few problems did occur. It was not very practical to keep the hydrogen gas completely sealed in the flask as some did escape when the rubber bun was being placed. To overcome this problem, I would have the bun already placed on the conical flask but with a small hole through, enough to pass through the magnesium ribbon. This hole in the bun would mean that there would be a smaller hole for the gas to escape, which can easily be covered with a thumb right after the magnesium is through.
Even though I tried my best to keep the temperature constant there is always a room for criticism. There was a one degrees centigrade increase at the end of each procedure due to transfer of body heat. Fortunately this happened to the whole investigation so I did not encounter any anomalies due to this factor. Now my results may not be too reliable because the reaction received extra heat, which meant that the hydrochloric acid molecules had more energy to react.
I personally think that 15 seconds interval before taking a reading was not suitable for the first concentration, 2M, as the reaction was taking place very quickly. If I were to perform this experiment again, I would give the first concentration at least 10 seconds of interval before taking a reading.
There was a result that I did not trust. On the results table on the last concentration, 0.5M, the gas produced from 120 seconds-135 seconds stays the same at 61cm3. This result arose when I dropped in the magnesium strip but it landed vertical and not all of the acid was able to react with the magnesium strip. I had to shake the flask which eventually made the ribbon lay flat at the bottom of the flask.
To overcome this problem, I would fold the magnesium ribbon before dropping it in the conical flask. The following diagram shows the situation.
I would suggest improvements to this experiment by:
- The repetition amount could be increased. This would improve the reliability of results.
- Undertaking the entire experiment in a water bath, ensuring that the temperature would not fluctuate.
- Widen the range of concentrations, so that there is a wider base of results to analyse.
- Cut the need for human involvement by using electronic measuring devices. This would eliminate much sign of ‘human error’.
My prediction came out correct, that fact that stronger acid increases the rate of reaction. But I was wrong about the results being proportional. I thought that by doubling the concentration we double the molecules of the acid, but this is not true as proven by my results.
I would further expand on this experiment on by using zinc instead of magnesium. The same plan would be used but the refinement, which I have realised and identified in my evaluation, would come in circulation. By undertaking this experiment again, but with the new amendments and one major change of metal, I will be increasing the reliability that I can successfully prove my prediction correct.
From scientific knowledge I know that I cannot use anything more reactive than Magnesium, because of reaching unsafe levels of safety. Also I could not use anything less reactive than lead. Zinc lies near the middle of that reactivity series, so it was the obvious option.
Ehtasham Junaid 11GD1
Fitzalan high school
Tutor: Mr Dennis
