So the easiest factor for me to investigate would be changing the concentration of sulphuric acid. This is quite easy as the actual experiment is quite straightforward and I can make my own dilutions of sulphuric acid.
Variables
The independent variable is what I am going to investigate; I will do so by changing the concentration of the acid.
The dependant variable is what I will be measuring, that is the time taken for all the magnesium to dissolve.
The controlled variables is what are going to stay the same to keep the investigation fair; i.e. the temperature of the acid, the temperature of the water, the total volume of acid solution, constant swirling and the size of the magnesium ribbon.
Prediction
My hypothesis is that the higher the concentration of sulphuric acid the faster the rate of reaction with the magnesium ribbon.
I have predicted this because:
All acids contain hydrogen, and hydrogen dissociates when it dissolves in water to produce ions. Sulphuric acid is one of the acids that dissociates completely to form a strong acid. Sulphuric acid will be very acidic on the pH scale and have a high concentration of hydrogen atoms as all its molecules will dissociate.
Its word equation is:
H2SO4 (aq) 2H+ (aq) + SO42-(aq)
As long as there is enough activation energy then a chemical reaction will take place between magnesium and sulphuric acid and magnesium sulphate and hydrogen will be produced.
The more successful collisions there are, the faster the rate of reaction.
So if the concentration of sulphuric acid is increased and as long as there is enough activation energy then the rate of reaction will be faster.
Apparatus:
2 burette holders
2 retort stands
2 waste beakers
1 stop clock
1 30cm ruler
1 pair scissors
1 small plastic filter funnel
1 piece sand paper
1 pair safety goggles
1 white tile
Chemicals
Sulphuric acid 100g/dm3
10cm length magnesium ribbon
Wash bottle of distilled water
Method
- Firstly I shall sand the piece of magnesium down to remove the oxide,
- Then I shall accurately divide my piece of magnesium into 10 1cm strips using scissors and a ruler.
- I shall then zero two burettes, one with water and the other with sulphuric acid.
-
I shall then accurately add 25cm3 of sulphuric acid into my conical flask.
- To this I shall add a single 1cm strip of magnesium starting the stop clock as soon as it enters the flask.
- I shall listen and watch until the magnesium completely disappears then I shall stop the clock and record the time.
- I shall then repeat this using the same volume of sulphuric acid making sure to zero the burettes.
- I will then repeat the above from the third point until now using the values in my table.
- By dividing the two times I recorded for a piece of magnesium to dissolve by two I can find out the average time taken for each different concentration.
-
By inverting my times as fractions (i.e. a time of 20 second which is 20 will become 1
- 20)
I can find out a decimal number which represents Rate of
Reactions-1.
Safety
During this experiment I shall wear safety goggles as Sulphuric acid is corrosive and can burn skin, and, as some may be taken up with the hydrogen ions as they escape as a gas (i.e. acid spray) it is only sensible to wear safety goggles.
Strategy for dealing with results
I shall display my results in a table like the one below:
I will graphically demonstrate these results in a graph which I believe will look similar (if the times have been recorded accurately) to the one below:
I have predicted the positive correlation in the graph above as I believe that the rate of reaction is directly proportional to the concentration. I believe that as the concentration of acid doubles the rate should also roughly double. I believe this as when the concentration doubles there are double the number of hydrogen ions present, so there are double the number of ions to collide with. This would mean the reaction should occur in half the time, so the rate at which the reaction took place doubles.
I will make sure that the results I take are as accurate as they can be by using burettes instead of measuring cylinders as they are more accurate and have a control on how much liquid is poured.
I will make sure I read the meniscus at eye level as to make my reading more accurate. I will also use a white tile and put it behind the meniscus to help me see it clearer.
I will sand the magnesium as this will remove the oxide layer which forms and help the reaction to run smoother.
I will make sure to start the stop clock at the exact moment the magnesium drops into the sulphuric acid as the reaction will tart then. As well as this I will stop the clock as soon as the magnesium has completely dissolved as this is when the reaction is over.
Each different concentration shall have results taken twice to not only find an average time but to see if there has been an error. If an error has occurred I shall repeat the experiment a third time to find out which of my two previous times is incorrect.
Also I shall have to make sure the controlled variables I mentioned earlier are kept the same throughout, i.e.
The temperature of both liquid shall have to be kept the same, as will the total volume of the acid solution. I will have to make sure that all the pieces of magnesium ribbon are as close to one centimetre as I can possibly make them. The hardest variable I will have to keep the same shall be the constant swirling as it will be hard to keep it the same speed as the time before. All of the above have to be followed to make this experiment a fair test.
Obtaining Evidence
My Table of Results
The above is my filled in table of results. All the times are my original times and as they seem quite consistent I felt no need to have to repeat an experiment.
I calculated the concentration by using the following calculation:
Volume of sulphuric acid x100=Concentration
Volume of water and acid i.e. 25
I calculated average time by using the following formula:
Time1 + Time2 = Average Time
2
I calculated Rate of Reaction by inverting the Average Time in Fraction and then I calculated its value as a decimal.
As you can see from my graph they appear to be quite accurate, but I shall go into more detail in the interpreting section.
Interpreting
I have drawn a graph to show the rate of reaction against the concentration of the solution. This graph is at the end of the interpreting section.
From the table you can see quite clearly that as the concentration of the solution decreases, the time take for the reaction to take place increases. This is what I predicted in my hypothesis.
Conclusion
I conclude that the rate of this reaction is directly proportional to the concentration of the acid which can be seen clearly from my graph. At a concentration of 40% the rate of reaction was 8.85s-1. The rate then quite steadily increased until, at a concentration of 100% the rate of reaction was 62.5s-1. These results strongly backup my conclusion. So, to increase the rate of reaction, the concentration of acid must be increased.
My original prediction was:
“The higher the concentration of sulphuric acid the faster the rate of reaction with the magnesium ribbon.”
And I believed, if I drew a graph, it would look like this:
I can now say that my original prediction was correct and my actual graph strongly matches the graph I made to show how I believe my results would look, i.e. there would be a strong trend showing that the rate of reaction is directly proportional to the concentration of the solution used.
Scientific Knowledge
Magnesium is high up the reactivity series (a list showing how reactive the elements are) and as it is higher up than Hydrogen it means magnesium is more reactive and can displace hydrogen in order to gain stability (a full outer shell of electrons). By placing a piece of magnesium in a aqueous solution of sulphuric acid, the magnesium displaces the hydrogen, which escapes as a gas .Meanwhile, the magnesium is forming into magnesium cations which bond with the sulphuric anions and together they form a salt. The hydrogen ions, move randomly throughout the solution, some of which, if there is enough energy collide and successfully react with the magnesium.
When the magnesium atoms loose atoms they become cations.
This is known as an oxidation reaction as hydrogen ions take the place of the displaced electrons and join together to form molecules.
As electrons are gained the below is a reduction reaction. The hydrogen molecules can escape as a gas which can be viewed and heard escaping from the solution.
The full equation of the experiment is
This experiment is an example of a redox reaction, due to the fact oxidation and reduction both taking place.
The magnesium is the reducing agent as it gives away its electrons quite easily.
The hydrogen ions are the oxidising agents in this experiment as they remove the electrons. Should the number of hydrogen ions increase, then more electrons can be removed and the reaction can occur faster. This can be observed when I used a 100% concentration of sulphuric acid, as there were a large number of hydrogen ions which increased the number of collisions with magnesium where electrons were lost, which increased the rate of reaction.
So, to conclude my experiment:
If the concentration of the solution is increased, and there is more sulphuric acid present then the rate of reaction with the piece of magnesium is increased.
Evaluating
My results
On my graph there is on point that is clearly off the line and two that are just barely on it. The one point that was off was my measurement for the rate of reaction taken for the solution with the concentration of 40gmd-3. It is this point however that I believe is anomalous, as the others are just on the line of best fit.
The point recorded for a concentration of 40gmd-3 has a rate of reaction faster than the expected trend. There are several reasons for this. The first is that possible I made a mistake in the measuring and added too much acid to the solution which would have given me a result that was too fast. There is also the chance I swirled the solution faster than I had with the other volumes, which would also explain a result off the trend line. Also there is the possibility too little magnesium was added; either I sanded the strip too much, and removed magnesium as well as the oxide layer, or I cut this particular piece too small, but either way it would explain a fast rate of reaction, or through no fault of my own this piece of magnesium was thinner than the rest, i.e. a fault of the supplier.
The last possibilities I can think of lie with the stop clock; either I stopped the stop clock too soon as perhaps the effervescence had dropped in noise level and the reaction was continuing when I thought it was over, or I started the experiment too late, while I attempted to drop the magnesium in, start swirling and start the clock all at once.
I will now evaluate the two points that are just on the line. To me they are not anomalous, but they do stand out so I will evaluate what could have gone wrong. For the point with a concentration of 92gmd-3 the above possibilities that I have for the point at concentration at 40gmd-3 apply.
However for the point at a concentration of 64gmd-3 there must be other reasons as it has a rate of reaction lower than the expected trend. It is possible that I didn’t swirl the solution as fast as I had done for the other concentrations. Also it is possible I made a mistake in the measuring of the magnesium and added a piece that was too big which would have meant the reaction would have gone on for longer. It’s also possible that I may have made a mistake in the volume of acid and added too little or I could have added too much water. Both of which could have given to a slower rate of reaction than expected. Then there is also the possibility that I may have not sanded the magnesium enough which could have left some of the oxide layer on, which would have slowed my reaction down. Finally it is possible that I stopped the clock too late.
Analysis of Apparatus
Measuring Apparatus
Burettes – I still think using the Burettes were a good choice as I could very accurately control the amount of acid and water I was adding.
Ruler- I felt this was a good choice as it enabled me to accurately measure out a 1cm strip of magnesium. However in combination with the scissors it was quite difficult to mark the length I needed to cut on the magnesium.
Stop Clock- The stop clock I felt was awkward, as I felt an extra pair of hands were needed to start the clock while I put the magnesium into the conical flask and began swirling. Unfortunately I cannot think of another piece of apparatus I could have used in its place, so it was and is still the best choice open to me.
Other Apparatus
Conical Flask- I feel a wider necked conical flask may have been better, as the piece of magnesium had a tendency to get stuck in the neck of the conical flask.
Scissors- I don’t really fell these were adequate in conjunction with a ruler to measure the strips of magnesium off with as I had to scratch the 1cm lengths on the piece of magnesium with these and then remove the ruler to cut the magnesium.
What would I do differently?
If I had to repeat this experiment there are several things I would do differently.
First of all I would use a magnetic stirring device instead of swirling the solution by hand which would remove the possibility of error.
Second of all, in conjunction with the magnetic stirrer I would use a beaker as there would be no need to worry about the acid spilling over the side with the magnetic stirrer. This would also mean the piece of magnesium couldn’t get stuck as the beaker has a very wide neck.
Finally, I would have used some fresh, powered magnesium. This would mean there would be no oxide layer to worry about. Also I would by able to measure it accurately with a scale which could eliminate error that I could have got from having to use a ruler and scissors.
Advice to the Medicine Company
I would advise the company to conduct some large scale tests to find which concentration of sulphuric acid would be safe, as to speed up their reactions I am sure they would use other factors which would increase the rate of reaction; which with a high concentration of acid could cause an unstable reaction.
An idea for the distribution of magnesium sulphate would be in the form of wipes which are easy to transport. The magnesium sulphate could be soaked into the wipes which would also prevent any magnesium sulphate being lost in the process because if any remained, it would be absorbed when more material is added.
