Once I have the right concentration I need in the beaker I will add 2cm of magnesium ribbon and time using the stopwatch how long it takes (to the nearest 10th of a second) for the magnesium ribbon to completely disappear and then recording this information on a results table. I will use the average of the two results for the same concentration to complete a graph that shows the results.
Preliminary Experiment:
To try and discover the length of magnesium ribbon that I would need I did a preliminary experiment in which I used 40cm3 of hydrochloric acid. which was at 4molar, and I used 4 different lengths of magnesium ribbon each time I would drop the mg into the acid and record the time it takes for the magnesium ribbon to completely disappear. The lengths of mg ribbon I used were;
2cm, 3cm, 4cm and 5cm long.
Here are the results that I got;
As the results show the length of the magnesium ribbon does not make a difference in the time taken for it to disappear because the concentration of acid is so strong. If we were to use the same lengths of ribbon but use a lower concentration I think that the results would be different. In my experiment I decided to use 2cm of magnesium ribbon, this decision was made purely on the basis that the less magnesium we use the more we save, so if there is no need in using more than 2cm of magnesium then it would just be wasting it if we did.
Fair Test:
To keep this experiment as fair and as accurate as possible I will have to keep all the variables involved in the experiment except the on I am changing (the concentration of the acid) exactly the same. The variables in this experiment, which will be kept the same, are called the controlled variables.
These controlled variables are:
Mass of magnesium ribbon
Volume of solution
Surface area of Mg ribbon
Stirring/Irritation
Mass of any catalyst used
Temperature
Variables:
The factors, which control the outcome and results of an experiment, are called the Variables. To keep the experiment as fair and accurate as possible these variables have to be controlled or kept the same throughout the whole experiment, these are called the controlled variables.
Length of Magnesium Ribbon:
Throughout the whole of this experiment the length of the magnesium ribbon should not be changed from 2cm long. The length of the ribbon will be measured using a ruler. If we ensure the length is kept the same then the results should be as accurate as possible.
Volume of Solution:
During the whole of the experiment the 8 different concentration tests the overall volume of both the Hydrochloric acid and the water when added together should not be changed. It should always be at 40cm3 even when different amounts of hydrochloric acid and water are being added together. This would stop and inaccurate results and make the test as fair as possible.
Catalyst:
In this experiment a catalyst will not be used to speed up the rate of reaction because it is pretty fast already, but if one was used then the mass of this catalyst and its surface area should not be changed at all!
Irritation (stirring):
In this experiment to speed it up slightly and to help the reaction get started the solution should be stirred slightly as the magnesium is dropped into the acid. If I do this then the number of times I stir the mixture should be kept the same in every different experiment so that the test is fair!
Temperature:
The solution of Hcl and H2O and the magnesium ribbon used in this experiment shouldn’t be chilled or heated at all throughout the experiment or before I start the experiment because this would not be accurate or fair, and cause the results to be inaccurate. This is because temperature effects the rate of the reaction so if the reactants were heated or chilled the rate of reaction would be faster or slower. All the apparatus, both solutions and magnesium will be left at room temperature before and during the experiment.
Prediction:
In this experiment I think I will find that the rate of the reaction is greatly affected by the concentration of the hydrochloric acid. And that there is also a definite correlation between the concentration of the acid and the time it takes for the magnesium ribbon to disappear. I hope to find that the speed (rate) of the reaction is proportional to the concentration of the acid. This means that if the concentration of the Hcl is halved then the speed in which the reaction takes place is also halved. The speed of the reaction will be slower if the concentration is lower because if the concentration of the acid is lower it means that there are fewer ions in the solution and they are further apart. This affects the rate of reaction because of Collision Theory. The theory states that if; more concentrated the reactants, the number of collision s between the particles in these reactants increases. Collision theory is the name given for the amounts of ions, which collide in a certain reaction. This explains that if there are more ions in a solution then there will be more collisions between the ions in the solution and more collisions between the reactants in that reaction. This is because the ions are closer together and there is more of a chance that they will collide with other ions in the solution. Apart from increasing the amount of ions in a solution (increasing the concentration), there is another way in which the collisions between ions can be increased! This is by giving the ions already in the solution more energy.
We must consider what happens when a reaction takes place. First of all the particles of the
reacting substances must collide with each other, and secondly a fixed amount of energy
called activation energy (Ea.) must be reached if the reaction is to take place. If the
particles can produce the right amount of energy (i.e. if they collide fast enough and in the
right direction) a reaction will take place. The reaction is speeded up if the number of
collisions is increased. This energy is heat, when heat is added to the experiment it increases the energy in the ions, which makes them move around in the solution a lot faster.
This means that the ions will collide more frequently and more vigorously increasing the rate of the reaction and the amount of collisions in the reaction. This also
explains why the greatest rate of reaction is usually as soon as the reactants have been
mixed; i.e. they are both at their highest concentrations. As the reaction continues, the
concentration of the reacting substances decrease and so does the rate of reaction. The higher the concentration of HCL you
use, the less time it takes for the magnesium to disappear and so the rate reaction
increases. The concentration of HCL you use is the independent variable because it will
vary, and the time taken for the rate of reaction to take place (i.e. the magnesium to
disappear) is the dependent variable because it depends upon the concentration of
hydrochloric acid. Other variables throughout the investigation, which will vary are the
volume of water used, and the volume of hydrochloric acid. The variables which will
remain unchanged are the temperature (room temperature) will stay the same in order for
it to be a fair test, because if the temperature changes it will effect the rate of reaction
between the reactants, either by speeding it up if the temperature rises because the
particles move faster and travel a greater distance in a given time and so will be involved
in more collisions. In this experiment I think that as the concentration of the hydrochloric acid decreases then the rate of reaction will also decrease, meaning that the time it takes for the magnesium ribbon to react and completely disappear will increase as we go down the table! This is all because of the ions in the solution of Hcl and H2O, the lower the concentration, the smaller the number of ions in the solution. The smaller the number of collisions between the ions therefore decreasing the rate of reaction and increasing the time it takes for the magnesium ribbon to disappear. This also works the other way around. If the concentration is higher so is the number of ions in the solution and so is the number of collisions between these ions. This means that the rate of reaction increases and the time it takes for the magnesium ribbon to disappear is shorter!
Safety:
Whilst doing this experiment I will be using some quite dangerous chemicals such as hydrochloric acid! Although the acid is dilute it is still corrosive and if left in contact with human skin would burn and maybe even cause blistering. In the event of any of the hydrochloric acid being spilt on clothes, skin benches etc. water should be poured straight onto the spillage so it cannot cause any damage to what it is in contact with. Acid is a corrosive liquid, this means that it causes a chemical reaction with whatever it comes into contact with, i.e. burning of skin and corroding of clothes and benches! The only way to combat this is by putting water onto the acid, what this does is dilute the acid making its concentration so low that it is not dangerous any more. Also safety precautions such as goggles should be worm to protect eyes from any acid which may splash into them, if this does happen it would be quite dangerous!
Results:
Here are the results I got from the experiment.
Graph:
Here is a graph showing the average time taken for the magnesium ribbon to disappear.
Conclusion:
From the results which I have obtained you can easily see that there is a clear pattern between the concentration of the hydrochloric acid and the time it takes for the magnesium ribbon to completely disappear. This pattern or ‘trend’ shows that as the concentration of the hydrochloric acid decreases the time it takes for the magnesium ribbon to disappear increases. This correlation is the same was what I predicted would happen in this experiment. In the prediction I have noted that the rate of reaction is greatly affected by the concentration of the acid. This part of the prediction was also correct as the rate of reaction was affected greatly by the concentration of the acid. But what I did find out in the experiment which does differ from my prediction is that, from the results which I obtained it is clear that the rate of reaction, despite being greatly affected by the concentration of the acid it is not whatsoever proportional to the concentration of the solution.
There are four things which affect the rate of reaction – temperature, concentration, surface area and adding a catalyst. In this experiment the thing we changed to affect the rate of the reaction was the concentration of one of the reactants. This reactant was Hydrochloric acid.
The concentration of a solution is another word for the strength of it. The concentration of the solution is all to do with the particles inside of it, the more concentrated a solution is the more reactive particles are inside of it. And the less concentrated a solution is the less reactive particles there are inside it. This is shown in the diagram on the next page:
The main reason in which the concentration directly affects the rate if reaction is because of a thing called collision theory. Collision theory is the name which is given to the amount of ions which collide into one another in a certain reaction. Considering that there are more ions in a more concentrated solution there is more of chance of the ions that are inside the solution colliding with one another. If there are more collisions in a reaction then the rate of reaction will increase! This is why when the concentration is increased the rate of reaction increases:
More ions = more collisions = faster rate of reaction!
This also means that if the concentration of the solution is lower then there are a lot less reactive ions inside of the solution so there are fewer collisions between these reactive ions. This means that fewer collisions also means a slower rate of reaction. So if we use this in our experiment we see that when the hydrochloric acid was less concentrated then there was less collisions in the solution so the time it takes for the magnesium ribbon to disappear was a lot longer, this is shown in the results table and also in the graph!
Evaluation:
I think that this test was as fair as I could have possibly made it considering the time and apparatus I had. All the controlled variables were kept exactly the same throughout the whole experiment and they were not changed once. All the measurements and volumes were kept the same and each time they were measured using the same burette but between each experiment this burette was washed and so were all the other equipment which I used more than once, these were washed using warm water in a sink. This prevented any contamination to the experiment which would have affected the accuracy of the whole experiment. In using a burette to measure out the volumes which I needed I made the measurements more accurate than say if I used a measuring cylinder because the burette is a more accurate piece of measuring equipment. The way in which I measured the length of magnesium ribbon was by using a ruler and a pair of scissors, this was not the most accurate of ways of measuring the lengths but t was the simplest and it was easily accurate enough for this experiment.
From my results table and graph you can see that there are not any really outstanding anomalous results except for the last concentration which was 0.5 molar. But this was to be expected as 0.5 molar is such a low concentration it contains not many ions at all so there aren’t many collisions taking place so it takes an awful long time for the length of magnesium ribbon to completely disappear!
From the results table and graph I have gathered that the results which I have obtained through this experiment are pretty accurate. On the graph all the results (except 1.5m) were on or very close to the line of best fit. This shows that the results were quite accurate and pretty reliable. This is mainly because of the accuracy and fairness of the experiment. This also shows that I succeeded in keeping the experiment fairly accurate!
To be 100% sure that the results I have obtained were the most accurate I could have redone another set of the results. This would mean that the average would be more accurate and that the results would be fairer because any anomalous results would be counted out because the results would be averaged to get the final set of results. This would also have implications on the conclusion, which could be slightly wrong because of the inaccuracy of the results! Maybe the rate of reaction was proportional to the concentration of the acid but because of the inaccuracy of the results it might not have been shown that the results actually are proportional to one another. These are all things that I could do to improve the accuracy of the experiment. Another thing which would increase the accuracy of the results, although not dramatically would still add to the accuracy and fairness of the experiment would be to keep the beaker with the reacting solution in it in a water bath which would be kept at the same temperature throughout the whole experiment, this would ensure that there would be no change in the temperature of the solution. A change in temperature could have happened whilst the experiment was taking place so this water bath idea would account for any temperature changes and get rid of them.
These three factors bellow could also effect the fairness and accuracy of the results I obtained but I could do nothing to change them.
1) When the reaction takes place bubbles of H2 are given off, which might stay around the
magnesium which therefore reduces the surface area of the magnesium and so the acid
cannot react properly so this effects the results.
2) We could have controlled factors in the investigation better (e.g. the stirring of the
solution because if this isn’t done properly it can lead to incorrect results).
3) Using larger concentrations of acid would give a bigger more accurate conclusion
instead of just using 10ml test tubes use 1litre test tubes, this way graphs would be more
spaced out and give an accurate form or curve
Chemistry Coursework Completed By: -
Colin Stuart
Form 11-2
