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When the Magnesium has just completely dissolved stop the stopwatch. It is vital that the watch is stopped as soon as the Mg has dissolved (when it is no longer visible) so accurate results can be recorded. Add the time to the table and repeat the experiment for another five different concentrations of acid.
Apparatus
- Two measuring cylinders (one for the acid and one for the water).
- Iron wool with which to remove the oxide layer from the magnesium.
- Test tubes in which the reaction will take place.
- Scissors with which to cut the magnesium.
- Ruler with which to measure the length of magnesium.
- Thermometer with which to measure the temperature of the acid and the water.
- Stopwatch with which to time the reaction.
Safety Points
- Wear safety goggles at all times during the experiment. This is because acid is being used which is corrosive and can damage the eye if the two come into contact.
- Stand up when carrying out the experiment.
- Put all bags and stools under desks so they cannot be tripped over.
Clean the work surface of all acid after experiment so it cannot come into contact with any other chemicals that may be on the desk.
Recording Results
As the experiment is being performed the results should be plotted onto a graph after every result is obtained. This is so any anomalous results can be recognized and repeated until a result that fits the line of best fit on the graph is achieved. The results should be recorded in the following table:
The middle columns show how the acid concentration will be varied. In order to create a lower known concentration of acid, and therefore a lower collision frequency, add specific amounts of water. So in order to create a higher concentration of acid, and therefore a greater collision frequency, have less water present in the mixture. However, the total volume of both acid and water must be 100cm3.
Each test should be done twice and the result recorded is the mean average between the two. This cuts down on anomalies and ensures more accurate results.
I will test concentrations from 0.6 M to 2.0M Hydrochloric acid, going up in increments of 0.2M. This will give 8 results, which is sufficient evidence with which to make a conclusion with strong evidence to support it. I will not test 0.2M and 0.4M HCl because the reaction takes too long as there are not many collisions between the H+ ions and the Magnesium particles, due to the low concentration of HCl. This became apparent in the preliminary experiment.
Rate of Reaction
The rate of reaction can be calculated when the results have been obtained using the following equation:
Length (cm) / Time (s)
This tells us the average length of magnesium which is being dissolved per second in the acid.
Factors for the experiment
Constants:
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Temperature of hydrochloric acid (room temperature – about 20°C).
- Length of Magnesium (1cm long with the oxide layer wiped off).
- Volume of hydrochloric acid and water mixture.
Variables:
- Concentration of acid (altered by the volume of water present).
The Collision Theory
‘The more collisions between particles in a given time, the faster the reaction.’
Four main factors affect the rate of a reaction.
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Surface Area – if a greater area of the reactant is exposed, therefore more particles exposed; there is more surface area available for a collision. So the above rule applies.
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Concentration – There are more reactants present in the solution of the same volume, so there will be a greater chance of two different reactants colliding. So the above rule applies.
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Temperature - When the temperature is increased, the particles gain more kinetic energy. This means that the particles are traveling quicker so more collisions will take place. The particles also collide with more force; they crash together harder. This means that the reaction is quickened on two counts.
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Catalyst – a catalyst is a substance that speeds up a chemical reaction. At the end of a reaction, the catalyst is chemically unchanged .A catalyst works by lowering the activation energy needed for the experiment to start.
Preliminary Experiment Report
When testing 2 M Hydrochloric acid the magnesium reacted so violently with it that the Magnesium ribbon jumped out of the Hydrochloric acid and stuck to the side of the beaker. This meant that it was not fully immersed in Hydrochloric acid so took longer to dissolve than 1.8 M Hydrochloric acid, which, scientifically, should not happen. To stop this occurring I held the Magnesium under the surface of the Hydrochloric acid with a pair of tweezers (that I had previously tested to make sure they do not react with acid). This gave more accurate results as the Magnesium was now fully immersed in Hydrochloric acid.
Concentrations of greater than 2 M were tested but the reactions were too quick to measure accurately. Lower concentrations were also tested but they took too long to be practical (i.e. over 5 minutes).
Therefore it has been decided to test concentrations between 1M Hydrochloric acid and 2M Hydrochloric acid and to test 6 different concentrations between these two values so a suitable extent and range of evidence will be collected.
However, all of the other points stated in the method and apparatus were correct so no other changes needed to be made.
Calculating the maximum length of magnesium that will dissolve in 100cm3 1M Hydrochloric acid
(in minimum concentration of Hydrochloric acid)
This is to make sure that the 1cm length of magnesium that is to be dissolved in the acid will actually do so.
2HCl (aq) + Mg(s) MgCl2 (aq) + H2 (g)
1 Mole Magnesium = 2 Moles Hydrochloric acid
- Number of moles of Hydrochloric acid = 100 / 1000 x 1
∴ Number of moles of magnesium = (100 / 1000) x (1 / 2)
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Mass / Ar = Number of moles
Mass of Magnesium = 24 (the Ar ) x ( (100 / 1000) x (1 / 2) )
= 1.2g
(20cm of Magnesium has a mass of 0.18g – with a defined width)
20cm = 0.18g ( x 6.66)
133.33cm = 1.2g
The above calculation has shown that 133cm of magnesium will dissolve in 1Mole Hydrochloric acid. This means that the 1cm lengths used in the experiment will easily dissolve in the acid.
Obtaining Evidence
The following method was used when performing the experiment:
- Measure out a defined volume of 2M Hydrochloric acid into a measuring cylinder using a dropping pipette. The level should be read from the bottom of the meniscus and should be read at eye-level with the measuring cylinder on a level surface. This ensures that the exact amount of acid is used every time so the results will be more accurate.
- If the concentration of acid being used is not 2M acid, a defined volume of water will need to be added to the acid. To find the volume of water needed use the following calculation:
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Volume of water = 100cm3 – Volume of acid.
- Pour the liquid into a beaker and measure the temperature of the solution using a thermometer. Record the temperature, the temperature should be the same for every test because if the temperature is increased, the ions have more kinetic energy so will move around faster resulting in harder collisions and more frequent collisions with the Magnesium atoms. This would affect the results.
- Measure 1cm length of magnesium, and re-measure it before cutting. It is important that the length is exact as the length of Magnesium is a constant in the experiment.
- Wipe the magnesium with iron wool to remove the oxide layer that will have formed when the magnesium was exposed to the air prior to the experiment. This has to be removed because the oxide layer would affect the results of the experiment.
- Add the magnesium to the acid and start a stopwatch as soon as it is added to the acid. It is important to do this as soon as the Magnesium is added because if it is not, the results will be adversely affected and will not be as accurate as they could be.
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When the Mg has been completely dissolved stop the stopwatch. It is vital that the watch is stopped as soon as the Magnesium has dissolved (when it is no longer visible) so accurate results can be recorded. Add the time to the table and repeat the experiment for another five different concentrations of acid.
Apparatus and chemicals needed:
- Hydrochloric acid (2 Molar).
- Two measuring cylinders (one for the acid and one for the water).
- Iron wool with which to remove the oxide layer from the magnesium.
- Test tubes in which the reaction will take place.
- Scissors with which to cut the magnesium.
- Ruler with which to measure the length of magnesium.
- Thermometer with which to measure the temperature of the acid and the water.
- Stopwatch with which to time the reaction.
Using instruments with care and precision
- When measuring out the Hydrochloric acid I read the level from the bottom of the meniscus at eye level. This ensured that accurate amounts of Hydrochloric acid were used for every test, which is important for the accuracy of the experiment.
- The length of Magnesium was be measured and then re-measured before cutting, so the length used will always the same.
- The stopwatch was started as soon as the Magnesium was added to the acid and stopped as soon as the Magnesium had just completely disappeared (dissolved).
Safety Points
When performing the experiment the following safety points were observed:
- Wear safety goggles at all times during the experiment. This is because acid is being used which is corrosive and can damage the eye if the two come into contact.
- Stand up when carrying out the experiment.
- Put all bags and stools under desks so they cannot be tripped over.
Clean the work surface of all acid after experiment so it cannot come into contact with any other chemicals that may be on the desk.
Results
The results recorded are as follows:
The results in the above table are using the tweezers to hold the Magnesium down under the surface of the Hydrochloric acid. The following table shows the contrast between the experiment using tweezers and the experiment without using tweezers:
As can be seen from the above table, the results were quite similar for the lower concentrations of Hydrochloric acid. This was because the reaction was not as violent for the lower concentrations so the Magnesium was not jumping out of the Hydrochloric acid. However, the difference between the results increased as the concentration increased because the reactions were more violent and the Magnesium was jumping out of the Hydrochloric acid more. This meant that the reaction was slower without using the tweezers as the magnesium was not full submerged in the acid all of the time.
The following graph shows the difference between the two sets of results:
The line for the tests with tweezers is much smoother than the tests without the tweezers. This shows a greater accuracy in the results when tweezers were used. None of the above lines are lines of best fit.
Rate of reaction
The rate of this reaction can be calculated by dividing 1cm / time in seconds. This gives the amount of Mg that reacts per second. The reciprocals (1 / time in seconds) can be shown in the following table (using tweezers):
The following graph shows the rate of reaction using the reciprocals of the results using tweezers:
.
Analysis
The experiment proved comprehensively that as the concentration of acid is increased, the rate of reaction also increases. This shows that my prediction was correct, as I predicted that as the concentration of acid is increased, the rate of reaction would increase. My results clearly show this:
The results can be explained scientifically: when the hydrochloric acid is more concentrated there will be more H+ ions in the acid. If there are more H+ ions in the acid there will be more collisions between the H+ ions and the Mg atoms, a greater collision frequency. Approximately 10% of all collisions result in a reaction, so if there were to be more collisions (as there will be with a greater concentration of acid), there will be more reactions.
I also predicted that the graph for the results would look like this:
Time
Conc of acid (moles)
This prediction was accurate because the results gave the following graph, which is also a curve:
The black line on the above diagram is the line of best fit. The line of best fit is close to the red line, which shows the actual results recorded. This shows that the results recorded were accurate.
The following graph shows the rate of reaction using the reciprocals of the results:
The rate of reaction graph above is a curve. This does not fit with my prediction as I said it would be a straight line. This shows me that the relationship between the rate and the concentration of acid is not directly proportional. The line given by the above graph is very close to y = 2x2. This means that the rate is equal to 2 x concentration2. From this I can see that the graph for rate against concentration2 would give the line y = 2x because the x is already squared. This would give a straight line.
As can be seen from the graph above, the line is straight (y =2x). The line is straight because in the reaction it takes 2 collisions of H+ with Mg atoms to dissolve the Mg (the two axis are now directly proportional). This can be shown in the following reaction scheme:
1 e
H+ H
Mg H2
1 e
H+ H
Mg2+
Conclusion
My results prove beyond doubt that as the concentration of the acid increases, the time taken for the reaction decreases. For example, my results show that the time taken is 99, 45, 24, 16, 12 and 10s when the concentration of Hydrochloric acid is increased. This supports the prediction to a high extent because the results follow the trend that was predicted. The only part of the prediction that was not entirely accurate was the sketch graph for the results. It was predicted that the line would be straight when in actual fact it was curved.
Prediction: Actual:
Rate Rate
Conc of acid (moles) Conc of acid (moles)
Evaluation
I would say that the results comprehensively support the prediction. However, the best results were obtained using different apparatus to those originally specified. In the preliminary experiment it was noticed that with more concentrated HCl (in particular the 2M HCl), the reaction was so violent that the Mg jumped out of the HCl and stuck to the side of the beaker. This meant that the Mg was not completely immersed in the HCl and this was affecting the time taken for the reaction. The experiment was then re-done using tweezers (that had been checked to make sure that they did not react with HCl) to hold the Mg in place, fully submerged in the HCl.
As can be seen from the graph below, the results recorded using tweezers were of a greater degree of accuracy than the results recorded without using tweezers. The graph below does not have a line of best fit.
Anomalous results
There are two anomalous on the yellow line above, the result for 1.6M HCl and for 2M HCl. I have already explained why the result for 2M HCl was anomalous, but the result for 1.6M HCl has no obvious explanation.
These results were not to an acceptable level to me, but the results using the tweezers were. As can be seen, the line is a smoother curve, which indicates a better consistency in the results.
Possibly improvements to the experiment
However, the results recorded are not perfect. This can be seen by the curve not being entirely smooth. Testing each concentration of Hydrochloric acid two or three times and then calculating the mean average could improve the results. Testing the concentrations in 0.1M increments would give a greater fluidity to the graph line, and would enable the prediction of results for concentrations not yet tested by being able to calculate a more accurate formula for the line.
Conclusion
My results prove beyond doubt that as the concentration of the acid increases, the time taken for the reaction decreases. For example, my results show that the time taken is 99, 45, 24, 16, 12 and 10s when the concentration of Hydrochloric acid is increased. The results are good enough to support a firm conclusion because they all follow the trend of the time for the reaction decreasing as the concentration of acid increases, and they are all very close to the line of best fit. However, if more concentrations were to be tested, a greater range of results could be recorded, so the results would support the conclusion even more strongly.
The results are very reliable because they can all be explained using science. Even if a person with no knowledge of science at all were to view the results, he would say that they are accurate because of their being in very close proximity to the line of best fit. This shows that the results are consistent, which adds to their reliability.
Further Work
There are numerous ways of obtaining more evidence for the conclusion or extending the investigation. Some of these ways are as follows:
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Carry out the experiment using a different acid, for example H2SO4 (sulphuric acid) or H2NO3 (nitric acid). Will the results be the same as for Hydrochloric acid?
- Carry out the experiment for a different group 2 alkali earth metal, for example Calcium (Ca) or Barium (Ba). Will the results be the same as for Magnesium?
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Carry out the experiment at a different temperature. Will the times for the various reactions be faster or slower than at 20°C?
