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During the reaction each magnesium atom gives each hydrogen ion, H+, 1 electron. (Two hydrogen ions are needed because magnesium forms an ion with the charge Mg2+ and hydrogen an ion with the charge 1-, so in order for magnesium to lose two electrons to achieve a full outer shell, and therefore stability, it needs two hydrogen atoms because they can only take one more electron each.)
- When the electron transfer has taken place a magnesium ion and two hydrogen atoms have been formed.
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The hydrogen atoms also want to achieve stability so the two atoms join together and share their outer electrons in a covalent bond. This is the hydrogen molecule, H2.
Below is a table showing which bonds are broken and which bonds are formed during the reaction.
First of all the metallic structure of the magnesium is broken up.
- Then each magnesium atom gives each positive hydrogen ion one electron, forming magnesium ions and hydrogen atoms.
X X X X
Mg H H
- Lastly, the hydrogen ions join together in twos to form hydrogen molecules joined with a covalent bond.
X
H X H
The reaction between magnesium and hydrochloric acid is an exothermic reaction .The definition of an exothermic reaction is ‘a reaction that gives out heat.’
In a reaction heat is taken in when the bonds are broken and heat is given off when bonds are formed. So, in the reaction between magnesium and hydrochloric acid, less energy is taken in during the bond breaking than is taken off in the bond forming. This can be show in the following exothermic energy level diagram:
Mg2+ + 2e + 2H+
Energy
(KJ)
Mg + 2H+
Mg2+ + H2
For the above reaction there is no activation energy needed as the reaction begins immediately when the magnesium is added to the hydrochloric acid.
Apparatus & Chemicals
- Hydrochloric acid of a defined molarity.
- Measuring cylinder with which to measure the volume of acid.
- Polystyrene cup in which the reaction will take place, also provides insulation.
- A lid with hole to put over the polythene cup to prevent heat loss.
- Thermometer with which to measure the temperature of the acid and the solution formed.
- Safety glasses to prevent acid spilling into eyes.
- Safety mat to stop acid spillage onto desk.
- A teat pipette with which to accurately adjust the volume of acid used.
- A ruler to measure the length of the ribbons of magnesium.
- A pencil to mark the length on the magnesium before it is cut.
- Scissors to cut magnesium.
Method
- Put on safety goggles to prevent acid splashes to the eyes.
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Measure out 30cm3 of hydrochloric acid into the measuring cylinder. Hydrochloric acid of the same molarity must be used every time so it is a fair test. The acid should be adjusted using the teat pipette and viewed at eye level with the measuring cylinder on the table. This ensures that the same amounts of acid are used for each test so it is a fair test as the results would be affected if the volume of acid were different.
- Measure 2 cm of Magnesium using a ruler. This measurement must be double-checked before cutting, as it is imperative that the measurement is correct.
- Pour the acid into the polystyrene cup and measure the temperature of the acid, this will give the temperature before the reaction.
- Add the measured strip of magnesium to the acid.
- Immediately after doing this add the lid to the top of the cup and stir the acid with the thermometer which is to be placed through the whole in the wooden lid. The wooden lid is to be added to prevent any heat loss to the surroundings as this would affect the results of the experiment. The acid is to be stirred with the thermometer to avoid any hot-spots in the acid. This is when there is a build up of heat in one part of the acid, this would affect the results if the thermometer reading were taken form one of these points. By simply stirring the solution all hot-spots are eradicated.
- Record the reading on the thermometer once the temperature has stopped rising.
- Pour the acid solution down the sink once the test is complete. Dry the measuring cylinder, replace the polythene cup and allow the thermometer to return to room temperature before performing the next experiment. This helps to standardize the test and ensure that it is a fair test.
- Repeat the experiment using other lengths of Magnesium, e.g. 4cm, 6cm, 8cm, 10cm and 12cm.
Safety Points
As acid is used in the experiment it is absolutely imperative that the following safety points are followed:
- Wear safety glasses whenever handling acid. This is because acid is extremely corrosive.
- Stand up when carrying out the experiment.
- Put all bags and stools under desks so they can not be tripped over.
- Wipe any spillages.
- Clean the work surface of all acid after experiment so it can not 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.
As well as displaying results on a graph, they should also be recorded in the following table:
A column for the molarity and volume of the acid is not included because it will be kept at a constant.
Analysis
It has been found that as the length of magnesium increases, so does the temperature change. The evidence of the results from the experiment strongly confirms the predictions. This is so because the prediction was ‘as the length of magnesium is increased, the temperature change of the Magnesium Chloride solution will increase’, this was confirmed by the results. The results recorded show clearly the temperature increase, e.g. in my results the temperature change was 3, 6, 9, 12, 15 and 18ºC.
It was also predicted how the graph would look, this graph was identical to the graph produced from the results.
The results support the prediction well but the changes are not as exact as we would have liked because we did the experiment with apparatus that was not entirely error free.
The class results are as follows:
The result in red was an anomalous result as it is far different from the other results for 6cm length of magnesium. It was ignored when calculating the mean average, as it would have had an influence on the result.
The above results have been obtained by processing the raw data. This was done by:
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Recording the start temperature (s) of the acid.
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Recording the finish (f) temperature of the acid after the reaction has taken place.
- Subtracting the start temperature from the finish temperature.
So the temperature change given in the table above was calculated using the formula: Change in temperature = f – s.
The results shown in the below table are the mean averages for the class results.
These results are not perfect, if they were they would show that as the length of magnesium is doubled, the temperature increase also doubles. However, they are very close to perfect as the result for 4cm of Mg is double that of 2cm of Mg (3.41 x 2 = 6.82). If the results were perfect, I would predict that the results would be the following (assuming that the value of 2cm of Mg is correct):
The results in red are the ones that are changed.
The following is a graph showing the results found:
The line of best fit has been added (in black). This shows that the results obtained were accurate because none of the points are far from the line of best fit. However, if the experiment could be done again, the test for 12cm of Mg should be repeated as it is a noticeable distance from the line of best fit.
The line of best fit goes through the origin because if there is no Magnesium, no reaction will take place. If no reaction takes place, no bonds are broken so no bonds are formed, thus no heat is given off. The line is straight because the temperature change is directly proportional to the length of Mg.
Using the results recorded, it is possible to calculate the results for lengths of magnesium that have not already been tested. This can be done in two ways:
- The graph can be extrapolated to show values for lengths not yet tested.
- Mathematics can be used to predict values by multiplying results from lower values to find results for higher values.
Using the results already recorded, I will predict the results for 14cm of Mg (which has not yet been tested).
I will calculate the result mathematically because it is more accurate than using a graph.
To find the result I will multiply the result for 2cm of Mg by 7, as this will give a value for 14cm.
3.41 x 7 = 23.87
The test for 14cm of Mg was carried out after the prediction was made, and the result was a temperature increase of 23°C. This shows that my results were within 4%, which I consider to be reasonably accurate, but could be better.
Evaluation
When performing the experiment, I made sure that I followed the method exactly, especially the fair test points. It was absolutely imperative that the tests were carried out fairly so I followed the following points to ensure a fair test was carried out:
- The Hydrochloric acid used was of a defined molarity, this was important because if the acid used for one test were of a different molarity, the results would be affected.
- A measuring cylinder and a dropping pipette were used to measure the amount of HCl. The reading was taken from eye level with the measuring cylinder on a horizontal surface from the bottom of the meniscus to ensure the same amount was used each time.
- The reaction took place in a polystyrene cup with a lid which acted as insulators to prevent the heat from the experiment being lost to the surrounding areas. This was important as any heat loss would have affected the results.
- The lid used had a hole in the top through which the thermometer was put. This also cut down on heat loss and enabled the stirring of the solution with the thermometer to prevent the formation of any hot spots, which would have affected the results. In retrospect I think the hole should have been a tighter fit to reduce heat loss.
- A ruler was used to measure the length of Mg. Before it was cut, the length as re-measured to make sure the correct length was used.
The results recorded were very accurate; this is shown by how close all of the points are to the line of best fit on the graph. The only result that could be repeated to achieve a more accurate reading would be the test for 12cm of Mg, and even this result was acceptable.
This shows that the results are reliable, and are sufficient to support the conclusion that as the length of magnesium increases so does the temperature change.
The fact that I made a prediction for 14cm length of magnesium prior to performing this test, and the value obtained was within 4%, suggests that this experiment provides powerful evidence for the conclusion.
Also, the results being as close as they are to the line of best-fit shows consistency in the recordings and therefore that they are of a high quality.
Improvements
Certain aspects of the experiment could have been improved upon. These points are listed as follows:
- The magnesium used had an oxide layer so was actually magnesium oxide (also known as magnesia). This layer could have been removed by rubbing the ribbon with some iron wool. This would have scraped off the layer of oxide and given more accurate results.
- Although the polystyrene cup was effective in reducing heat loss, it could have been made even better if there had been two polystyrene cups instead of just the one. Also on the subject of insulation, the hole in the wooden lid for the thermometer was considerably larger than it needed to be, so heat will have escaped that way also. To prevent this heat loss, a smaller hole should be used.
- The same polystyrene cup was used for each test. This meant that there would still have been some acid from the previous test in the cup when being used. This could have affected the results. To stop this, a new, sterilized cup should be used each time.
All of these points are fairly insignificant own their own, but together they could make considerable changes to the results recorded.
Numerous other investigations could be done in relation to this one. These include:
- Investigating how other metals react with the hydrochloric acid. This would show where metals fit into the reactivity series, i.e. a metal higher up the reactivity series than magnesium would cause a bigger temperature increase and visa versa.
- Investigating how other acids such as Sulphuric acid, nitric acid and carbonic acid react with magnesium. Do they produce a greater amount or a smaller amount of thermal energy than hydrochloric acid?
- Investigating how magnesium would react if the weight were regulated instead of the length. Would a change in surface area affect the amount of heat produced? (However, this would be incredibly difficult to do as 2cm of magnesium has a very small mass.)
- Investigating the volume of hydrogen being given off as well as the amount of heat being given off. Are the two directly proportional?
