My preliminary method:
Firstly I got all the equipment needed, and then I measured 2cm of magnesium strip. I measured out the required volume of hydrochloric acid (12ml) using a pipette to extract the liquid and measuring using a 25ml measuring cylinder. I measured out different amounts of water (4ml, 8ml, 12ml, 16ml, and 20ml) in the same way. I made sure that the stopwatch was reset and poured the measured out water and hydrochloric acid into the conical flask and stirred using a stirrer for five seconds. Then I put the magnesium strip into the conical flask whilst starting the stopwatch and then quickly plugged the bung on top. Then the stopwatch is stopped as soon as the magnesium strip is fully dissolved.
Safety: Always wear lab coats so that corrosive chemicals, like hydrochloric acid which will cause irritation if in contact with skin and could burn through clothing, are visible. Also wear goggles so that chemicals do not get in contact with your eyes which could potentially blind a person. The easier and safer option is to wear gloves.
Also do not touch magnesium if you know you have some hydrochloric acid on your skin as it will fizz and burn your skin, just wash off the hydrochloric acid and if burnt hold your hand under cold water and request the aid of staff or technician. If any hydrochloric acid gets into your eyes wash with clean, cool water and make sure that a technician or a member of staff knows what has happened.
Variables
- Temperature
- Concentration
- Pressure
- Surface area
- Catalysts
Temperature was measured before the experiment and was normally around 25 degrees which is at room temperature and it was measured using a small thermometer. The concentration of the solution varied as we experimented on how the concentration of hydrochloric acid affected the reaction rate. The pressure was a factor which would increase when the concentration of hydrochloric acid was high and decrease when the concentration was low due to more hydrogen gas being produced in the same volume of space. We tried to minimise the effect of pressure by using conical flasks that were of the same volume, and this was because if there was a certain amount of gas in a big volume of space there will not be as much pressure as it would have if it was in a smaller volume of space. Surface area was kept the same as we used magnesium strips which were of the same length, thickness and height therefore they had the same surface area. We did not use any other forms of magnesium like balls of magnesium which would affect the rate of reaction. The surface area is a major factor due to a higher surface area having more space for the hydrochloric acid to attack. I only used one magnesium strip which was 2cm long and did not use two strips which were 1cm long as this would also affect the rate of reaction as the surface area will be bigger. Catalysts speed up chemical reactions, therefore the presence of catalysts in an experiment would increase the rate of reaction unfairly. As long as there are no chemicals already in your equipment and there is no cross contamination you are not likely to introduce a catalyst unknowingly.
Safety: When the pressure in the conical flask increases it could pop, therefore make sure that you are not directly on top of the conical flask and that you are a safe distance away from the conical flask.
My preliminary results, with the time taken converted into the reaction rate for each trial.
Trial1 Trial 2 Trial 3
GRAPHS
TRIALS 1, 2 and 3
Method
WEAR EYE PROTECTION!!!
Before carrying out experiment make sure all naked flames or fires are extinguished.
I changed my method after my preliminary experiment a lot in order to get more accurate results that will follow the trendline better. We decided to change what we recorded; instead of recording the time it took for the experiment to finish, I measured the volume of hydrogen produced in time intervals of 15 seconds. We realised that in our preliminary we had not changed the concentration of the solution but the volume of the solution (the concentration is the amount of solution in a certain volume of space and the volume is the amount of solution there is. In the concentration the volume of solution must be the same for all tests). We changed the maximum volume of solution to be 50ml. The highest concentration was hundred percent and the smallest concentration was twenty percent. The amount of water added constantly increased by ten millilitres, therefore the amount of hydrochloric acid constantly decreased by ten millimetres for each test.
In order to measure the volume of hydrogen produced I would need more equipment such as:
- Burette- could be used to contain the hydrogen gas produced, as it is filled with water I could find out how much water was present before the experiment and how much was left after the experiment then work out the difference. This will be the volume of water that has been displaced (replaced) with hydrogen. A burette was used instead of a measuring cylinder because a burette can measure much more accurately than a measuring cylinder.
- Funnel- It is hard to get the water from a tap into the burette without a funnel.
- A trough of water- a bowl filled halfway with tap water under the burette filled with water, so that the displaced water can be collected in the bowl. The bowl of water also prevents the water from the burette escaping when I have to lift it to feed in the delivery tube.
- Delivery tube- to get the hydrogen from the conical flask into the burette filled with water to measure the amount of hydrogen gas produced.
- Stand with a burette clamp- A burette has a small base and is very tall so a stand and a burette clamp is needed to hold the burette upright and to hold it just under the water line so that a delivery tube can be sent underneath it.
- Distilled water- used to clean all equipment before use and used to vary the concentration of the hydrochloric acid solution. I used distilled water instead of tap water because there is almost no chance of impurities mixing with distilled water so you can be sure that there are no different ions unlike tap water (which gives it a good taste).
- Large thermometer- to measure the temperature of the solution before and after the reaction to get an average temperature of the solution during the reaction. It is important as a small difference in temperature causes a big difference in reaction rate. I used a large thermometer because it measures the temperature to a more accurate degree.
Firstly I wore a lab coat and goggles. Then I collected all the equipment needed and cleaned using distilled water. Then I started to fill the bowl with water and the burette (with the other end of the burette, with a tap, closed to prevent water escaping from the other side.) with the aid of a funnel, and overturned the burette (covering the opened side) and placed it into the bowl. Then I measured out the necessary volume of distilled water and two molar hydrochloric acid using separate pipettes for each. I measured the volume of the liquids using twenty five millilitres measuring cylinder. Then mixed together in a conical flask using a stirring rod (stirring for five seconds) and measured the temperature of the solution using a large thermometer. Then I measured out two centimetres of magnesium ribbon using a ruler and cut out many strips of magnesium that were all two centimetres long with a pair of scissors. This had to be kept constant to find out the real impact of changing the concentration, as the length of the magnesium strip would affect our results. Then I took a stand and burette clamp and attached it to the burette and fixed one end of the delivery tube into the bung and the other end was sent up the burette. Then I put the magnesium strip into the conical flask, plugged the bung into it and started the stopwatch. Then in intervals of ten seconds I measured the volume of hydrogen gas produced. At the end of the experiment I measured the temperature of the solution to get an average temperature of the reaction. The average temperature of the reaction is twenty eight degrees Celsius.
You can check to see whether the product is hydrogen or something different by collecting the gas in the test tube by covering the opening of the test tube with your thumb. Then put a burning splint into it, if it goes out then there is carbon dioxide produced. If it burns brighter then there is oxygen produced and if there is a small explosion and you hear a squeaky pop then hydrogen is being produced.
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Data analysis
The graphs that I have produced are the graphs of the averages for each concentration tested for 80 seconds. The averages were attained by finding the mean of three trials.
The ionic bonds (a type of chemical bond formed through electrostatic attraction between two oppositely charged ions, H+ and Cl-) between the chlorine and hydrogen are quite strong and when the hydrochloric acid collide with the magnesium strip the bonds between the hydrogen and chlorine break up and the chlorine ion bonds with the magnesium producing magnesium chloride and the hydrogen gas is given off.
Mg (s) + 2HCl (aq) = MgCl₂ (aq) + H₂ (g)
In the graph of the hundred percent concentration (50ml) of hydrochloric acid, it is clear that between ten and twenty seconds there is a dramatic increase in the volume of hydrogen produced and this is due to the high concentrations of hydrochloric acid means that there are more collisions and therefore more successful collisions. In a successful collision the hydrogen and chlorine from the hydrochloric acid break apart and the chlorine forms new bonds with the magnesium, the hydrogen is given of as a gas, this is shown in the equation above. This means that if there are more collisions then there will be more reactions which means that more hydrogen gas will be given off . The graph also seems to curve from twenty seconds to thirty seconds. This means that the reaction is nearly over and that most of the magnesium strip has been used up, there is also less hydrochloric acid at this time as some of the hydrochloric acid molecules have reacted with the magnesium. The reaction took forty seconds to finish and this is because there is not any hydrogen being produced and in the graph this is represented by a flat line.
The graph of the eighty percent concentration (40ml HCl 10ml water) of hydrochloric acid shows a similar trend to the graph for the hundred percent concentrations. The graph rapidly climbs in the first ten seconds due to the high concentrations of hydrochloric acid and then starts to curve as the reaction is near the end. This reaction is slightly different to the reaction with hundred percent concentration of hydrochloric acid because the increase in the volume of hydrogen produced in the first ten seconds seems to be less hence why the slope is less steep. This is because the concentration of hydrochloric acid is less than the reaction with a hundred percent so there are less molecules of hydrochloric acid to react with the magnesium strip of the same length. The reaction takes about the same amount of time to end. The duration of the reaction was also forty seconds long as the results after forty seconds were the same as it was for forty seconds.
The graph of the sixty percent concentration (30ml HCl 20ml water) of hydrochloric acid lasts for longer than the previous two concentrations as it lasted for fifty seconds. The reaction starts slowly and from twenty to thirty seconds the reaction starts to get faster. From twenty to forty seconds the reaction stays at a steady pace, until it gets to forty seconds where the reaction starts to slow down and the reaction ends by fifty seconds. This graph is quiet similar to the graph of the previous two concentrations because of the shape of the curve , but what is clear is that this reaction takes 50 seconds to produce 25cm3 of hydrogen whereas the previous graph took two and a half seconds. The 100% concentration took two seconds. This is evidence that as the concentration decreases the rate of reaction decreases too.
The graph of the forty percent concentration (20ml HCL 30ml water) of hydrochloric acid lasted for more than eighty seconds but had to stop the experiment before the reaction could finish because it took a lot of time and we would not be able to do all the experiments in time otherwise. The graph has a strong, positive correlation as all the points are very close to each other and fit the trend very well. This is proven with a good r squared value of 0.9983. The r squared value tells you how well your results fit the line of best fit and the closer the r squared value is to 1 the better it is. The reaction seems to have gone almost at a steady pace throughout the experiment. As the amount of molecules of hydrochloric acid decreases the longer it takes for the magnesium strip to have fully reacted. For the eighty seconds that we measured there seems to be a uniform increase in the volume of hydrogen produced. This time it takes seven and a half seconds o produce 25cm3 of hydrogen.
The lowest concentration of hydrochloric acid used was twenty percent (10ml HCl and 40ml water. This reaction also took longer than ninety seconds which was inevitable because it was the lowest concentration so it will take a lot of time for the magnesium strip to have fully reacted with the small volume of hydrochloric acid. The graph shows that the reaction started of slow and increased for about ten seconds and then carried on at a lower steady pace until it reached fifty seconds where the reaction speed up a bit and continued producing hydrogen at that speed. The r squared value is 0.973 which is not a bad r squared vale but there seems to some outliers during twenty, thirty and forty seconds.
There are no surprises in my data as the reasons for the reaction in different concentrations can be explained and it is expected that in lower concentrations the reaction takes longer than it would in higher concentrations due to the number of collisions between the reactants (the chemicals that react together in a reaction) are low due to less hydrochloric acid molecules whereas in higher concentrations there are more. It is also expected that the graphs of the higher concentrations react quickly at the start therefore the graph will have a steep gradient because towards the end there are less reactants (like magnesium) because they have mostly reacted with the hydrochloric acid.
If I had recorded the volume of hydrogen produced for lower concentrations such as five percent the reaction might take hours to finish and will only react with some of the magnesium before being used up. This is because there are not enough collisions taking place and only a small fraction of those reactions are actually successful.
Evaluation
Evaluation of Method
My method involves a range of different equipments that are best suited to do the job. An example of this is using a burette instead of a 50ml measuring cylinder. A 50ml measuring cylinder measures in intervals of 1cm3 whereas a burette measures in intervals of 0.1 cm3. Using distilled water instead of tap water means that there are fewer impurities which mean that you are confident that there are no catalysts which affect the rate of reaction.
In my method I did not measure the mass of the magnesium but instead I measured the length of magnesium ribbon needed using a 30cm ruler and cut it using scissors. This means that the magnesium ribbon could have been more or less than the length needed (2cm) so this could affect the results of my experiment because having different lengths of magnesium strip means that the surface area is also different. If the magnesium strip was a little longer than it should have been then there is more surface for the hydrochloric acid o react with on the magnesium which means that there are more collisions. More collisions mean more successful collisions which in turn mean more hydrogen gas is produced. If the lengths if the magnesium strip were shorter than 2cm then there will be fewer surfaces for the hydrochloric acid to react with so there will be less successful collisions and less hydrogen being produced. What I could have done was cut out 2cm of magnesium and make sure that the length is exactly 2cm by measuring the length again. Then I could weigh the strip and find out its weight, so in the next experiment I cut out the strip, weigh it and compare the weight. If it weighs too much then I know that it is longer than 2cm so I can make it smaller, and vice versa. For this a weighing scale that measures correct to 3 decimal places will be needed to ensure the weight and because it is more sensitive to small changes in weight than a weighing scale measuring correct to 1 decimal place.
I would also maybe clean the magnesium strip next time with distilled water or try rub of any oxidation and lumps of other elements o prevent contamination so that I am confident the magnesium strip does not have any other elements that could be catalysts or could affect the rate of reaction.
I did not continue the experiment if the reaction went on after 80 seconds because it consumed a lot of time and continuing with the experiment after that time would mean that I would not be able to finish all the experiments in time. In future experiments, provided that I have enough time, I would make sure that the reaction continues so that I can see if there are any changes if the experiment continues and in the 20% concentration graph I can see that the volume of hydrogen produced rises steadily but I do not know how the graph will look like towards the end of the reaction. This also means that I can be more confident in the conclusions I make because I know what happens throughout the whole experiment.
The temperature throughout the experiment stayed in-between 28 degrees and 28.5 degrees Celsius, which is quite a bit higher than room temperature and will affect my results a lot, but it will affect all my results. Because it affects all my results they are still in proportion and I can compare them to each other and see the differences between them, for example; the rate of reaction decreases as the concentration decreases. I don’t think it will affect the reliability of my conclusions but a bit of the reliability of my results. The changes in temperature were only 0.5 degrees between all the different concentrations and the temperature was the mean of the temperature before and after the reaction took place.
During the final stage of the experiment where you have to put the magnesium strip into the solution and place the bung and start the stopwatch. There will be delay when doing all those things so having two people doing the experiment helped. One person put the magnesium strip in and was ready to start the stopwatch and the other person put the bung onto the conical flask as soon as the magnesium strip was put in. The stopwatch was started as soon as the bung was placed so the time between starting the reaction and timing it was kept to a minimal. There still was a delay but I think it was so insignificant because it was probably 1 or 2 seconds and the reaction would have just started.
The pH of a solution is the number of hydrogen ions in it and because the volume of hydrochloric acid decreased as the concentration decreased here were less hydrogen ion from the hydrochloric acid. Water even though has hydrogen does not form ions as it is formed covalently and is neutral. I could have just checked the pH of the solution using a pH meter which I could do in future experiments.
Reliability of results
During the experiment, the variable that was being tested was the concentration of HCl. Therefore, to try and maintain reliability, all the other variables that can affect the rate of the reaction were kept constant. The variables that were deliberately kept constant were temperature, length of magnesium strip, volume of solution (HCl + water) and duration of experiment (time). These were maintained for each of the repeats at the different HCl concentrations.
In the 100% concentration graph the mean error for the upper quartile is 3.3 and the mean error of the lower quartile is 2.9.
In the 80% concentration graph the mean error for the upper quartile is 2.4 and for the lower quartile it is 2.2.
In the 60% concentration graph the mean error for the upper quartile is 1.2 and the lower quartile is 1.3.
In the 40% concentration graph the mean error for the upper quartile is 0.9 and the lower quartile is 1.
In the 20% concentration graph the mean error for the upper quartile is 0.23 and the lower quartile is 0.2.
The mean errors for both quartiles decrease as you go down the concentrations from the highest to the lowest. This means that the results become more reliable because the range is narrowest in the lowest concentration which means that the results are close for all the trials whereas in the highest concentration the range is not as narrow which means that the results for the three trials are not as close. Therefore the results are less reliable than the results for the lowest concentration. This is probably because the change in volume of hydrogen is not so fast in lower concentrations than it is with higher ones. Therefore reading the volume of hydrogen produced at a ten second interval is more accurate, due to the volume of hydrogen produced does not change so quickly.
There may have been impurities on the magnesium strip and I did not clear the magnesium strip before it was used so any impurities could have affected my results. If there are impurities that react are able to react with the hydrochloric acid then they will use up some of the hydrochloric acid which means that the volume of hydrogen produced is lower. This also means that there could be other gases being produced and as I did not check o see if the gas was actually hydrogen so it could have been any gas that was formed. Therefore I could have made sure that the gas from the burette did not escape and put a burning splint into it. If the burning splint goes out, here is carbon dioxide present (maybe calcium carbonate is an impurity, if the flame burns brighter then there is oxygen present as it is what you need to make a fire stronger (maybe there are impurities such as hydrogen peroxide or manganese dioxide), and if there is a little explosion and you can hear a squeaky pop then there is hydrogen produced.
At 80seconds in the 40% and 60% concentration there are results that do not fit the rest of my data and it goes against my prediction. This is not an anomaly but it could be because there was an impurity or which made the 60% concentration result different to the rest or because I used a longer strip for the 40% concentration so there was more surface area for the hydrochloric acid to react with. There aren’t many specific outliers in my results as you can see in the graphs so in general they all follow the positive correlation trend. The variables that were meant to be kept constant as proven by a narrow range or variation within repeats which makes my results look quite reliable.
In the first ten seconds of the 100% concentration results there seems to be the only clear outlier because the results from trial 1 (9cm3) to rial2 (4cm3) is half the volume of hydrogen produced and it rises back to 7cm3 . This could be due to the time taken to put on the bung because sometimes I could not put the bung into the test tube quickly. Therefore the volume collected was less in the first ten seconds because some hydrogen gas escaped.
Conclusion and the reliability of the conclusion
My conclusion is that from the results that I have attained and from the clear trends shown on the graphs that the relationship between the increase in concentration and the rate of reaction is that as the concentration increases so does the rate of reaction, this is because the volume of hydrogen produced is higher for the because there is more hydrochloric acid to react with the surface of the magnesium and her are more successful collisions between the reactants so more hydrogen is made from the reaction. In low concentrations there are not as much hydrochloric acid molecules to react with the magnesium so there are less successful collision so the reaction takes place slowly.
The results at 80seconds for the 40% and 60% concentration do not follow the general trend of the other results, which is hat as the concentration increases the volume of hydrogen produced also increases. Here the 60% concentration produces on average 25.7 cm3 of hydrogen whereas the 40% concentration produces 26.2 cm3 of hydrogen. This challenges the reliability of my conclusion because it goes against it but it could just be an error because of the length of magnesium strip put in being too long for the 40% concentration which increases the surface area and therefore there will be more collisions. It could also be because of impurities in the magnesium strip for the 60% concentration which also reacts with hydrochloric acid and reduces the surface area of the magnesium and uses up some of the hydrochloric acid by reacting with it. This would decrease the volume of hydrogen gas produced. It could be both of those factors together that caused this effect.
I could be more confident in my conclusion if I had other concentrations to work with and enough time to do all these concentrations. Then I could show that the conclusion applies for most concentrations and it would make my conclusion more reliable. I would also continue with the experiments that I have been doing and not stopped at 80 seconds so that I can see more results and see if this is true throughout the whole experiment and show that my conclusion is true and reliable because even when carrying on with the experiment it follows the same trend.
I would also check the pH of the water added to make sure that the water added does not alter it in any way and also keep a record of the Ph of the solutions and compare it against what the actual pH should be for that particular concentration and comment on whether it was at the right pH or whether the pH is slightly higher or lower than it should be.
