I also predict that if you double the concentration of hydrochloric acid, then you will double the rate of reaction. This is because there is twice the number of acid molecules in the same volume therefore you are doubling the amount of collisions between magnesium and hydrochloric acid resulting in doubling the rate of reaction. However, the exothermic factor will have an influence on the rate of reaction. This is because if you double the concentration there is more heat being given out, because the higher the concentration the more heat is given off. The heat will have an effect because that heat is used up in the reaction as energy for the particles which will increase the rate of reaction. This extra heat together with the double hydrochloric acid concentration will lead to more successful collisions per second and so I think that the reaction will not be twice as fast if the concentration is doubled, but will be faster that double due to the extra heat
Planning
Before doing the actual experiment I decided to do some preliminary work. These were to tell me details that I would need to know for my investigation to be successful. I decide to use 5 different hydrochloric acids, they are:
0.5m 1m 1.5m 1.75m 2m
For my mini experiment I measured 10cm3 of hydrochloric acid and poured it in a test tube. I then measured some magnesium and put it into the test tube filled with acid. As soon as the reacting started I started to time with a stopwatch, I continued timing until all the magnesium had reacted.
From this preliminary experiment, I noticed that when the magnesium came into contact with the hydrochloric acid, it started to react immediately and when the magnesium had reacted with the acid, the reaction would stop. The results I found were:
I realised from this experiment that my results were not accurate because I did not time them accurately. In this experiment when the magnesium strip was dropped in the hydrochloric acid, I did not switch on the timer straight away and so my results were not accurate. Also the ranges of acid I used were very constricted so I decided to use a wider range of acids. I also noticed that with one magnesium strip the reaction was too fast and it would not allow me to collect the amount of gas I wanted as the magnesium strip would have been eaten away.
After this trial experiment I decided that I would use a wider range of hydrochloric acids, instead of using one magnesium strip I would use two strips for this reaction. I also decided that I would start timing the reaction as soon as the magnesium strip was dropped in the hydrochloric acid, this would help to give me more accurate results. The other problem was that this was not an accurate procedure because the magnesium strip sometimes floated at the top of the acid or got stuck at the side of the test tube. Therefore the results could be unreliable and less accurate.
In this experiment, I used the following apparatus:
- Conical flask
- Delivery tube
- 2 measuring cylinders – Measuring acid.
Collecting gas.
- Washing up bowl
- Safety goggles
- Stopwatch
- hydrochloric acid
- magnesium strips
- water
The name of the method that I am going to use is called collecting gas under water. To do this experiment I will need to fill the washing up bowl half way with water, fill the conical flask with 25cm3 of hydrochloric acid using a measuring cylinder to measure the amount of acid. I will get the second measuring cylinder and fill it with water from the bowl, making sure that there are no air bubbles in the measuring cylinder, and then I shall make the cylinder stand upside down in the washing up bowl and insert one end of the delivery tube at the bottom of the cylinder in the water, while the other end will be attached to the conical flask. As soon as I put the magnesium strips in the flask I will shut the flask with a bung and at the same time I will start the stopwatch. I will repeat this experiment three times for each acid.
The ranges of hydrochloric acid I intend to use are:
0.5m 1.5m 2.5m 4m 5m
To ensure a safe experiment and working environment I need to have at least one metre squared of working space around me and wear goggles at all times to protect my eyes from the harmful acid. I will tie my hair back to avoid contact with any harmful substances. I will wipe away any spillages immediately to avoid any reactions. I will make the experiment accurate by using a pipette to measure the volume of acid correctly and accurately and I will also look at the measured acid from eye level. I will measure the time in seconds so that the results are more accurate and precise. I will always wash out the measuring cylinders to prevent the acid concentration being mixed up and giving us strange results, but I will also dry the measuring cylinders to prevent cross contamination, so that the water does not dilute with the acid. I will know that my results are reliable because I will repeat this experiment three times and I should get a similar result as before because the results should follow a pattern.
To make this experiment a fair test, I will weigh the magnesium strips, to ensure that I am using the same amount and length of magnesium each time. I will also use the same amount of acid for each experiment to make it fair. I will start the stopwatch as soon as the magnesium touches the acid and stop the stopwatch as soon as the magnesium has disappeared and the reaction has stopped fizzing.
Before doing this experiment I did try and weigh the magnesium strips but the scale was not powerful enough to weigh it and therefore it may have given me inaccurate
results.
The amount of acid I will use for this experiment is 25 cm3 and the amount of hydrogen I will be collecting is 25cm3 for each experiment. I will use 2 magnesium strips for each experiment.
Results
The results I got for my experiment are:
My results table shows 6 columns. The first column shows the hydrochloric acid concentration in molar. The next 3 columns show the results from each experiment. The next column shows the average time it took for the reaction to take place and the last column shows the value for 1/T.
When the magnesium came in contact with the hydrochloric acid, it started fizzing immediately. The higher the concentration the faster the magnesium strip was fizzing. The temperature rose proving that this was an exothermic reaction. The products I was left with were magnesium chloride and hydrogen.
Magnesium+ hydrochloric acid = magnesium chloride + hydrogen
Mg 2HCL MgCl2 H2
Conclusion
From my results and graphs I can see that 0.5 molar acid was the slowest to react with the magnesium as it took 80.86 seconds for the reaction to take place. The fastest reaction was with the 5 molar acid as it took 4.65 seconds for the reaction to take place. This is because 5 molar acid concentrations has more acid molecules in the same volume, therefore there were more particles in the acid leading to more collisions per seconds with the hydrochloric acid and magnesium and thus resulting in a faster rate of reaction.
I predicted that as the hydrochloric acid concentration increased, the rate of reaction would also increase, this is because as the concentration increased, the number of acid particles in the acid solution would also increase, therefore there would be more acid particles of magnesium to collide with, leading to an increase in more successful and frequent collisions thus increasing the rate of reaction. According to my results and my graph, my prediction was proved correct as it shows in the results table that as the concentration increased, the time it took for the reaction to take place decreased, indicating that there was a faster reaction. It specifies the same thing on my average time graph.
I also noticed that when you double the concentration, the rate of reaction was less than half the previous rate of reaction. For example when the concentration was 2.5 m the average time was 10.01 seconds and the 1/T value was 0.10, but when the concentration was doubled to 5 m the average time was 4.64 seconds and the 1/T value was 0.22
2.5 acid molar concentration x 2 = 5
2.5 molar average rate ÷ 2 = 5 molar average time.
10.01 seconds ÷ 2 = 5.01 seconds.
This shows that if the 2.5 molar concentration is doubled to 5 molar then the rate of reaction will be exactly half and will be 5.01 secs. But in my results table it shows that it took 4.64 secs for the reaction to take place with the 5 molar acid. Therefore the rate of reaction was less than half of the rate of reaction for the 2.5 molar acid concentration. The reaction was faster due to the exothermic factor. The 5 molar acid concentration released more heat than the 2.5 molar acid concentration therefore this extra heat from the 5 molar acid concentration provided the particles with more kinetic energy than the 2.5 molar did and thus the particles moved around more, collided with each other more, which led to successful collisions per second and thus leading to a faster rate of reaction.
The exothermic factor did affect the results because the extra heat from the reaction was used by the reactant particles that hadn’t already reacted, giving them more energy. This energy was making it vibrate more, move around more and collide more often which led to more successful collisions per second. It was due to this extra heat that the reaction was faster, because this extra heat gave the particles more energy to collide more often. At the start of the reaction there were the greatest number of particles to react with, and so therefore there was a much greater chance of the particles colliding and reacting with each other. The more particles there were the faster the reaction was. As the reaction proceeded, the reactants particles were being used up and so there were fewer particles left to collide with, which led to slower collisions and to a slower rate of reaction. Eventually, all the magnesium particles had reacted. When this happened, there were no more collisions between the reactants and so there were no more reactions.
While I was doing the experiment, I got an anomalous result that did not fit the pattern of the results I usually got. In first trial, the 5 molar acid concentration gave me a value of 7.72 seconds. This did not fit the trend and so I repeated the experiment again. This anomalous result may have been caused due to many reasons:
- The rubber bung – the bung may not have been closed tight enough on the flask and therefore some of the hydrogen gas might have escaped into the air, which would have meant that it would take longer for me to collect the amount of hydrogen I wanted.
- The measuring cylinder may not have been dried properly and therefore there may have been some water left in the cylinder which would lead to cross contamination as the hydrochloric acid would have been diluted with the water.
- The length and weight of the magnesium strip may have been measured inaccurately and therefore this would affect the time it took for the reaction.
The anomalous results may have been due to one of these mistakes that could have happened in the experiment.
Evaluation
Overall, I am happy with the results I obtained as they follow a pattern that I predicted, but there are many ways that I could improve my results. The size and weight of the magnesium strips was not measured accurately for this experiment, so the result may have been inaccurate and unreliable. To improve on this, I would make sure that I weighed and measured the magnesium strips with a powerful scale. Also, in this experiment it was very difficult to collect the gas as you had to hold onto the measuring cylinder in the washing up bowl during the reaction, therefore some of the gas may possibly have escaped and so the result would not be accurate. I could improve on this by using a gas syringe to collect the gas in the next experiment.
In the reaction, the exothermic factor caused a rise in temperature and therefore this will have affected the results. This is because the unreacted particles will gain more energy from this extra heat and therefore will collide more often with successful, effective and frequent collisions, leading to a faster rate of reaction. I could control this temperature by doing the experiment in a water bath which includes a thermostat, which is set at a constant temperature. To see what temperature it should be set at, I would do the 5 molar acid experiment and measure the maximum temperature rate at the end of the reaction. I would set the thermostat at that temperature, this discounts the exothermic factor.
I also found out from background information that the magnesium we were using was not 100% pure magnesium; it was magnesium oxide where the magnesium had reacted with the air. This may have affected the results because it may have taken longer for the acid particles to reach the magnesium itself and then react with it. To improve my results, I could clean the magnesium oxide from the magnesium by using sandpaper to sand away the oxide layer. Also, to make the results more reliable and accurate, I could weigh the magnesium strips after the oxide has been cleared.
In my investigation, I used concentration as the variable, to improve my investigation further, I could concentrate on the lower ranges of acid such as 0.1, 0.2, 0.3 etc. and see how they affect the rate of reaction. I could also use other variables such as, surface area, temperature, pressure for gas and the presence of a catalyst and observe how they affect the rate of reaction.
