To use one or more catalysts in my experiment may leave me with inaccurate results as catalysts are hard to control and there is not a known catalyst for this experiment anyway. If you introduce a catalyst to a solution all you are doing is giving the reacting particles a place to stick to so they can collide into each other.
Considering these possible choices, I have decided to change the concentration of the hydrochloric acid, as this is one of the most straightforward of the possible variables to adjust. An additional reason was that it would be easier to execute the experiment in less time as I have only a short time to complete the experiment so I believe that it is best to use this variable. Advantages of using this variable include the benefit of experiments overlapping into the next lesson - so if I am unable to collect all results in one lesson, then I can simply continue without affecting my experiment’s validity. One extra gain that is available is that there are a wide range of concentrations available - any value from 0% concentration to 100% concentration.
Preliminary work
I need to do some preliminary work before I start my main experiment. This is so I can check that my measurements of magnesium and hydrochloric acid are correct, and that the range of concentrations I have chosen are suitable. I will be particularly interested in the lowest and highest concentrations, checking to see whether they have produced enough, or too much hydrogen gas. The optimum rate that I am trying to find is a reaction that is neither too short nor too long, so I can obtain enough results to plot a good, accurate graph. The hydrochloric acid also needs to be in excess for the duration of the reaction.
I set up the apparatus (see diagram in method section). I carried out several separate experiments to see which quantities, concentrations and equipment would be best to achieve the most accurate results. I collected the hydrogen in the measuring cylinder so I could accurately calculate the amount of hydrogen given off at a given solution, but the cylinder was filled with hydrogen after a few seconds. I tried to time the hydrogen produced every minute but that was too complex and most of the reactions took less than a minute to empty anyway. Next, I tried taking a reading every fifteen seconds, but for the higher concentrations, the reaction was too violent to take that many readings. Finally, I decided that the best way to carry out the experiment was to count how long it took for the cylinder to fully empty of water and fill with gas (the gas taking the place of the water). I had a few problems dropping in the magnesium, putting in the bung and starting the stopwatch quickly in sequence, so a friend had to help, leaving one less person to check the results for validity.
In the preliminary work I carried out an initial trial. I found the suitable measurements for the hydrochloric acid and the magnesium.
From these results, I have decided that the 25:25 combination of acid and water is too slow so when I’m collecting my final results, I will not use this experiment, choosing instead to proceed with the other five.
The temperature variable has to be fixed. I need to decide upon the exact temperature to use. Once we find the right temperature, we must maintain it throughout the experiment in order to conduct a fair test. I tested this variable with a standard concentration of 2M of hydrochloric acid.
Room temperature (25ºC) is a reasonable temperature to work with. As I discovered whilst performing the preliminary investigations, it is a complicated variable to alter since to maintain a standard temperature throughout one experiment requires special equipment so temperatures that are above or below room temperature are unsuitable. If I was to change the temperature, the practical work would become very complicated, and with the time allocated for practical work in class, I would be likely to run out of time.
I decided not to test the amount of magnesium ribbon and immediately decided on a value of 0.20 grams as I have frequently used this substance in the past and I am sure that this will not create a violent reaction and also that the results will be noticeable. All preliminary work results were conducted using 0.20 grams of magnesium ribbon.
Prediction
I expect that the quickest and most vigorous reaction will be when the concentration is 2M. I would expect the order from the most vigorous reaction to the slowest reaction to be, 2M, 1.8M, 1.6M, 1.4M, 1.2M. This idea originates from when I analysed my preliminary work. At higher concentrations there are more acid particles, therefore more chances of effective collisions resulting in a faster reaction.
The 2M hydrochloric acid concentration is the concentration that I would expect to react quickest because it possesses the most hydrochloric acid molecules. With more molecules in the solution, it is considerably more likely that more collisions will occur between the two molecules, as some molecules must collide if the solution is to react. This means that a reaction is more likely to take place in a shorter time, making the rate of reaction quicker. The more successful collisions that take place between the hydrochloric acid and the magnesium, the greater the amount of hydrogen produced. The concentration of acid and the rate of reaction are in proportion to each other, - when the concentration is increased, the rate will also increase. When the concentration is decreased, the rate will also decrease.
As the concentration of the hydrochloric acid increases there will be more successful collisions between particles therefore a quicker rate of reaction. I predict that the rate of the reaction will continue to increase as long as the concentration of hydrochloric acid increases (providing all other factors remain constant). I also predict that the greatest concentration will complete the reaction in the shortest period of time and therefore have the quickest reaction rate. This prediction automatically means that the remainder of lower concentrations will take a proportionately longer time to complete the reaction.
I think that the graph will form straight lines and that the lines will be steeper as the concentration rises up to 2M. The line with the steepest gradient on my graph will be the highest concentration of hydrochloric acid - every line next to that line will represents a decrease in the concentration. I expect the line pattern to be similar to this prediction as there is initially a constant reaction as particles from both reactants are abundant. The reaction finally will stop when all the energy has been exhausted (this will appear as a straight line approximately half way along my graph near the top right).
To summarise, I think that the reaction with the highest concentration of hydrochloric acid will produce the most hydrogen.
Method
I will quickly drop the length of magnesium into the flask and connect the rubber bung to it. I will then start the stop-clock and watch carefully until enough gas has entered the measuring cylinder so that all water has been completely emptied from it. As soon as all water disappears, I will stop the stop-clock and record the volume of gas produced. I will repeat each experiment three times so that my results are reliable and informative.
The concentrations that I’m going to use are:
I set up my apparatus as shown in the diagram overleaf. I then measured out the correct concentrations of hydrochloric acid and water, which were then added to the conical flask. To produce the desired concentrations, I had to add water to the solution. In order to reach the correct concentration – for example 1.0M, I added one part water to one part hydrochloric acid. I repeated this process for all the concentrations I used. Next I weighed out 0.20g of magnesium that was about 7cm long. I was careful not to bend the magnesium before dropping it into the flask so the acid would cover it completely. The magnesium was then dropped into the conical flask, the bung added before any gas could escape and the stop-clock started. I timed how long it took for 100cm3 of gas (the whole cylinder) to collect in the measuring cylinder and I recorded this in my results table. Each experiment was carried out three times so that I could take an average later.
Apparatus
- A syringe
- Small beaker
- Initial Cylinder (for storing Hydrochloric acid)
- Measuring cylinder
- Large water container (so gas cannot escape)
- Long measuring cylinder
- Safety goggles
- Stopwatch
- Small container (for storing magnesium)
- Bung
- Delivery tube
- Conical flask connected to Bung and Tube
- Magnesium
- Hydrochloric acid
- Scales
Safety
To ensure a high level of safety in my experiment I will ensure that:
- When carrying out the experiment I will wear safety glasses in case of spillage or if the substances spit during reaction. At smaller concentrations, hydrochloric acid is relatively harmless but hydrochloric acid is corrosive and as the concentrations that I am planning to use rise up to 100%, precautions need to be taken to avoid severe injury.
- All bags will be kept out of the way; I will ensure this by putting them under the desk.
- The experimental work surface will be kept neat, tidy and uncluttered.
- In this experiment, if only in small amounts, hydrogen gas is being produced. This gas is explosive so I will be careful not to combine it with any other form of gas, flammable material, or indeed fire itself.
- I will ensure safe disposal of reactants and chemicals.
- Care will be taken when returning all used glassware and equipment at the end of the experiment.
The apparatus was erected as shown in the diagram above. I will measure out a rough amount of hydrochloric acid into one small cylinder. Next I must add the magnesium to the hydrochloric acid and quickly insert the bung into the conical flask before any reacted hydrogen can be lost. These two pieces of apparatus will be placed at the bottom of the water in the tub. I will start the stopwatch as soon as the magnesium is added to the solution. I will time how long it takes for the cylinder to empty of water. I will read the result by using the scale printed on the side of the tube as a guide.
I will perform each separate concentration experiment three times, so that I will be able to obtain a series of results to create an average for each concentration. In addition, I will be comparing the averages once the experiment has concluded. I will do fifteen different experiments in total (five times three equals fifteen). If any mistakes are made during any experiment, I will discard that individual reading, and redo that single experiment again.
I considered adding an extra concentration of 0% (water) but this would not provide a reaction therefore the result would be useless, as the graph line for this concentration would simply run the length of the x-axis.
Fair Test
To make sure that my experiment is fair to create precise results, I will thoroughly clean each piece of equipment or use a different set for each experiment to avoid contamination. Also to prevent contamination, I will keep each reactant in separate beakers. I will make sure more than one person is watching each experiment so that inaccurate results are not recorded. The hydrochloric acid temperature, the volume of the hydrochloric acid and the length of magnesium ribbon must all be kept the same, unvaried. I will take three readings for every reaction and calculate an average from those three results. I will scrape the magnesium with sand/emery paper, before weighing it, so that no other substances are inadvertently included in the reaction. All of my measurements will be made as precise as possible to keep the experiment accurate and fair. I will make sure that the gas connection tube is correctly connected and put in place firmly, quickly and tightly so that no hydrogen gas escapes. If I fail to do this, I will restart the experiment. I must be cautious of all the glassware I am using, as it is breakable and can cut through skin. I will also be careful to dispose of all chemicals safely once I have finished using them.
An important point to mention involves air unintentionally entering the measuring cylinder before the reaction starts. If this happens, again the experiment should be stopped and restarted. When I measure the volume of hydrochloric acid before and after the reaction, I will take the reading from the bottom of the meniscus consistently to prevent any inaccuracies, however small. The temperature I have used is room temperature, because this will stay constant. The reaction is exothermic and so will heat up, but this change will be the same for all the experiments so it should not affect my results. I will not locate my experiment near any open windows, heaters or any other sources of temperature change. I will try and complete my whole experiment at the same time so there isn’t any time for temperature changes. I will not use a catalyst in any of my experiments. The only variable I will change is the concentration of Hydrochloric Acid.
Results
The results I gained from the experiment are shown in the table below:
Adding up all three results and dividing by three (the number of experiments) calculated an average time.
Conclusion
In total, I tested five different concentrations of hydrochloric acid. With 1.2M, which was the lowest concentration of acid that I used, there was a slow reaction. 2.0M hydrochloric acid was the highest concentration I used so it came as no surprise when it produced the fastest reaction. I repeated all five concentrations three times to be sure that the results I had collected were reliable - in all cases the results were almost perfect meaning I encountered no anomalous results. The results I have obtained and the graphs I have plotted have showed me that as I increase the concentration of the acid, the rate of reaction increases - that as I increase the amount of acid, the magnesium ribbon disappears at a faster speed, which indicates that more hydrogen is being produced.
The gradient on the graph for all reactions is noticeably steep. This shows that the reaction in general is very fast. The results then show that as the concentration lowers, the reaction rate lowers too. My graph shows that the reaction rate for the higher concentration is quickest because it has the steepest line that once again shows that more hydrogen was produced than when there was a lesser concentration of hydrochloric acid.
The 2M concentration (highest concentration I used) produced the most hydrogen in the measuring cylinder. This results in a higher production of hydrogen bubbles than my other results, which in turn shows that my prediction was indeed correct: ‘the reaction with the highest concentration of hydrochloric acid will produce the most hydrogen gas’. I said in my prediction that the higher concentration the hydrochloric acid, the more hydrogen would be produced. I also said that the concentration and the rate would be in proportion to each other, (i.e. when one increased the other increased too). My results follow an almost perfect pattern so I can positively say that my results support my prediction. My anticipated results that I produced in my preliminary section were proved correct. As I said, I encountered no anomalous results.
Evaluation
I intended to discover how the concentration of hydrochloric acid affects the amount of hydrogen produced in a reaction involving a consistent amount of magnesium. The results (see results table) weren’t unusual - they were as I predicted and expected from the start of the experiment.
My experiment was not 100% reliable, although it’s generated quite accurate results. There are things that could have altered my results so that they became less accurate. However these errors would only be small compared to the overall results. My results definitely show that the higher the concentration of hydrochloric acid, the quicker the rate of the reaction and therefore the more hydrogen produced.
I could have made the experiment more accurate in a number of ways such as developing a way to prevent any hydrogen escaping by perfecting a way to attach the bung instantaneously, or perhaps creating a completely different technique altogether such as using equipment that had a pre-fixed bung and a tube leading through the bung, where the magnesium would enter, preventing any gas from escaping. I could have also included more concentrations in between the existing ones (as I incorporated the even concentrations into my experiment – 1.2M, 1.4M etc, perhaps in a future experiment I could use odd concentrations like 1.1M and 1.3M as well as the even concentrations) and maybe let the experiment run for longer too, in order to get more accurate results. One final inaccuracy was when we cleaned out all the various pieces of equipment; they could (and almost certainly did) still contain traces of hydrochloric acid, water, unreacted magnesium, magnesium chloride or hydrogen. This would give some individual experiments an advantage or a disadvantage, depending on which experiment was receiving extra solution and also what substance had remained to be included in the next experiment. The most obvious way to combat this dilemma was to use new equipment every time but this is impractical as there simply wasn’t that much equipment for everybody to do this at the same time.
Some other important points I feel I need to add include:
I know that temperature can affect reactions from my preparation work. The experiment was conducted over two days. The ambient temperature was substantially different on both days, as one of the days was significantly warmer than the other. This may have affected the results to some extent. More conclusive results would have been obtained if the temperature could have been stabilised, perhaps by conducting the temperature within a heated box where the temperature could easily be controlled accurately for long periods of time. The air conditioning system was tampered with during one practical lesson. This could have caused a bit of an interference with the results since temperature is a variable that can affect rates of reaction. This variable was a factor that was impossible to control. Another improvement would be to repeat each concentration more than three times. This would make results more accurate when finding averages but time is once again the main concern.
The equipment we were provided with is old and overused, such as stop-clocks that frequently break. When measuring the volume of acid and the volume of water, I could have been a bit hasty due to the lack of time. Things may have been rushed towards the end of the experiment.
Atoms in all substances vibrate more as they have more energy to move about in a freer fashion. Consequently, when the temperature rises, the atoms collide more, which consequently produces a faster reaction. When the temperature is lowered the opposite occurs - the atoms have less heat energy, the particles vibrate less and the reaction inevitably slows down (providing no catalyst or other substance is added).
As all the points on my graph lie more or less on the line, I can say that there are no anomalous results. I think my results are reliable because I did each experiment 3 times. These repeats accounted for any anomalous results (although there were none). I believe that my results are sufficient enough to support my prediction and conclusion.
Extension Work
If I wanted to add proof to my results, or find other trends and patterns I could carry out some further work. This could include:
- Trying the same reaction again but with a different metal (e.g. potassium) to see if I acquire any new patterns compared to magnesium.
- Investigate a completely different reaction to find other patterns that relate to the original experiment (hydrochloric acid + hydrogen) regarding the rate of reaction.
- I could keep a constant temperature for my reaction by using a water bath. This would ensure that temperature is not a factor affecting the rate of reaction.
- The first is to find out what the rate of reaction would be for greater concentrations of acid but this would be more dangerous. It would be interesting to see how the rate increases compared to my greatest concentration of acid though.
5. Another experiment that I could do is change the type of acid that I use. I could use sulphuric acid. This acid has a different molecular build-up to hydrochloric acid so I would find new results to analyse and compare. I could also use the less reactive metals of the reactivity series - (zinc, aluminium, iron, lead etc) to find the rate of reaction for these metals. I would have to use higher concentrations of acid, such as 3.00M or 3.50M, when working with these metals, as they are lower in the reactivity series than magnesium.
Bibliography
The Supreme GCSE Chemistry Guidebook
The World Around Us - Michael Roberts
Various Science Revision Guides - Lonsdale
GCSE Bitesize Revision Website
Extension Chemistry – Bryan Milner, John Mills
I obtained most of the information that I have included from my teacher and exercise book. I also collected information from several science textbooks and computer encyclopaedias. I found a small amount of information from various Internet sites similar to the website I have included above (GCSE Bitesize Revision).