From my preliminary I learnt a few things that, in order that I can obtain the best and most reliable results, I will have to enforce. Firstly I realised that the length of the magnesium I should use in the main experiment should be ten cm and that the time interval between the readings should be five seconds. Secondly, I realised that when I was measuring out the solutions, and when I was reading off the results, I would have to be careful of the meniscus. So in the real experiment I will have to be very careful to read off at the bottom off the meniscus and to read off the results so that my eyes are level with the meniscus. Thirdly, when I dropped the magnesium in to the acid, it occasionally landed flat down on top of the acid. This would mean that there was a smaller surface area of magnesium to be reacted and so it would be an unfair test. To prevent this I coiled up the magnesium and dropped it in so that it pierces the acid, so that it’s all covered. Lastly, I thought that it would be a good idea to rinse out the beaker after experiment. If I didn’t do this then there might be a bit of acid left in the beaker which would make the next solution more concentrated and so it wouldn’t be a fair test.
Apparatus:
- Magnesium Ribbon,
- Hydrochloric Acid 2 Mol,
- Distilled water,
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100cm3 Measuring Cylinder,
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Two 100cm3 beakers
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50 cm3 measuring cylinder
- Plastic bowl
- Side-arm flask and delivery tube,
- A Balance,
- A ruler,
- Timer,
- Metal Clamp stand, boss, and clamp,
- Pair of scissors,
- Pair of safety goggles.
Diagram:
Method:
I will set up the metal clamp stand, boss, and clamp. So that it can hold the
100 cm3 measuring cylinder directly vertical upside down full of water in the in the plastic bowl which will also be full of water. To put the measuring cylinder into the bowl so that no air gets in and no water falls out I will have to seal off the end with a small piece of paper, then when it’s in the water no air will be able to get into it. Next I will attach the delivery tube to the side arm of the side arm flask. With the apparatus I will use the delivery tube will be a very tight fit on the side arm and so it may need some lubrication to slide it on. Then the other end of the delivery tube will be placed into the bowl of water and under the 100cm3 Measuring Cylinder, so that any gas given off in the reaction displaces the water in it, so that I can see how much gas is given off. I will then fill one of the 100cm3 beakers with hydrochloric acid and the other beaker will be filler with distilled water. I will also cut a 10 cm strip of magnesium ribbon, and weigh it with the balance. Having taken it all to the bench where I am working I will make up the acid solution, in the 50 cm3 measuring cylinder to the concentration needed. I will be using four different concentrations in this experiment these are 0.5 Mol, 1 Mol, 1.5 Mol and 2 Mol. To make up the different concentrations I will need to add different amounts of distilled water to the 2 mol hydrochloric acid. To find out the amount of acid I need I will use the equation: concentration of solution, divided by concentration of acid, multiplied by volume of solution. So to find the amount of acid for a concentration of 1.5 Molar, I’d do 1.5 / 2 x 50 = 37.5. Then to work out the amount of water needed I minus the volume of the acid water from the total volume of the solution. So 50 – 37.5 = 12.5. The results are shown in the table below:
Next I pour one of the solutions into the side arm flask. I get the timer ready along with the bung and the coiled up magnesium. As soon as I drop the magnesium into the flask along with the acid I place the bung on. I will then record the amount of hydrogen given off every five seconds and stop timing when all the reactant, in this case all the magnesium, is used up. When the experiment has finished I will pour the cid into an organic waste flask and wash out all the equipment, and having set up all the apparatus again as before I will repeat the experiment three times for each concentration.
Safety:
In my experiment it’s vital that at all times I make sure that there is no chance that I can cause any injury to either myself or anyone working around me. So in this experiment the cautions I will have to undertake are:
- The magnesium ribbon is flammable and so at all times the Magnesium should be kept away from flames.
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The Hydrochloric acid is corrosive and so when I am handling it in the experiment I will have to be very careful about not spilling it on my self, and very wary of the people around me. If any is spilled it should report it and it should be mopped up straight away.
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Because the acid is corrosive I also need to be wearing safety goggles at all times.
Variables:
In my experiment there are a number of variables that might affect the results. So to keep my experiment a fair test I will have to make sure that all these variables are kept the same all the way through. The one variable I will be changing is the concentration of the acid, but the ones that have to be kept the same are:
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Changing the surface area. If the surface area of the magnesium is increased then the rate of reaction will be quicker. This is because, when the magnesium is in the form of a strip or ribbon the acid can only react with the atoms on the outer layer of the magnesium ribbon, but if it powdered or put in the acid in the form of lumps then there are a lot more magnesium atoms exposed and so the chance of a successful collision.
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Changing the temperature. If the temperature is increased the particles take in energy. This causes them to move faster and to collide more often. Then also because they have taken in energy when they collide there is more chance of them colliding with enough energy to break the old bond, therefore there will be more successful collisions.
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The use of a Catalyst. A catalyst is a chemical substance that speeds up a reaction, without actually being used up itself. It does this by lowering the amount of energy needed to break the old bonds. In other words a catalyst lowers the activation energy. It also provides a surface for the molecules to attach to, thereby increasing their chances of bumping into each other. In this experiment I won’t actually be using a catalyst at any time.
Fair Testing:
When I’m carrying out the experiment it’s vital that I keep all the conditions the same and make sure that everything I do it applied to all the experiments. If I do this then there will be no reason for the results to be anomalous or wrong, therefore this should produce very accurate results. The different things I will have to keep the same to keep it a fair test are:
- Use the same apparatus every time, this is because different apparatus may have different volumes and so the results will be inaccurate.
- Clean the apparatus after each experiment otherwise some acid may be left in the side arm flask and so the next solution will have ore acid and so will be more concentrated.
- Put In the bung very quickly. The slower I put the bung in the more hydrogen is lost to the air. But if I push the bung in too hard lots of air is pushed through into the measuring cylinder at one go and so the results may be affected by this.
- Cut the magnesium ribbon just before I actually need it this is because if it is left in the air too long then it will start to oxidise and so might prevent the amount of hydrogen given off and it might affect the speed at which the magnesium and acid react.
- Coil the magnesium ribbon every time so that it is all covered by acid.
- Keep the same amount of magnesium and acid otherwise the magnesium will have a larger surface area and the acid will have a different concentration.
- Keep the room temperature and the temperature of the acid the same, otherwise the particles will move faster and so the rate of reaction will be quicker.
References:
[1] Gallacher, R. and Ingram, P., New Coordinated Science: chemistry (for higher tier), (2001)136, Oxford.
Results:
Analysis:
Looking at my results on the graph, all the graphs are curves which the level off as all the reactant is used up, which in this case is at 100cm3, this shows that there is a relationship between the amount of hydrogen produced and the concentration of the acid. The graphs are a visual image showing me how quickly the hydrogen was produced. The graph showing the acid of 2 Molar occurs very quickly and has produced 87 cm3 of hydrogen in ten seconds where as the acid of concentration 0.5 molar had produced only 3 cm3 is 29 times less than the 2 molar concentration. Also looking at the graph the 0.5 Molar concentration doesn’t differ that much from the 2 molar concentration but as soon as I got to the 1 molar concentration there was a huge leap between the times. For example, the difference between the 2 molar and the 1.5 Molar concentration finishing was about 5.5 seconds. Then the difference between the 1.5 Molar and the 1 Molar concentration from finishing was about 25 seconds which is just under five times longer. The difference between the last two concentration finishing is 310 seconds, which is 12.4 times longer.
From the graphs I can work out the average rate of reaction with the formula:
Total volume of hydrogen (cm3) / total time for the reaction. Therefore I can work out the average time for each reaction.
This table of results shows the average reate of reaction and so over the whole reaction on avrage how quick it went. But from my grpahs I can also work out the intial rate of reaction which is the quickest the reaction can go, when time = 0. so if I draw a tangent on each of the graphs where time = 0 and find out the gradient of the tangent. To work out the gradient of the tangent, I will pick any point on the tangent and do y/x, in other words calculate amount of hydrogen / time. So I choose a set amount of hydrogen, see how long it took to produce that muc and divide them. These are the results:
This graph then proves my prediction where I stated that I thought that as the concentration increased the rate of the reaction would also increase. This is because as the concentration increases I believe this to be the case because as I increase the concentration of the hydrochloric acid then the number of acid atoms will be greater and so the chance of one of these atoms colliding with a magnesium atom will be higher and so the rate of reaction should be higher. My prediction has therefore been proved right. If I then plot these results of average rate of reaction against time a pattern might emerge. When I drew the line of best fit, I saw that if you double the concentration then the rate of reaction doubles. It is a positive correlation in direct proportion. This again proves my prediction as the rate of reaction is shown to increase when the concentration increases.
Evaluation:
Overall I’m relatively pleased with my results and I believe them to be very accurate. But when I looked at the graphs I saw that there was a possible anomaly. This was in the experiment where I was using a concentration of acid of 1 Molar. It looks it was just the results from the first run as this results is so much lower than the others. This might be due to the fact that when I put the bung in I didn’t put it in tight enough and so some gas could escape. It may also be due to human error. There is quite a good possibility of this as I had to put the magnesium, put the bung in and start the timer all at the same time. Then I had to record the measurement at five seconds and so it might have been down to me having slow reactions. I found this to be a big problem in the experiment, if I could do the experiment again I would have liked to have device where I when the magnesium touched the acid in the flask the time would start and I could just put the bung in. this would mean that the human error factor would be cut down if not eliminated.
Also run three of the 0.5 Molar experiment looked quite a lot faster that the other two runs. This was probably because this was the last run I did on a different day from the other two runs, and therefore the air and therefore the acid could have been warmer and so the reaction would have been quicker due to the fact that the particles have more energy and are moving quicker. If I was to do the experiment again this would be another area where I would, make a change. I’d use a thermometer and make sure that the acid was the same temperature each time I did the experiment. Also I could have done all the experiments in on day and so this would have limited the temperature range as the temperature can’t fluctuate to a large degree all in one day. Apart from that looking at the tables of results they all look very accurate as there isn’t too much variation in each run for the different concentrations.
If I could do the experiment again I would have also liked to do five runs for each concentration. This would then mean that there was an even larger average, so that any anomalous results were averaged out amongst the other runs. Also I would have liked to have done a larger range of concentrations for example going up in 0.2 Molars from 0 to 3 Molars, this would give more graphs and so more chance to spot patterns.
I would have like to use an electronic stirrer device. This would be more appropriate than me stirring it as I may stir it more or less for each of the other concentrations and so the test would be unfair. But I would have liked to use a stirrer because when I have the acid in the side arm flask if it has been concentrated then the distilled water may not be completely mixed with the acid and so some areas of the solution may be more diluted than others and so affecting the results. Also when the reaction was taking place the hydrogen that was being given off was sometimes sticking to the Magnesium ribbon, thus reducing the surface area of the ribbon. This would mean that there are less magnesium particles for the acid particles to collide and react with.
Although there are a few things that I would have liked to have changed, there wasn’t enough time to repeat the whole experiment again with the new improvements. But I think that the results are actually very accurate and very adequate for drawing up concise conclusions.