These are important as if they are not observed the experiment will not be a fair test. For these reasons we will perform the experiment the same way each time we do it. We will do the experiment at:
Room pressure and temperature, although we cannot keep these exactly the same all the time; but have to rely on them being almost the same.
There will be no catalyst added.
We will use the same powdered marble each time.
We will use 1 gram of powdered marble each time.
We will use distilled water.
We will use 2M HCl each time.
We will use 20 cm cubed or HCl solution each time.
We plan to take 4 sets of results, ranging from a 2.5 mls of HCl in the 20 mls to 20 mls of HCl in the solution. We will take a reading of how much gas has been produced in the gas syringe every 10 seconds for 1 minute. We will then take and average from the 4 sets to give us a truer reading.
Preliminary tests
In this experiment my main task was to find out the correct amount of mass for the marble powder to use, by using different masses of marble chips, but needing to keep the concentration of hydrochloric acid the same.
Prediction: Before I carried out my preliminary task I had predicted that an increase in marble chips would lead to an increase in the production or CO2 given off.
I used different masses of calcium carbonate for my preliminary work, so that I could determine which mass would be appropriate for the final experiment. The masses of the calcium carbonate were as follows:
1 g of calcium carbonate, 2 g of calcium carbonate, 4 g of calcium carbonate, 6 g of calcium carbonate, 8 g of calcium carbonate, and 10 g of calcium carbonate. I followed the same method as used in the real experiment, shown below, and eventually decided on 1g or calcium carbonate as no more was needed and it was just wasteful. We also decided on using 2M HCl because it was strong enough to make the reaction work and would still create a reaction when diluted, as we were going to be doing.
Method
To keep ourselves safe, we will wear safety goggles throughout and we will use the HCl carefully so that we don’t burn ourselves. We will tuck in all stools so that n0 one falls in the lab, and do the same with bags. We will never run in the lab, and we will always carry out the experiments standing up so that if we are in danger we can move away quickly.
First, we will collect out materials and apparatus. These include:
30 1g samples of medium sized marble chips, measured using a top man balance
2 M hydrochloric acid
Some distilled water
A gas syringe, with delivery tube and split bung attached
A burette
A conical flask
A stopwatch
A 10 ml measuring cylinder
A clamp
A funnel
Pen and paper to record results
We set up the experiment apparatus by clamping the burette to the desk, and then filling it with HCl through the funnel. We made sure that the tap was always shut to prevent spillage. We then measured out the HCl from the burette into the conical flask, and the water in the small measuring cylinder, which we then added to the conical flask. This changed the concentration of the HCl. We then measured out several separate grams of marble chips, so that we had a plentiful supply and didn’t run out. We made sure everything was ready, then we put the marble chips into the flask and as quickly as we could we put the bung of the gas syringe into the flask. We had to do this so that the least amount of gas possible would escape and the results would be accurate. At the same time, the other person started the stopwatch and recorded how much gas had been produced every 10 seconds, until a minute had gone by. We repeated the experiment with 20 mls of HCl and no water, 17.7 mls of HCl and 2.5 mls of water, 15 mls of HCl and 5 mls of water,12.5mls of HCl and 7.5 mls of water, 10 mls of HCl and 10 mls of water, 7.5 mls of HCl and 12.5 mls of water, 5mls of HCl and 15 mls of water, 2.5 mls of HCl and 17.5 of water. We then repeated all these results 5 times so that we could take an average and give us a more correct result. I think we need at least this many to give us a fair average- otherwise it might be biased by anomalous points. We then put all the results into tables, took averages and drew graphs.
Safety: Where any of the results went straight up to 100 cm cubed, after 5 seconds we removed the bung from the test tube. If much longer had been left, this could have been dangerous due to a build up of pressure resulting in the shattering of the test tube.
Prediction
The experiment we will do will involve the concentration of 2M HCl when it is added to powdered marble. I predict that the lower the concentration of HCl the less gas will be produced in the 60 seconds we will be timing for. I think this will happen because water does not react with marble, so the less HCl there is in the conical flask, the smaller the reaction and so less gas will be produced.
Obtaining Evidence
The most important thing in the whole experiment is that we performed it in a safe way. For this reason, we wore safety goggles throughout and we used the HCl cautiously to make sure that we didn’t spill anything. We both made sure that we knew that if we did spill any on ourselves or anything else, we would halt the experiment immediately and wash our hands thoroughly and wipe up any HCl that might have been spilt.
When we perform the experiment, we will take 5 sets of results so that we can take an average from them. This will give us a more likely set of results and we will then be able to say what we found out. If we only took one set of results, we might have anomalous points and not realise it. This would give us an unfair set of results and therefore the wrong conclusion. One of us wrote the results down in clear tables so that only one pair of hands was wasted, and the other took control of most of the experiment.
To get as accurate results as we could, we had to make sure that we performed the experiment carefully. This meant we had to keep all the factors that might affect the experiment the same every time. We also had to make sure that nothing else affected the experiment. This meant that we had to wash out the conical flask every time we performed the experiment. We also made sure that we read the results off the gas syringe as close to exactly ten seconds between readings, and we used a stop watch to keep the time. We made sure all the gas from the syringe was expelled before repeating the experiment. We took amounts of HCl from a burette very carefully so as not to go over or under the specified amount. The results we took were spread over a god range, from 2.5 mls of acid in the solution to 20 mls in the solution. Results too close together would not give a very obvious conclusion. We repeated the experiment several times in case our results varied a lot. We then worked out the averages and plotted them on a graph.
Analysing evidence and drawing Conclusions
We have found out:
We have discovered that the higher the concentration of HCl, the more gas (oxygen) is produced when it is reacted with marble chips.
Processing Results
I processed my results by making tables for all the results. From these, I calculated averages for each concentration of acid, and made another table to show these. I then made a graph showing all the averages. From this graph, I could that a pattern had developed. It showed that the lower the concentration of acid, the less gas was produced.
I also calculated the gradients of each line on the graph.
0.25M. 13 divided by 10 = 1.3
0.5M. 15 divided by 2 = 7.5
0.75M. 11 divided by 1 = 11
1M. 15 divided by 3 = 5
1.5M. 10 divided by 1 = 10
1.75M. 13 divided by 1 = 13
This generally showed that the higher the concentration of the acid, the higher the gradient of the graph.
The Collision Theory
When a reaction takes place, the particles of the reacting substances must collide with each other and the activation energy must be reached for the reaction to occur. The reaction will be speeded up if the number of collisions is increased. This can be done by increasing the temperature, by which the particles gain more energy and so the reeaction happens quicker, and also by changing other variables, of which can be read about in my planning section.
However, the variable I am interested in is concentration, and if a solution is more concentrated this will result in more particles of the reactant hitting about between water molecules, which creates collisions between important molecules more likely. If a solution is more dilute, there are less important molecules for th reactant to collide with and so the rate of reaction is slower.
Conclusion- prediction proved?
My prediction was proved right. This was shown by our results, which showed that the lower the concentration in a set amount of HCl, the less gas was produced when it was added to marble chips. We did expect this as we knew there was less acid in the set amount of 20 mls as it was diluted. This meant there was less acid for the marble chips to react with, and thus less gas was produced. We realised this might happen because of similar experiments that we have done in the past which show us that the more diluted things are, the less strongly they react.
Evaluation
I think our experiment worked quite well but there was definitely room for improvement. I think that a couple of our results were inaccurate because my graph and tables show the 1.5M acid managed to produce more acid than the 1.75M acid, when it should have produced less. I think this is because we did not get the bung into the flask quick enough and this made a significant difference to the results. We also had an old syringe and this could have meant that it got stuck at one point and we had to free it, which would have made a small difference to our results but I think that this is unlikely because I have no recording of it and I do not remember it happening either. We also would have performed the experiment again if this had happened.
However the reason may have been because of a slight fluctuation in the temperature of the room. I think this or the reason that we didn’t get the bung in quick enough are the most likely reasons because we wouldn’t have noticed anything odd happening. We could have prevented the temperature from fluctuating by perhaps performing all the experiments in a water bath, keeping the temperature at a constant, for example 30 degrees. It was probably not the best method to carry out the experiment but it was the best we came up with with the equipment we had. If we had had more advanced apparatus, perhaps we would have had a better system so that no gas escaped and the results were precise rather than the “close” ones we got. If I did the experiment again, I would like a better system of preventing any gas from escaping so that the results were more accurate and it was a fairer test. We could have used something that had a separate container inside it which we could have put the marble chips in and the HCl in the main container. Then we could have tipped the small container inside the large one to release the chips. No gas could have escaped.
We also could have done more experiments between the ones we performed, at say 0.3M, 0.4M etc. This would have made our results more reliable and would have backed up our result with more evidence.
I could have drawn my graph on the computer which would have made it more accurate, but the facilities for drwing line graphs mean that the line follows the points exactly and does not allow for anomolous results and so does not give a smooth curve. This would not have allowed me to draw a reliable graph or find the gradient of the lines, which meant that although it was not a great option, I had to do it by hand.
I think we had enough results to draw a sensible conclusion because we still managed to get clear results to prove our prediction right even though we did have a couple of anomalous results. However we only had one slight anomolus result, on the 0.5M line on the graph, and the fact that the tables showed that the 1.5M acid produced more gas than the 0.75M acid, which shows that although our capabilities were limited, we managed to prove our prediction, even if it was not as accurate as we would have liked.
