This is the method that I used to conduct all of my preliminary experiments. I changed the measurements of some substances so I could investigate them. I have replaced the values of reactants that I will be changing with “x”, so the method can be general for all of the preliminary work.
- Fill a tray three quarters full of water. (This will create the water pressure needed to keep the measuring cylinder full until the carbon dioxide displaces it.)
-
Fill a 100cm3 measuring cylinder full of water, place your hand over the top and tip upside down. Place in the tray of water.
- Fill a conical flask of xml of hydrochloric acid
- Measure x grams of small/medium/large calcium carbonate chips.
- Place a delivery tube on the conical flask, making sure it is on tightly so no air can escape. Put the other end of tube under the measuring cylinder in the water.
- Take off the tube, place in the marble chips and start timing. Put the delivery tube back on the conical flask.
- At ten second intervals, record how much of the water the carbon dioxide has displaced.
- Do this for a minute and record the results in a table.
My first preliminary experiment will show which size chips I should use in my main experiment. I have used two concentrations of acid but kept the weight of the chips the same to ensure a fair test.
50 cm3 0.5 mol/dm3 HCl
2g calcium carbonate chips
50 cm3 3.0 mol/dm3 HCl
2g calcium carbonate chips
Conclusion of results
These experiments were done so I could decide which size of chip I should use in my main experiment. After compiling my results, I have decided to use medium chips. This is because they gave reasonable results in both concentrations. The small chips in the 3.0m acid was too fast whereas the large chips in 0.5m acid was too slow. Medium chips are just right because they are the median size.
My second preliminary experiment will decide how many grams of medium chips I will use. I have already used 2g of chips and they gave me quite good results. Therefore, I shall investigate masses above and below 2g of medium chips, as medium chips gave me the best results in my last experiment. This will help me to see which one gives me the best set of results. I will also use the same two concentrations of acid as I did in my first experiment, which will also help me to make my decision.
50 cm3 0.5 mol/dm3
Medium calcium carbonate chips
50 cm3 3.0mol/dm3
Medium calcium carbonate chips
Conclusion of results
From both tables I can see that using only 1 gram of calcium carbonate chips gives me a very slow reaction and so it is not as measurable. On the other hand, 6 grams gives a very fast reaction, as the surface area is much greater, especially in the higher concentrated acid, and so this weight would give me a much quicker reaction than I want. Therefore, 2 grams seems the right weight to use, as it also worked well in my first preliminary experiment.
My next and final preliminary experiment will decide which volume of acid I should use in my main experiment. As I have used 50ml of acid in my first two experiments, I will experiment with another two volumes as well as 50ml. This will help me to see which volume of acid will give me the most measurable results for my main experiment. I will use 2 grams of medium calcium carbonate chips, as these are my decisions from my previous 2 experiments. I will also use both concentrations again.
0.5 mol/dm3
2g medium calcium carbonate chips
3.0 mol/dm3
2g medium calcium carbonate chips
Conclusion of results
From the first table I can clearly see that 25 cm3 does not give enough surface area to give me good results in a lower concentration. The rate of reaction for 25 cm3 was better in the second table but as I am going to investigate changing the concentration in my main experiment it will be unsuitable. On the other hand, I think that 75 cm3 was too great a volume, as the rate of reaction was very fast, especially in the higher concentration. So I think the most measurable volume of acid will be 50 cm3, and so I will use this in my main investigation.
After carrying out all of my preliminary experiments, I have found out suitable measurements for the reactants. They are:
- 2g medium calcium carbonate chips
-
50 cm3 hydrochloric acid
Now I have collected the measurements I need, I can now begin to conduct my main experiment.
Safety
Whilst carrying out my experiment, I must take steps to ensure my safety. I must take care when pouring the stronger acids as they can be corrosive, and must also wear safety goggles so that acid cannot get to my eyes. This will ensure I carry out a safe experiment.
A fair test?
To get accurate and reliable results, I must make sure that my investigation is fair. Fist of all, I am changing the independent variable (the concentration of the hydrochloric acid) and measuring the dependent variable (volume of gas produced). I must keep every other possible variable the same. This will make sure that in each experiment, the conditions are the same. I will also repeat each experiment 3 times so that I get reliable results. Taking these steps will ensure that I get good results so that I can make a firm conclusion based on my prediction.
Prediction
Based on my background information and the knowledge I gained from doing my preliminary experiments, I predict that as the concentration of hydrochloric acid increases, so will the rate of the reaction. This is based on that there will be more particles in a higher concentration and so there is a much higher chance of a collision and a reaction. I also predict that if I double the concentration then the reaction should be twice as fast. This is logical, as there will be twice as many particles for the chips to react with and so there should be twice as many collisions. This was also the main trend in my preliminary experiments and so under a more precise environment, the results should show a similar trend.
Results
50cm3 0.5mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
50cm31.0mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
50cm3 1.5mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
50cm3 2.0mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
50cm3 2.5mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
50cm3 3.0mol/dm3 Hydrochloric acid
2.0g Medium calcium carbonate chips
Analysis
My first reaction to the results is what I expected. I can clearly see that as the concentration increased, more carbon dioxide was produced. Also, each time the result was quite close to the average, and so the average results should give me a reliable reflection when I draw my graphs.
To get a better idea of the trend of my data, I will put the results onto graphs. My first one will show the rate of reaction, and the second will show the gradient of each line from the first, against the concentration. This will help me to conclude my experiment and to decide whether my prediction was right or wrong.
My graphs are on the next pages
From the first graph I can clearly see that as the concentration increased, so did the rate of reaction. Each time the concentration was increased, the line was higher than the one below. Also, I can see that every reaction was quicker at the start. This is because that there would have been more calcium carbonate at the beginning. As it was reacted it turned to carbon dioxide, calcium chloride and water, so it was being used up during the reaction and so less was left nearer the end. This meant that my lines of best fit were steeper at the beginning but were shallower towards the end. I can also see two anomalous results that I have circled. These might have appeared for a number of reasons, which I will explain in my evaluation.
So I could draw a second graph to interpret my results from, I had to work out the gradient of each of the six lines. The problem was that none of them were straight and so I had to pick a point from which I would work out each line’s gradient from. I chose 40 second on the “x” axis. To work out the gradient of a line you have to make a right-angled triangle with it, then divide the vertical side by the horizontal side. I went down 5 squares from each line, then went across until I met the line again. Each equation can be found next to its respective line.
At first glance at the gradients, I can see that each line is steeper when the concentration was increased. This was as expected although some of the gradients are fairly similar. This could mean that the second part of my prediction might be wrong.
After drawing my second graph, I can see the results of the similar gradients. I have two very obvious anomalous results, under 1.0 mol/dm3 and 1.5mol/dm3; these are probably the result of my curved lines of best fit on my first graph. However, the other four points are relatively close to the line and so those four results are fairly accurate.
After analysing my results, I will now come to a conclusion about my prediction.
Conclusion
From my tables and my first graph I can see that the first part of my prediction was correct. As the concentration increased, so did the rate of reaction. This is because there are more particles in a close space in a higher concentration, and so the number of collisions increases. I think that anyone doing the same experiment would get the same trend. However, I am not sure that I have sufficient evidence to prove the second part of my prediction correct. If I look back at my tables, and I look at the average result for 60 seconds, I can see that on most occasions the result is near enough double than what it was for half the concentration. This is true for
0.5 mol/dm3 and 1.0 mol/dm3, and 1.5 mol/dm3 and 3.0 mol/dm3. This supports the second part of my prediction, although it is about the only evidence I have. If I had predicted correctly then my second graph would have been an almost perfect y=x correlation. However, I had two very strong anomalies and the line is too steep for me to say that I had predicted correctly. So, in conclusion, the first part of my prediction was correct and I can almost be certain that anyone else would have collected similar results. I don’t have sufficient evidence to prove the second part of my prediction, although flaws in my experiment could have had an affect. I will now explore these in my evaluation.
Evaluation
Although my results followed the expected trend, a very badly carried out experiment probably would have found that as the concentration increased, so did the rate of reaction. It is the anomalous results on both graphs that show my experiments weren’t perfect, and so I have listed some possible reasons and how to improve on them below.
- As we could not place the chips in the conical flask as well as having the delivery tube placed over the top at the same time, some gas may have escaped when we were putting the tube on. We also started the time as soon as the chips hit the water, but if the tube wasn’t on the flask, then no gas would get to the measuring cylinder until it was. This may have led to inaccuracies in the timing. One way to overcome this would be to not start the stopwatch until the tube was placed over the flask, this would ensure that no gas would escape when the stopwatch was timing. This could give us more accurate results.
- As there were several sets of results to collect, we had to do the experiments over a number of days. The temperatures may have varied from one day to another, varying the reaction rate due to the kinetic energy theory explained in my scientific knowledge. If we had done the experiment on one day, making sure the temperature was relatively constant, we might have obtained more reliable results.
- We also had different people doing different jobs on the different days that we did the experiment. One person might have done their job differently to another and so that may have varied the results we got as well. If we kept the same person doing each job then our results would have been more reliable.
- We could have also used a wider range, e.g. we could have timed the reaction for 2 minutes which would have given us more results to analyse.
- The acid used may have been slightly different from day to day as the lab technician has to water down a stronger concentration to get different dilutions. Acid on one day could have been weaker than the day before but this is out of our control, as we are not allowed to dilute our own acid.
Overall our results were fairly reliable as we repeated each experiment three times; repeating them even more probably would not have made much difference. Accuracy could have been improved, as I said above in the list, but with the given equipment and the school limitations we probably could not have done it much better.
I could have expanded my experiment by fully investigating the change in the rate of reaction under different variables, like the ones in my preliminary experiment. This would have given me a broader understanding of how changing variables can affect the rate at which things happen.
The results I did obtain were sufficient enough to support my first prediction, and, as I have decided that they were fairly reliable, I can be quite sure that I was right. However I obtained little evidence to support my second prediction. This may be because it was incorrect or because the faults in the experiment were more exaggerated than normal. Although I predicted wrongly once, another experiment may have shown differently. I did prove the expected trend which shows that my experiment was reliable and of reasonable quality and so I think that I was fairly successful in what I did.
