Safety/ Risks
- Be careful when handling the graduated pipette as it is made of fragile glass. If the pipettes are used incorrectly, they may cause injuries. Always handle this equipment with care!
- Hydrochloric acid is corrosive and therefore it should be treated with care. Wear gloves! If HCL is spilt, clean the spill up immediately
- Safety goggles should be worn at all times.
- Tuck in any loose pieces of clothing or hair so they won’t get in the way of the practical.
- Any other breakages of equipment should be reported and cleaned up appropriately.
Equipment
Marble Chips (x10g): Marble chips are the source of calcium carbonate for this experiment
Different concentrations of HCL (0.5, 1.0, 1.5, 2.0, 2.5mol/dm3): These concentrations of HCL after being varied to see how the rate of reaction varies with concentration.
Cotton wool: Used to seal the top of each conical flask to prevent any of the reactants from spitting out. The cotton wool will still permit carbon dioxide to escape.
Electrical balance (x1, correct to 2 decimal places): The flask with the reactants on top is used to measure the change in mass as the reaction proceeds
Stopwatch (x1): Used to see how long the reaction continues on for and tells us which times we should measure at.
Conical flask (x5): Used to keep the reactants while the experiment is carried out.
Thermometer (x5): Used to measure the temperature at which each reaction takes place at. This will show us if there is an external or internal change in temperature.
Beakers containing different concentrations of hydrochloric acid (x5): These will store each acid concentration before the practical begins.
Weighing boat (x1): Stores the marble chips on the electrical balance whilst we get the appropriate quantity of marble chips.
Spatula (x1): Used to carry the marble chips from one place to another.
Results table (x1): This can be used to jot down the result you get from your practical.
Graduated Pipette (x1): Used to accurately measure the quantity of hydrochloric acid needed for each concentration
Pipette filler (x1): Used to fill the graduated pipette with HCL.
Method
- Firstly, set up all the apparatus so the equipment is ready for the experiment (Make sure all equipment is clean).
- Before measuring out the quantity of HCl acid for each solution, dip the thermometer into the first HCL acid solution. Note down the temperature.
- Take 25ml of a certain concentration of HCl acid (start with the lowest concentrated solution of HCl) using the graduated pipette and the pipette filler. Be aware that you are using a graduated pipette. This apparatus is fragile and need to be handled appropriately.
- Pour this liquid into 1 of the conical flasks. Leave this aside.
- Take a weighing boat containing some marble chips. Place this on top of the electrical balance.
- Measure out 2 grams of marble chips accurately using the spatula. Make sure that during the process of handling marble chips, you do not contaminate the chips.
- Before you place the conical flask with hydrochloric acid on the balance, use the “tare” function to make the balance read 0.00 grams.
- After placing the conical flask on top of the balance, put in the 2 grams of marble chips into the flask. Seal the conical flask immediately by placing the cotton wool on the top of the flask.
- Start the stopwatch once the marble chips are in.
- Record the starting mass of the equipment and reactants on the results table.
- Record results every 30 seconds for approximately 12 minutes. It would be more suitable if recording results can keep on going until the whole reaction is complete however for time, 12 minutes should be fine.
- Repeat the same procedure for the next 4 concentrations of hydrochloric acid.
Secondary Evidence showing the average time taken to produce certain amounts of carbon dioxide (1)
Results table showing the change in mass of the contents of the conical flask per 30 seconds
Results Table showing the amount of carbon dioxide gas produced at each concentration of hydrochloric acid in grams
Table showing the average mass of CO2 lost against time (secondary evidence)
Table showing rate of reaction and average rate of reaction of each concentration of hydrochloric acid
Conclusion
Use scientific ideas to explain conclusions you draw from your evidence
The results I obtained through my practical, I conclude that the higher the concentration of HCL, the greater the mass loss in a given period of time therefore the higher the rate of reaction. According to my line graph showing the mass of CO2 produced, we can clearly see that the amount of CO2 produced decreased as the reaction progressed. At greater concentrations, there are more reactants per unit volume; there will be more collisions per second, so more particles reacting per second. The rate of reaction is steady for the first few minutes but then it decreases since the acid is diluted by water produced by the reaction of HCL and marble chips; the concentration of reactants decrease. Particles are less likely to collide as there will be fewer HCL particles (collision theory). (The curved lines of mass loss illustrate this). The marble chips also lose surface area as the reaction progresses so HCL particles react on a smaller surface. However, this effect is negligible. My secondary evidence from the Internet showing the average amount of CO2 produced for each concentration show that the amount of CO2 produced, decreases and starts to level off. This again supports that the rate of reaction is affected by these factors.
I also plotted a line graph showing the average rate of reaction from all of our group’s experiments. This graph gave me a consistent straight line showing that the rate of reaction was directly proportional to each concentration of acid in the first 2-3 minutes, just as I predicted. My results showed that at:
30 seconds: At 1 molar, mass of CO2 lost was 0.02g
At 2 molars, mass of CO2 lost was 0.04g
60 seconds: At 1 molar, mass of CO2 lost was 0.05g
At 2 molars, mass of CO2 lost was 0.09g
This illustrates that the concentration is directly proportional to the rate of reaction in the first 2-3 minutes in my results. The secondary evidence produced by the other class show that the rate of reaction doubled; at concentration 0.5mol is approximately half the rate of reaction when the concentration of the acid is 1.0mol. I got these figures by finding the gradients if the lines of best fit on my primary and secondary data.
Rate at 0.5mol: 2.22 x 10-1
Rate at 1.0mol: 4.66 x 10-1
For my primary evidence, the rates of reaction were the following:
....
I conclude that as the concentration of HCL increases, the amount of CO2 produced increases therefore the rate of reaction increases. The rate of reaction is also directly proportional to the rate of reaction.
Use your conclusion to justify your hypothesis
My conclusion fully supports my hypothesis. I conclude that as the concentration of HCL increases, rate of reaction increases.
State how well your evidence supported your conclusion
My evidence supported my conclusions fairly well. I did have to overcome a few anomalies that I got from my primary and secondary evidence. I ignored all anomalies when processing my data and evidence. There were only a handful of anomalies so my overall conclusion was still what I had predicted.
The results from our practical was set to carry on for between 13 and 15 minutes per molarity of HCL which gave us a broad set of results in which we can work out averages with. Our class also performed their procedures for 10 minutes on average for each concentration. This wide range of data made possible to avoid any unreliable or anomalous results.
When I drew my line graphs, all the points for each experiment laid close to the smooth curves We did our preliminary experiment to see what equipment, concentrations, etc were necessary for our actual experiment. This helped us to understand what was needed for the task in order to give us excellent evidence, which would in turn give a reliable conclusion.
What additional evidence could you have used to provide a stronger conclusion?
In the experiments that were used to obtain my secondary data, different molarities of HCL (such as 0.2, 0.8 and 1.6 molars) were used. In our practical, we used 0.5, 1.0, 1.5, 2.0 and 2.5 molars; I should have found secondary evidence which used our molarities of HCL. I could have found secondary evidence where the experiment is conducted until the reaction is over; we stopped our practical for each concentration at around 12 minutes due to time’s restrictions. A much broader set of results may have strengthened the evidence.
I could have found evidence where professional scientists performed the practical under controlled conditions. Professional scientists may provide better evidence rather than a student performing the same practical. I could have collected gradients from different points of my lines of best fit rather than at single points. By averaging these gradients, you can get the mean gradient for the whole line instead of one point. This broader range of gradients could possibly lead to more accurate results.
Describe the strengths and weaknesses in your method
Weaknesses: The marble chips were selected by inspection and weighed without measuring the surface area. The total mass measured will be 2 grams but the number and surface of marble chips taken will be different. This probably led to minor inaccuracies because if the surface area was larger, the marble chips would be more prone to collisions resulting in a faster rate of reaction. We should have marble chips that have been accurately cut into certain surface areas to minimise this effect.
Some of the marble chips did not react all once as they were in the solution. This may have been caused by contamination of the marble chips. We did use scalpels to measure our 2 grams of marble chips; yet they were contaminated. Since some chips didn’t react, we know that the full 2 grams of marble chip didn’t react. To counter this, we should get pure calcium carbonate. This may have affected the CO2 produced hence the rate of reaction. However, I do not believe that this would significantly change the overall conclusion.
Apparatus impurities could have led to some errors. We reused the same apparatus for each concentration of HCL, due to equipment restrictions. Some equipment such as the graduated pipette are difficult to clean properly, so there may have been drops of acids from different molarities of solution. This may have interfered with each concentration of HCL. If the concentrations of acid changed, it may well affect the results we obtain for each experiment. If the results did change, it could affect my hypothesis. To counter this problem, we should have separate sets of equipment for each concentration to prevent any contamination.
Once we saw that there was no change in mass of the reactants, we stopped the experiment for that concentration. However, the marble chips were still producing CO2 (marble chips were still fizzing). I think that we should have continued each concentration to around 20 to 25 minutes in order to see if there will be a further change in mass. This could have helped us to obtain a wider and more reliable set of results.
We only performed one practical for each concentration of HCL. We could have conducted repeats for our separate concentrations and averaged those results in order to get very reliable data. The more reliable results would have strengthened the conclusion made from my hypothesis.
Strengths: We strictly followed the control variables. For example: we kept the mass of marble chips and the volume of HCL the same. This minimised most problems in our method.
We used the same make of apparatus for every concentration. This meant that each experiment with different concentrations of HCL were similar.
The entire experiment was conducted in the space of two days resulting in a reduction in scope of errors (like temperature). In addition, we dipped the thermometers in each HCL acid and observed if there was any temperature change. Since a change in temperature would affect the rate of reaction, it was essential that this variable was controlled.
The results were taken correct to 2 decimal places. This meant that the results obtained were accurate.
The apparatus was handled carefully and the practical was conducted smoothly. This helped us avoid any unnecessary accidents or breakages.
I have looked through my primary data and I found a few points which did not fit my lines of best fit. These points have been circled on my graphs. My anomalies were at:
0.5mol: 390 and 460 seconds
1.0mol: 150 seconds
1.5mol: 210, 630, 660 and 690 seconds
2.0mol: 30 seconds
2.5mol: nothing
These anomalies may have occurred because of misreading of the mass at any particular time. The several of the anomalies aren’t very major so the presence of anomalies will definitely not change my hypothesis or my conclusion.