I believe that the graph that shows 1/time against the concentration will be a straight line through the origin partly because I know this already, but also because after 0 minutes the reaction has not started yet so no mass of CO2 will have been produced.
In my preliminary work I used the concentrations 1.0 and 2.0mol/dm3 and I tried to see which size marble chips I would use in regards to the rate of their reaction. I put 4g of small, medium and large into different beakers with 100cm3 of the different concentrations and I saw which reaction finished the fastest. It was the small chips. I also found out that if I use this mass of marble chips, the acid will be in excess.
I found out these masses and volumes with the following calculations:
CaCO3 (s) + 2HCl CaCl2 (aq) + H2O (l) + CO2
Ca = 40
C = 12
O = 16
H = 1
Cl = 36.5
Amount = mass / molar mass
Amount = 0.8 g of CO2 / 44 (molar mass of CO2)
Amount = 0.018 mol
Therefore as one mol of CO2 reacts with 2 mol of HCl I will need 2 x 0.018 mol of HCl = 0.036 mol of HCl
This calculation is based on acid with concentration 1.0 mol/dm3.
Amount = volume x concentration
0.036mol / 1.0 mol/dm3 = 0.036 dm3 = 36cm3
This means that I will need to use 36cm3 of acid if I am to make 0.8g of CO2.
Unfortunately this volume of acid is not enough to cover the marble chips and so I will have to put in 100cm3 of acid which means that the acid will be in excess.
Apparatus and Chemical List
Small size marble chips
Different concentrations of HCl (0.5, 0.75, 1.0, 2.0 and 2.5 mol/dm3)
Cotton Wool
Stopwatch
Electric Balance
200cm3 beaker
100cm3 Measuring Cylinders
Loose fitting watch glass
Safety Precautions
Hydrochloric acid is corrosive and so therefore it should be treated with care. Safety goggles should be worn at all times and ties and any loose pieces of clothing or hair must be tucked away. Any spillages or breakages must be reported immediately and spillages cleaned up, as HCl is corrosive.
Method
Before you take your readings you must compile a table with the following headings:
Concentration (mol/dm3)
Time (minutes)
Mass of CO2 Produced (g)
Also before you take the readings we need to make up the following concentrations of acid:
0.5 mol/dm3
0.75 mol/dm3
1.0 mol/dm3
2.0 mol/dm3
2.5 mol/dm3
To make these concentrations do the following:
For the 0.5mol/dm3 take 100cm3 of acid of concentration 1mol/dm3 and mix with 100cm3 of distilled water. You should now have 200cm3 of 0.5mol/dm3 acid.
For the 0.75 mol/dm3 take 150cm3 of acid of concentration 1 mol/dm3 and mix with 50cm3 of distilled water. You should now have 200cm3 of 0.75 mol/dm3 acid.
For the 1.0 mol/dm3 take 200cm3 of 1.0 mol/dm3 acid.
For the 2.0 mol/dm3 take 200cm3 of 2.0 mol/dm3 acid.
For the 2.5 mol/dm3 take 125cm3 of 4.0 mol/dm3 acid and mix with 75cm3 of distilled water.
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Take a 200cm3 beaker and place it on the top-pan balance.
- ‘Tare’ the balance so that the reading is 0g.
- Add 4g of the marble chips from the box labelled ‘small chips’ to the beaker on the balance.
- Remove the beaker and set it aside for later use.
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Measure out 100 cm3 of acid of concentration 0.5 mol/dm3 using a measuring cylinder.
- Now place the beaker containing marble chips back onto the balance and prepare to add the acid, tare the balance, fit the watch glass and start the clock.
- Add the acid and as fast as possible add the watch glass.
- Now immediately tare the balance and start the clock.
- At 0 minutes the mass should read 0g, as no gas has been given off.
- Every minute take and record a reading from the balance.
- After five minutes stop the experiment.
- Now repeat steps 1 to 11 with the same concentration for a repeat reading.
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Next repeat steps 1 to 12 using the concentrations: 0.5 mol/dm3, 0.75 mol/dm3, 1 mol/dm3, 2 mol/dm3 and 2.5 mol/dm3.
- Now repeat steps 1 to 13 and record results in an identical table labelled repeats.
Precision and Errors
With the balance it is perfectly easy to take readings to 3 decimal places.
With the measuring cylinder make sure that you measure to at least an accuracy of one decimal place.
Presentation of Results
The table of results will have 3 columns. The first will be the concentration of the acid, the second will be the time the reaction has been going for and the third will be the mass of carbon dioxide produced. There will then be a table of repeats with the same columns.
I will make two graphs, the first will be a graph of time against mass of CO2 produced and the second will be 1/time taken to give a given mass of CO2 against the concentration. As I believe that 1/time is proportional to the rate, this graph will enable us to compare the rates of different concentrations.
References
Longman GCSE Chemistry by Jim Clark. Published by Pearson Education Limited 2002.
Results
In all occasions I used: 4g of marble chips
100cm3 of HCl
Graphs on Separate Page
Conclusion
I have drawn two graphs; the first shows the mass of Carbon Dioxide produced against time. This graph shows that as you increase the concentration of the HCl, more CO2 is produced in a given time. This pattern is shown with all the lines and in most cases the rate of reaction increases with a higher concentration of acid, as I predicted. My second graph shows 1/time taken to produce 0.2g of Carbon Dioxide against the concentration of the acid. This graph also has a very distinct pattern. To get these results I had to measure the time taken to produce 0.2g of CO2 and then do 1/that time. As I predicted this graph shows a straight line through the origin. We can conclude from this graph that 1/time taken to produce a given mass of CO2 is indeed proportional to the rate of reaction. These results are shown because as the concentration increases, so does the initial gradient of the first graph. This therefore means that the rate increases as the rate equals the gradient. This therefore means that 1/time is proportional to the rate of reaction. Although my results do support my prediction, I do not think that 5 minutes was long enough to let the reactions last for. I would have preferred to have let them come to their natural end.
The increased concentration of acid in the beaker means that there are more particles per unit volume and there fore there will be more frequent collisions with the required activation energy and so the rate of reaction will increase.
Evaluation
The method I adopted and the results I got, do allow me to be confident about the results I got and the conclusions I have drawn from them. As I predicted by varying the concentration of the acid we have also varied the rate of reaction. As shown by my graphs, as you increase the concentration of the acid you also increase the rate of reaction. I feel confident about my results because they show almost exactly what I had predicted and also they back up the information I supplied in my background section. My results show a clear trend that follows my prediction consistently and my two graphs show patterns that were predicted. Although I do have some sources of error I do think that my method was good enough to support my prediction. The results I got support the theory and my prediction.
Sources of Error
This experiment has many sources of error and I will try to list these now. First of all, when measuring the mass of CO2 made, some may have stayed in the beaker and not been released. ‘Pouring away’ the carbon dioxide after the reaction has finished may eliminate this error. Also when planning this experiment we allowed for a small amount of error when measuring out the HCl but excess error could lead to strange results. We could make this better by using a more accurate measuring device such as a pipette. Also although there was a lid on the experiment, some of the solution may have been able to escape. We could eliminate this by maybe using cotton wool instead of the watch glass. This would allow the gas to escape but not the solution. Also the surface area and mass of each individual marble chip may differ. If this is only a small difference that it should be ok, but if the differences are large, then this could severely affect the experiment. Also instead of a beaker I could have used a conical flask with cotton wool in the top. This would have stopped any spray from escaping while letting the gas out effectively.
Anomalous Results
On my graphs I have ringed the results that I believe to be anomalous. On my graph showing concentration against time the 3-minute result for the 0.75 mol/dm3 concentration is the only one on that graph that I believe to be anomalous. Although I have not ringed it, the 1-minute result for the 0.75 mol/dm3 concentration is slightly dubious but I do not believe that it is anomalous.
These anomalies occurred because I misread the readings on the balance at that particular pointing time. Any other anomalies could have occurred because of any of the above sources of error.
Further Work
To continue this experiment I would like to do the same experiment while taking into account all the above sources of error and to use a wider range of acids. This would mean using much higher concentrations like 3.0, 4.0 5.0 and 6.0 mol/dm3. I would also like to able to leave it until the experiment actually finished naturally. This would give us a nicer and more complete set of results, without the anomalies.