CaCO3(s) + 2HCL(aq) = CO2(G) + H2O(l) + CACL2(aq)
I have decided to vary this factor because it is fairly easy to control; the other variables are less suitable for this experiment. Temperature is a very difficult variable to use in this experiment. I would have problems collecting accurate results because of the difficulty of keeping the heat source constant during the reaction. I also chose not to vary the concentration of hydrochloric acid because the highest concentration available for my use was 4M and I felt I could not achieve the desired variation with such limited concentrations. I could have investigated the rate of reaction by using a catalyst, but the lack of chemicals available to me made this impossible.
I will measure the volume of carbon dioxide produced by the reaction as a measure of the rate of reaction.
In order to ensure a fair test with reliable results, I will need to keep all variables constant apart from the one I intend to vary or else I could not be sure which of these variables was causing the change in rate of reaction. I will also need to repeat each test to allow me to take an average to ensure accurate results and allow for human error.
So I will keep constant:
∙ Concentration of solvent (dilute hydrochloric acid)
∙ The temperature during the reaction (temperature will be kept at room temperature approximately 23°C)
∙ Catalyst (no catalyst will be involved)
In each test I will have to ensure that the mass of the marble chips used is constant and that only surface area differs.
Prediction and justification
I predict that as the surface area to volume ratio increases, the speed of the reaction will increase therefore the gas will be produced faster. I believe this as more of the reactant will be exposed to the acid at a given time allowing more opportunity for collisions. Usually, when the surface area to volume ratio is doubled it will double the rate of reaction, which shows proportionality.
As more collisions will occur in a given time the rate of reaction should increase with surface area to volume ratio. However the surface area to volume ratio will be greater for small particles than larger ones as in a large particle the majority of the atoms are not exposed to the solvent outside.
Planning procedure:
In order to plan an appropriate strategy to justify my prediction, I will use my scientific knowledge together with careful planning and an appropriate range of surface areas to ensure both accurate results and a fair test.
Apparatus:
Hydrochloric acid – A reactant
Marble chips – A reactant
100cm³ Conical flask – To house the reaction
500 cm³ Beaker – Containing a specified amount of water to act as a water bath
Measuring Cylinder (100cm³)– To collect the carbon dioxide gas
Delivery glass tubing- To channel the gas under the measuring cylinder
Stop watch – To time the reaction
Diagram of apparatus:
Preliminary experiment
The rate of reaction can be observed by either how quickly the reactants are used up or by how quickly the products are forming. I am going to collect and measure the amount of carbon dioxide given off during the reaction of different surface areas because this is far less time consuming than observing the speed at which the reactant is used up. I have chosen this method as it gives the least error and is easy to collect accurately. I will record the time as each 20cm³ of carbon dioxide is given off up to 60cm³ of carbon dioxide. I will record the results cumulatively in minutes and seconds. I have first done a preliminary experiment in which I tested the smallest and largest surface areas that I intend to use. From the experiment I found that 3g of the marble chippings takes too long to react with the hydrochloric acid, so in the actual experiment I will reduce the quantity of calcium carbonate to 1g.
Method
- To begin I shall set up the apparatus as shown in the diagram above.
- I will then take the large group of marble chips (approximately 10.5mm in diameter) and measure out an amount equal to 1g. I shall do the same for all of the chips until there are 5 groups. By this point I should have a single group of the large chips, another group of the medium chips, 1 group of the small chips, 1 set of the roughly ground chips, and also 1 group of the very finely ground chips. (The smaller marble chips will be made by taking an extra amount of the small chips, weighing out 1g for each group and then either roughly or finely grinding, using the pestle and mortar. They roughly ground chips should be ground until each individual chip is roughly about 1mm in diameter and for the finely ground chips, 0.3mm in diameter).
- Next I will take one group of the chips and place into the hydrochloric acid with tweezers (this avoids any splash that may occur if merely dropped into the acid). As soon as the bung is fitted into the top of the conical flask, I shall start the stopwatch.
- As soon as the volume of gas produced reaches 20cm³, I will make a note of the time, I will allow the experiment to continue recording the time at 20cm³intervals until the gas reaches 60cm³.
- When one set of results has been completed, I will repeat steps 3-4, using new clean acid and calcium carbonate each time, and making sure that the measuring cylinder and glass bowl have been refilled with water. This, I shall repeat until three entire result tables are filled with a total of fifteen results each.
- When the experiment is completely finished, I will wash thoroughly all items that have been in contact with the hydrochloric acid, and any remaining calcium carbonate that did not completely dissolve doing the reaction I shall put into the bin or dispose of safely to avoid any harm to the environment.
Fair Test
To ensure a fair test I will not only make certain all variables are kept constant other than surface area, but also take other factors into consideration. The two repeats of my test will help make sure that any human mistakes are clear so as to be rectified. I shall also need to be certain that all the instruments I use are both clean and have no flaws such as cracks or chips that may cause inaccurate results.
Safety
I must ensure that my experiment is carried out in a safe manner, by making sure that safety goggles are worn at all times when the acid is present. This is because hydrochloric acid is corrosive so if it were to get spilt or splash, the eyes must be protected as it could have extremely harmful effects. I will also be sure to be certain that all equipment is kept away from the edges of the bench, to reduce the risk of items being knocked, spilt or broken, as this also would be dangerous. A further issue of safety that must be taken into consideration are the environmental issues. When the practical investigation is complete both the acid and any remaining calcium carbonate must be disposed via a sensible and practical method that is not harmful to the environment.
Results and graph
Original Test
Second Repeat of Experiment
Third Repeat of Experiment
Final Averages of Collected Results
Please see overleaf for the graph used to show these results. On this graph I have used the gradient of my line of best fit to work out the rate of reaction. I have done this by dividing the height of the line of best fit by its length. This gives a measure of the volume of gas produced per unit time, which is my rate of reaction. I can use these values to form a graph showing the rate of reaction for the different sizes of marble chip and hence how they vary.
Average Rates of reaction
The graph overleaf shows the average rates of reaction for each diameter.
To draw a conclusion however I need to know the surface area to volume ratio which is impossible. I can do a rough estimate though by modelling the chips as cubes with the relevant diameters. The volume for a gram of marble will be constant and so the surface area to volume ratio is proportional to the surface area of these chips. Therefore in order to calculate the relative surface area to volume ratios which is all I need to draw my conclusion I need only calculate the surface area of the largest cube and then the surface areas of each smaller cube multiplied by the number of these cubes necessary to give the same volume of marble this can be expressed as follows:
diameter2 x6x(10.5/diameter)3
The graph showing these results can be found overleaf.
Conclusion
My results prove my hypotheses correct as they show that as the surface area to volume ratio increases so does the reaction. The fact that my best-fit line is a straight line shows that they are proportional to one another. This is because as stated in collision theory, as the surface area for a given volume of solid increases so does the rate of collision with the dissolved reactant in proportion to this increase i.e. if twice the area is exposed to the solvent, twice the number of collisions will occur. This supports my hypothesis well however, my not results are not accurate enough on their own to be conclusive. I am confident enough in my results that if I was to be given a surface area of marble chip I would be able to use my line of best fit to calculate the rate of reaction without having to carry out an experiment.
Evaluation
The procedure I used was perhaps not the most accurate measure to find out the relationship between surface area and rate of reaction. I could for example have measured the change in mass to work out the rate of reaction; this may have been more accurate as balances are accurate to several decimal places a measuring cylinder is not. Another way to improve my experiment would be to extend my test. With only five results it is easy to have inaccurate results, if I had extended my results I could have been sure that my results were accurate. My results have supported my hypothesis however, and follow the trend I would expect so the experiment did work even if it was less accurate than the other option.
There are slight flaws in my results for instance the points of the graph do not all lie exactly on the line of best fit. This could be explained by the surface area approximations that I made. The marble pieces I used were only roughly the correct surface area so I made approximations when commenting on the surface areas, which means that there is a margin for error which can explain the lack of a perfect best-fit line. The same is true of the earlier best fit lines which can be explained by the fact that the surface area of the chips was constantly changing throughout the reaction, as it was reacted with the acid, and so did the surface area to volume ratio. Also as the marble was used up the reaction would slow, as there would be less marble remaining to react with the acid reducing the chances of a collision occurring. The evidence is sufficient to imply that my hypothesis is correct but I think to prove it successfully further evidence would be necessary.
I could have checked the rates of reactions I produced as a result of my experiment by carrying out a different test. If I had examined my results by doing a test measuring the mass change of the calcium carbonate I could have checked that I came up with sufficiently similar final rates of reaction, however I did not have time for this or to use a wider range of values.