To make it a fair test and make my results more accurate I will try and keep all variables constant except the concentration of the acid. This will include weighing the marble chips every time and trying to use lumps of roughly the same size to keep the surface area fairly constant as a change in surface area will affect the rate of reaction. I will try and avoid a change in temperature and I will use the same equipment throughout the tests. Each concentration will be repeated at least once to improve accuracy as an average can be taken.
The rate of reaction depends upon two factors, the number of collisions within a time between the reactants, and the fraction of these collisions that are successful. Therefore, the higher the concentration of the reactants, the higher the rate of reaction because there will be an increased number of collisions, this is called collision theory. However, for the reaction between the two reactants to be successful, they must collide with enough energy to react, the energy needed to react is known as activation energy. This can be shown on a graph like the one below, I believe this graph is similar to what the energy diagram would be for my investigation.
This shows that my chemical reaction will be exothermic, therefore it will release more energy than it uses to react. Therefore, at a higher concentration, there will be more reactions releasing energy in the form of heat that will enable more particles to reach their activation energy level and react. This shows that the rate of reaction will increase with the concentration of acid.
After having done some research I feel that I can make an education prediction concerning what will happen during my investigation. I predict that the higher the concentration of acid I use, the faster the rate of reaction will be. I think this because the higher the concentration is, the more acid particles there will be to collide with the marble chip with enough energy to cause a reaction as explained in collision theory. I also believe that the rate of reaction will increase with concentration of acid because the chemical reaction I am doing is an exothermic reaction. Therefore, it will release energy as the reactions take place, this will give more energy to other particles around it and will cause the reaction to speed up as more particles reach their activation energy and collide with enough energy to react.
Preliminary test results
I started with 10g of marble chip and 100cm3 of 2M acid, the reaction was too fast and could not be accurately measured. I then tried 3g of marble chip with the same amount of 2M acid (100cm3), this worked fine, however, when the 0.4M concentration was tested the reaction was too slow and could not be accurately measured due to our scale. I then tried 6g of marble chip and it gave 135 cm3 of CO2 with the 2M concentration and 5 cm3 of CO2 with the 0.4M concentration. I felt that this would give us a good range of results so I stuck with those measurements based on my preliminary tests.
When we had done our experiment we realised that we could benefit from some tests with other concentrations of acid to increase accuracy. Therefore we decided to do tests with 1.8M, 1.4M, 1M, 0.6M and 0.2M acid.
However, when we did these tests they produced a much higher volume of carbon dioxide than we would expect from our previous results. I came to the conclusion that this would have to be down to a change in one of the variables. Looking at the variables it was possible that it could have been temperature or surface area of the marble chips that caused the change, but as there had not been any large change in these variables it was unlikely that they would have such a clear affect on the results. After doing a few tests I found that it was the acid that I was using that had affected the results, I had started on a new batch of acid as the previous bottle had run out.
Even though both batches of acid were labelled as being 2M, it was clear that one was stronger than the other and therefore increased the rate of reaction as the chance of acid particles colliding with the marble chip had been increased as there were more acid particles. Due to this, I plotted there results as a separate line, however, the results still show the same pattern and are therefore still useful in concluding the relation between gas given off and the concentration of acid, as explained by collision theory.
As you can see on the graph, both lines go up in a smooth curve. This shows that there is a correlation between concentration of acid and the volume of gas given off. As the concentration of acid increases so does the volume of gas, therefore they are proportional. The gradient of the line gets steeper as the concentration increases, this means that there was a larger increase of gas produced between 1.6M and 2M, than there was between 0.4M and 0.8M, even though the increase in concentration of acid was 0.4M in both.
The graph shows that at higher concentrations of acid, more carbon dioxide was produced but in the same amount of time. This shows that at higher concentrations, the rate of reaction must have been faster to produce more products in the same time period. This can be explained by collision theory and the ideas on activation energy that I explained in my research and prediction.
In my prediction I said that more gas would be given off with a higher concentration, these results show that I was correct. This is because due to collision theory, in a low concentration of acid the acid particles are spread far apart. This means that there is very little chance of acid particles colliding with the marble chip with enough energy to react. In a high concentration, the acid particles are closer together as there is more of them within the same volume. This means that there is a much higher chance of collision with the marble chip, and there is more energy due to the energy being produced by the reaction, which is exothermic, this causes the reaction to be faster. This can be seen in my results, the 2M acid gave an average of 134.75cm3 of carbon dioxide, where the 0.4M concentration only gave an average of 4cm3 of carbon dioxide. The rate of reaction is reduced by a decrease of particles within the same space.
Accuracy of results
I feel that my experiment was accurate because the results made a smooth curve on my graph. I did have a problem with the 1.8M, 1.4M, 1M, 0.6M and 0.2M acid results, as the acid appeared to be stronger than it was labelled. However, I resolved this problem by plotting the results as a separate line on my graph and they still showed a very similar curve to the other one showing the same relation between concentration of acid and the volume of gas given off. This shows that it was probably just the concentration of the acid that caused the problem and not my execution of the experiment that caused the results to be slightly different than what was expected.
I think that my results could have been better as there was some variation between my four results for each of 2M, 1.6M, 1.2M, 0.8M, and 0.4M. However I do not feel this problem is major as the variations were relatively small as is shown by the error margins drawn on my graph, but there is still room for improvement.
If I was to do this investigation again I would try and keep some of the variables constant that I had less control over. The investigation was carried out over a few days so the classroom temperature may have varied. This could have had a slight effect on the rate of reaction making my results less accurate. Although the marble chips were roughly the same size they weren’t all exactly the same shape. If I could find a way to make the marble chips all the same shape and size then the investigation would be fairer as the surface area of the marble chips would be constant.
If I was to do this investigation again I would try and use only one batch of acid for the whole experiment because as I found out, the acid supplied isn’t always exactly the strength that it says it is. I would also try to use a more accurate piece of equipment to measure the volume of gas produced, using a measuring cylinder did not provide the best accuracy that was open to me. I could have achieved a higher level of accuracy with a buret.