However, this experiment is to find out if the concentration of hydrochloric acid affects the reaction. This means that all the other ways to speed up a reaction will not be used in order to prove that it is the concentration that causes any changes. Since the concentration of the acid will be the only variable, the mass of the marble chips will be kept at 3g, there will only be 5 similarly sized chips per reaction in order to keep the surface area about the same, the volume of the acid solution will always be 20ml and the temperature of the reaction will be kept at room temperature. Also, no catalysts will be used.
Because the concentration of acid will be changed, it is likely that the time in which 20cm3 of carbon dioxide will also change, as is proved by the preliminary experiment. As the acid becomes more and more concentrated the time for the gas to collect should get quicker, as there are more particles to collide and react, while a weaker concentration means there are fewer particles to react and so should take longer. Also, a concentration that is twice as strong as another should take only half the time, as there are twice as many HCl particles to collide with the CaCO3 particles and there are likely to be twice as many collisions with activation energy. Also, a concentration that is three times as weak as another should take three times longer, as there are only a third of the acid particles, and a third less collisions with the activation energy.
As mentioned before, the experiment will be done using 20ml, 15ml, 10ml, 5ml and 0ml of 2 molar HCl, and adding the appropriate amount of water to bring the total volume of the solution up to 20ml, each test being done as many times as required in order to obtain three concordant results. Any result that is not within a couple of seconds of the others will be regarded as an anomaly and will not be used when calculating the average. If the times become longer, a greater margin of error will be allowed, as very slight changes in any of the constants will become more exaggerated as the test progresses.
In order to acquire a set of fair results, the same equipment will be used for each test, and all volumes, masses, areas and temperatures will be kept constant, excepting the concentration of the acid. As mentioned before, the surface area of a reactant affects the rate of the reaction therefore the same number of similarly sized chips will be used for each test in order to keep this the same. Also, the clock will be stopped at a certain volume of gas instead of reading the gas at a certain time. This is done so that it is possible to just look at the amount of gas, instead of having to check the volume when the stopwatch reaches a specific time, as it may have changed making it less accurate.
Results:-
A Table To Show How the concentration of hydrochloric acid affects the amount of carbon dioxide given off by marble chips while reacting
Analysis:-
These results obtained from the experiment show that, like I predicted in my hypothesis, the reaction between the hydrochloric acid and the marble chips speeds up as the concentration of the acid increases. This is proven as, on average, the reaction took the shortest time, 15.8 seconds, and had the largest rate of reaction, 1.27 ml/s, when the solution was at its strongest with a concentration of 72.0 g/l. This happened because there were the most acid particles to collide and react with the CaCO3 particles and more with the activation energy. This explanation is supported by the fact that the second strongest acid with a concentration of 54.0 g/l was the second fastest, with an average rate of reaction of 0.94 ml/s, and a time of 21.3 seconds.
The rate of reaction is ascertained by dividing the amount of gas collected by the average time the reaction took, while the concentration of the acid is obtained by dividing the amount of acid by the amount of the solution, and multiplying the answer by the original concentration of the acid. The higher the rate of reaction, the quicker it took place, and the higher the concentration number, the stronger it is.
However, a concentration of 72.0 g/l gave a rate of reaction of 1.27 ml/s whereas one of 36.0 g/l gave a rate of just 0.45 ml/s, therefore doubling the concentration did not half the time. This disagrees with my hypothesis, which predicted that it would.
There is an exception to this, a concentration of 36.0 g/l had a rate of reaction of 0.45 ml/s, which is just over half of the rate of reaction for a concentration of 18.0 g/l, which was 0.25 ml/s. However, as the graph shows, this result is slightly off the line of best fit enough to be considered anomalous.
Reactions, such as this one between a base and an acid, cannot occur by simply putting two substances together. The substances must be reactant with each other, the particles must collide with each other, and they must collide with enough energy to break the bonds that keep the particles together. This amount of energy is called the activation energy, or the Ea. There are two ways in which to increase the rate of reaction: giving the particles more energy, which gives more particles the activation energy, and also by increasing the frequency of collisions. This can be done in a few ways. Heating up a reaction gives the particles more energy, making more particles collide with the activation energy. Another way to speed up a reaction is to increase the concentration of reactants, which means that there are more particles to collide with each other making collisions occur more frequently. Two other ways to speed up a reaction are using a catalyst to lower the activation energy needed for the particles to react, and grinding up a solid in order to increase the surface area and allowing more particles to react. In this case, the concentration was increased, meaning there were more acid particles to react with the marble particles.
Evaluation:-
Overall, the results ascertained from this experiment were reliable, as the factors that needed to be kept the same remained constant and the same apparatus was used for each test. This is reflected in the similar times of the trials of each concentration, despite having some anomalous results. For example, the first trial for the 54.0 g/l was 26.5 seconds, a couple of seconds out from the other three, which were 20.4 s, 22.5s and 20.9s, and the last trial for the 18.0 concentration was 95.4s, whereas the other ones were 83.2s, 79.6s and 78.4s.
There was only one result that was different from the others and looked out of place on the graph. The average for the 36.0 g/l concentration was slightly off the line of best fit on the graph with a rate of reaction of 0.45 ml/s, as was the result for the 72.0 g/l test which was 1.27 ml/s although not as much as the 36.0 g/l test. As mentioned before there were individual trials that were too slow or quick but these were unnoticeable overall as they were ignored as they were anomalous. For example, the first trial of the 54.0 g/l experiment took 26.5s whereas the others took closer to 21s.
I believe that the conclusion I came to from these results is correct as it describes everything that occurred in the experiment and explains why these things happened. The slight anomalies could have been the result of small measurement misreadings, the slight change in the surface area of the chips or a change in room temperature but the main factors that needed to be constant were done so and this is supported by the invariability of the final results.
This experiment could be improved and expanded in order to obtain a larger quantity of more accurate results. The method used to measure the volume of gas collected could be made more accurate by using a glass syringe-like piece of apparatus, and the measuring cylinders used to measure out the acid and water could be replaced with ones with smaller intervals to make measuring more accurate. Another way to make the test more accurate, and one of the reasons that all the results were not concordant, could be to grind up the marble chips in order to give them all the same surface area while keeping the same mass. The same number and total weight of the chips was kept the same throughout the test, but it was too difficult to ensure that all chips were of the same surface area, which could have altered the speed of the reaction. Grinding the chips up into a powder would guarantee that the same surface area is used for each test, which would help to prove that it is the change in concentration that causes the changes in the rate of reaction, not other unintended variables.
The experiment could be repeated with a larger number of concentrations at smaller intervals, maybe decreasing by two ml per test from 100ml of acid and no water down to no acid and 100ml of water. It could also be repeated with different masses of chips and volumes of carbon dioxide, such as seeing how long it takes for a reaction between 75ml of hydrochloric acid and 15ml of water, and 7g of marble chips to produce 50cm3 of carbon dioxide. Should these steps be taken then the experiment would provide better evidence to support the fact that the rate of reaction increases as the concentration of the acid increases.