Before choosing the quantities I was going to use for my experiments, I decided to conduct some preliminary tests to ensure that I was using reasonable quantities. To decide on a mass of marble chips, I decided that 5 chips, amounting to 2 grams, would be a reasonable mass to use, with a volume of 20cm3 of hydrochloric acid. I then tested these quantities for the 0.5 molar acid, and the 2.5 molar acid (as it was not necessary to test all the molar values – just the extremes).
Above are the results from my preliminary test. As you can see, the 0.5 molar reactions were slow, whereas the 2.5 molar reactions were faster, showing that the results are along the right lines. I have chosen to see what volume of gas is produced in 60 seconds, giving me a rate of reaction in 60 seconds/cm3. This is because 0 is too close to the beginning of the scale, 100 is too close to the end, and the fact that the 2.5 molar acid’s quantity of gas produced in 120 seconds went completely off the end of the scale makes it impossible to be able to record these results.
I have also decided that I will get the mass of the marble chips to a minimum of 1.95grams, and a maximum of 2.05 grams, as it proved too difficult to achieve perfect weights each time.
Below is a diagram showing the apparatus I will use through the investigation.
Using the apparatus shown, with 5 marble chips with a mass of 2 grams in the small test tube and 20cm3 of the 0.5M hydrochloric acid in the boiling tube, I will pour the chips into the boiling tube of acid, and immediately after that I will put the rubber bung of the delivery tube into the boiling tube, and set the stop-clock – both at the same time. After 60 seconds I will stop timing and take a reading of the volume of carbon dioxide from the gas syringe and note this down as my result at that concentration. I will then repeat this method using 1M, 1.5M, 2M and 2.5M concentrations of hydrochloric acid, and then after I have collected a set of results, I will repeat the whole process again, so that I can take the average of the 2 readings for my over-all results – as this will most likely be more accurate.
Before taking each reading, I will make sure that the gas syringe either reads zero, or else subtract whatever reading it gives from my final result to make sure I am only recording the gas given off during the actual reaction. I will also twist the plunger of the gas syringe slightly to verify that it can move with ease - as this could affect the final reading.
So that I am as safe as is possible throughout the experiments, I will be wearing safety goggles whenever I am handling hydrochloric acid, or am around this acid, and I will also have my hair tied back, as the acid is an irritant. I am also only experimenting using a concentration of 2.5M, because above this it is too dangerous to conduct in the school.
Analysis of Results
It can be seen that the results above show that as the concentration increases, the rate of reaction also increases. This is because in a more diluted acid, there are not as many acid particles. However in a more concentrated acid, there are more acid particles. This means that collision occurs more successfully. In my experiment, when the concentration of the acid is at 0.5M, there is a smaller acid particle to calcium carbonate ratio. When the concentration of the acid is 2.5M, the ratio is much greater, meaning there is a greater chance of a successful collision between an acid particle and a calcium carbonate particle. This therefore creates the pattern being that as the concentration increases, the rate of reaction also increases. Although my results are not very accurate, as can be seen from the first construction line, my graph does show that for every half mole of hydrochloric acid increased, the volume of gas increased by approximately 15cm3, so even though my results are not directly proportional by bisecting the origin, by taking into account the construction lines, they are. This proves my previous prediction. At 0.5M Hydrochloric Acid, the volume of gas was 5 cm3, at 1.0 M, it was 21.5 cm3, and at 2.0M it was 55cm3. This unfortunately means that when – in my prediction – I said that if the concentration is doubled, the rate of reaction is also doubled, I was wrong. This could be put down to a number of simple human errors, which I will reflect on later.
Evaluation
Within my results, there were no real anomalous results. However, my line of best fit did not intersect many of my marked points, even though they were reasonably close. Therefore I do not feel that my results were very reliable. Also, because my line of best fit did not bisect the origin, it means that my results were not strictly directly proportional – even though the distribution of the results after that implicates it.
Why these inaccuracies occurred, there are many possible reasons to consider. At the beginning of the experiment, when I had to put the marble chips in with the acid, insert the rubber bung and start the stop-clock, it would have been impossible for me to perform these 3 simultaneously, without having a short period of time within each action. This could have accounted for some inaccuracies, because the reaction could already have started by the time the rubber bung was put in – meaning that the gas syringe did not read all of the gas produced, and also because by the time the stop-clock was set, some of the reaction could have already occurred. The bung could also not have been inserted properly – given that I was in such haste to get everything started. This could have meant that some of the gas produced was released, and not caught in the gas syringe. I could have rectified this error by having a partner to help, but there was no one who was available at the time of my experiment. I therefore think that a group of 2 – perhaps 3 – people should conduct the experiment together – if I did the experiment again.
The temperature could also have played a part in disrupting my results. I know for a fact that at the beginning of the 1st experiment, the temperature at the beginning was 21°c, and at the beginning of the 2nd, it was 23°c. This already could have altered the results, as changing the temperature was one of the factors that I needed to keep consistent. Also, the temperature of the classroom was recorded shortly after the room had been vacant – therefore, after a while it would have got warmer, because of the body heat of the pupils, meaning that the later results recorded in each experiment would have been faster (both because of the increased concentration of acid – which I was testing; and the temperature – which I was not!). This problem could be overcome by using a thermostatically controlled water bath, which would have kept the temperature to a very close constant. Unfortunately, these water baths were not available for my experiments, but if I had to do it again, I would use one.
Because I made up the 1.0 molar acid and the 2.0 molar acids, these could have been inaccurate. By ensuring that the concentrations were ready made, this could have improved the accuracy.
Another problem I came across was that it was extremely difficult to measure out exactly 2 gram masses of the marble chips, and even when I did, the surface areas would not be the same – even though I used the same quantity of chips. This could be corrected by using marble cubes – which would all have the same surface area and mass, increasing accuracy greatly in this area. However, it is very expensive to use these, and we were unable to use them.
These are the reasons for why I think my results were so unreliable, with suggestions to resolve them. I could also expand my investigation. I could use an extended range of concentrations with smaller intervals between them. For example 0.25M, 0.5M, 0.75M, 1.0M, 1.25M, 1.50M, 1.75M etc. up to 5.0M could be used. Although this would take an extremely long time, my results would be very precise, and would give a wider range of results.
Generally speaking, I feel that the experiment has been quite successful, despite not being able to prove direct proportion. However more accurate methods of carrying out the stages have been found, meaning that if I were to repeat the experiment, I would be able to produce a much higher accuracy of data.
