Preliminary Experiment
I have chosen to investigate how concentration affects the rate of reaction. I was initially going to grind up 10grams of marble chips, but soon realised that this measurement was much to big, and reduced it to 1gram of powder. However, this lead to discrepancies considering droplets of water in our test tube would mix with the calcium carbonate when it was tipped in. So the test tube had wet powder all up along the sides of the test tube, and it could therefore not mix with the hydrochloric acid being poured in. Also the calcium carbonate powder tended to get wet before it was even placed in to the test tube and so this lead to some of it sticking to its plastic tray instead of being in the test tube and this would affect the rate of reaction as there would be a smaller amount of a reactant. Also the powder was reacting to quickly so I decided to use small marble chips of size 2-4mm. I chose 1gram to be the mass of marble chips to react with 10ml of acid. The hydrochloric acid was 2molar and I mixed it with distilled water to get my proportions of concentration. I chose to take readings of 100%, 80%, 60%, 50% and 40% of hydrochloric acid, because 5 readings suited my time span, as it left me with enough time to record at least 3 precise attempts without needing to rush. Also the 5 values cover a wide range and will give me a good line for my graph. I tested both the extremes of my readings in the preliminary experiment to make sure that a suitable time could be recorded with the amounts of reactants I was using.
Decided Variables
I worked in a pair and we wanted to eradicate as much error as possible, so we decided that I would put the reactants in and the cork on trying to keep exactly the same routine each time. My partner found it very difficult to know when to start the stopwatch in the preliminary experiment and so we decided I would tell him to start exactly after I had poured all of the hydrochloric acid solution into the conical flask already containing the calcium carbonate chips. I would then put the cork on as quickly as possible. I also decided that neither of us should shake or stir the mixed solution as it meant the carbonates will come into contact with more molecules of acid, increasing the chance of a carbonate molecule colliding with an acid molecule, with enough combined force to react. Although it is possible to stir/shake with the same vigour and for the amount of time, for each experiment, it is highly unlikely, so the easiest way to uniform the amount of shaking/stirring would be to simply do none at all.
Equipment
- Hydrochloric acid - Remember to wear safety spectacles when using hydrochloric acid, as it is an irritant if it meets the eyes.
- Distilled water - To dilute the hydrochloric acid to different concentrations.
- Measuring Cylinder 10ml - To measure acid and distilled water in order to produce accurate concentrations ready for the experiment. As I was measuring 20ml of solution, it would be the most sensitive measuring cylinder that I could use.
- 2-4mm calcium carbonate chips - I also chose the 2-4mm size as the bigger chips reacted too slowly (as they had a lower surface area: mass ratio) to record the values of concentration. I will be measuring out 1gram of chips for every reading.
- Plastic tray - To place chips in so they can then be placed on the digital scales and carried back to the experiment. The weight of the tray was deducted from reading on the scales each time. Also with a tray it is possible to slide all of the chips in at once, instead of having to drop them in one by one. The latter would cause inconsistencies, such as time, for one reading may have more chips dropped in after 5 seconds than another and thus it would have been able to produce more gas due to the collision theory. Ultimately it would be an unfair experiment.
- Scales - To find the total mass of the chips in the plastic tray. The scales are sensitive to the nearest 0.1g, which is acceptable as it has an error bound of only 0.05/2.0 x 100 = 2.5%.
- Conical Flask - The solution gets tipped into this after it has been measured out and the calcium carbonate after it has been weighed. It has a great advantage over other instruments in which the reaction takes places for this experiment, as the conical flask has a rubber tube exiting from it and a cork to stop any gas being lost. This means the precise amount of gas created in reactions inside the conical flask can be measured if the correct instruments are placed at the end of the rubber tube.
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50cm3 measuring cylinder - This is used to collect the gas produced. (These will be filled with water at the start of the experiment,) and I will time how long the carbon dioxide takes to displace 10cm3 of the water. A clamp will be used to hold this in place.
- Trough (filled with water) - This keeps the water in the measuring cylinder until the carbon dioxide gas passing out of the tube from the conical flask has displaced it. When this happens, it gives the displaced water a place to go.
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Stopwatch - This is used to accurately record the time it takes for 10cm3 of carbon dioxide to be produced from the solution.
Prediction
I predict that the higher the concentration the faster the reaction will take place this is because in a higher concentration there will be more hydrochloric acid molecules per set volume. This means that there will be a higher chance of the calcium carbonate molecules colliding with the hydrochloric acid and reacting. This should in theory increase the rate of reaction as the concentration is increased. The reaction rate, however, should decrease as the experiment progresses because as the reaction time increases the number of hydrochloric acid molecules present will decrease as they have been reacted to form water calcium chloride and carbon dioxide. The additional water and calcium chloride present as the experiment progresses should decrease the rate of reaction because of decrease in concentration. This should make a graph of the reaction curved as the reaction rate slows down. I can predict that there will be a positive correlation between the rate of the reaction and the concentration so that as the concentration is increased, the rate of reaction will increase, the increase in the concentration should be directly proportional to the increase of the reaction rate at a given time This is because by doubling the number of hydrochloric acid molecules present the chance of a collision should be doubled, as there is now twice the possibility of a collision-taking place initially.
Method
I firstly set up all of the permanent apparatus, I filled our trough to the brim with water and then placed the 50cm3 measuring cylinder within it thus filling it with water, when I was sure it was completely full, I lifted the base of the cylinder (making sure the top was still in the trough) and clamped it upright, we found out via the preliminary experiment that this made it much easy to take the readings. I then put the delivery tube from the conical flask into the upturned measuring cylinder. I placed my plastic tray of 2-4mm marble chips on the scales and weighed out a gram, I then put the measured calcium carbonate in the conical flask, as I found it was much easier to place the solid in first. I then measured out 10ml of the corresponding solution, which would be 100%, 80%, 60%, 50% or 40% of hydrochloric acid diluted by distilled water, by using a 10ml-measuring cylinder, which was therefore very precise. I then poured this solution into the conical flask from the measuring cylinder. Immediately as all of the solution had left the measuring cylinder I called for my partner to start the stopwatch, and I put the cork on top of the conical flask as quickly as I could in an attempt to let out the least gas possible. After I had taken the readings I washed out the conical flask and refilled the 50cm3 measuring cylinder with water and dried them, as it would have ruined the proportions of the solutions if there was an extra 1cm3 of water left in the bottom of the conical flask and resulted in an unfair test. I then weighed out more marble chips and measured out the next concentration I was doing, this process was repeated for every reading taken.
Obtaining Evidence
To obtain my results I measured the product of the reactants, which in this case was carbon dioxide. After the cork had been placed into the conical flask there was only one place that the gas could go and this was through the delivery tube and up into the upturned measuring cylinder. Here the carbon dioxide displaced the water molecules and I timed how long it took for each concentrated solution to produce 10cm3 of gas so as soon as the carbon dioxide had bubbled up to this amount I called to my partner to stop the clock and the readings we took for all of our experiments are as follows.
I think enough readings were taken to ensure reliability; as there are at least three valid results for each concentration and most are roughly the same, which reflects an accurate procedure. However, there were a few anomalies (which I have put a cross by), which were quite far off of their equivalent readings, thus I have not included them in my averages. I have used the averages to figure out the reactions rate shown in the table below.