Fair Testing
In order to carry out a fair test the only variable I will change is the concentration of the acid. I will ensure that the following variables are kept constant:
Ø Surface area of solute (marble chip)
Ø Amount of reactants (5g of marble chips and 20cm3 of hydrochloric acid)
Ø Catalyst (no catalyst will be involved)
Ø Temperature (room temperature throughout)
A fair test will also require the experiment to be repeated several times at each temperature for reliable results.
Since I will be working with strong acid, I will wear safety goggles while conducting the experiment.
To be sure I can rely on my results I will repeat he experiment twice or three times if necessary, because there is chance of inaccurate results for many reasons.
Prediction
CaCo3 (s) + 2HCl (aq) = CaCl2 (aq) + H2O (l) + CO2 (g)
I predict that the higher the concentration the faster the reaction will take place this is because in a higher concentration there will 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. This should make a graph of the reaction curved as the reaction rate slows down.
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. This can be thought of as like people in a refined space, if there is twice the number of people there will be twice the chance of people colliding. This situation may change over time, however, depending on the situation.
Equipment List
Ø Conical Flask
Ø Bung With Delivery Tube
Ø Water Bath
Ø Measuring Cylinder(s)
Ø Clamp and Stand
Ø Stopwatch
Method
Method for collecting results for one concentration of acid
Collect equipment as stated previosly and assemble as per diagram
Collect 20ml of correct concentration of Hydrochloric Acid and 5g of Calcium Carbonate Chips
RESET STOPWATCH. Fill measuring cylinder(s) with water and place downside-up in water bath. Make sure there are no air bubbles present in the cylinder.
Place CaCO3 in conical flask. Add HCl and push the bung tightly into the neck of the conical flask. Start the stopwatch as soon as the bung is tightly inside the neck of the flask.
Take readings at 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 minutes, swapping measuring cylinders over if one fills up. Complete my pre-prepared table at the correct intervals.
Analysing Evidence and Conclusions
From looking at my results and graph I am able to conclude that the more concentrated the acid the quicker the reaction happened and the more gas was produced. When I looked at the graph I saw clearly see that in all five results, the beginning the reaction happened more quickly than at the end, where I saw that it started to tail off.
Before I did the experiments I made some predictions they were that the more concentrated the acid was the quicker the reaction would happen and more hydrogen would be produced. This was completely correct because what I thought would happen did happen.
At this point I am able to answer my main hypothesis that was 'how does changing the concentration of acid affect the rate of reaction between hydrochloric acid and calcium carbonate.' The answer to this would be that changing the concentration of the acid will either make it go slower or quicker and more or less gas would be produced depending on whether the concentrations were more or less. If the concentration of the acid was less then the rate of reaction would be slower however if the acid was more concentrated then the reaction rate would be faster.
Overall this was a very successful experiment and everything went very smoothly, and that was why my results were very good, and I am able to make a conclusion like this.
Evaluation
I feel that this was a successful experiment. My results seem to indicate a clear pattern from which I was able to confidently draw a conclusion. This conclusion confirmed my hypothesis. The method produced results accurately and quickly enough to finish the experiments in the allotted time.
The method did show the relationship between the concentration and the rate of reaction but there was a slight margin of error because when turning the cylinder upside down a small volume of water was lost. To solve this problem we could use a gas syringe but the problem with this is we could not test the higher volumes of gas, which would mean the range of results would be lower which would mean the pattern we got would be less obvious.
I had a problem with my method in that I had to drop the marble chips into the conical flask, place the bung into the top and start the stopwatch. This is such a long sequence of activities to do in the time it takes the marble chips to hit the surface of the solution. There is a potential for error when carrying out a long sequence of activities such as this, alone.
I have already mentioned the problem of the gas having to push the water down the delivery tube before the gas was collected. This added time onto the true time taken. This means my results are higher than they should be. This does not question the truth of my conclusion however; as the same thing happened each time I performed the experiment. I could alleviate this problem by only starting the timer when the gas starts to fill the measuring cylinder. This would not include the time taken to push the water down the delivery tube in my results, and I would not have to start the stopwatch at the same time as putting the marble chips in the flask.
If the experiment was undertaken more than once for each concentration of acid and an average was taken then the accuracy of my results would have been greatly improved. This would be a serious consideration if I had more time for collecting my results. The ambient temperature of the surroundings could also have affected the results. The room heated up as the lesson went on so the reactions may have gone faster than they should have.