If you add a catalyst to a reaction this will also speed up the reaction. The important factor about using a catalyst is they do not use up after being added to a reaction. Catalysts work because they lower the activation energy, which is the minimum energy to bring about a reaction. If you lower the energy level more particles will have the minimum level that is needed. This doesn’t mean there will be more collisions it just means they are more likely to be successful. Examples of using a catalyst are the ‘cracking’ of hydrocarbons using broken pottery and the manufacture of Ammonia (Haber Process) using iron.
If the solid is finely divided the surface area will increase which therefore means there will be more particles around to react with other particles. The rate of reaction will then increase, as there are more collisions occurring.
Safety
During the experiment you will need to wear safety goggles. As harmful gasses will be given off the experiment has to be in a well-ventilated area. If you have a tie or clothing that is loose tuck them in to avoid contact with experiment. When handling hot beakers and measuring cylinders a pair of tongs will be used. A gauze and heatproof mat will be used while heating to avoid any damage to the equipment.
Apparatus
Conical flask
Stop Watch
A sheet of white paper
Sodium thiosulphate-50ml
Hydrochloric acid-5ml
Two measuring cylinders for sodium thiosulphate and hydrochloric acid quantities.
Distilled Water
Fair Test
In order for the experiment to work it has to be carried out fairly. In order to make my experiment fair I will control the following variables. I will use the same standard each time for judging when the X has disappeared. I will make sure that the measuring cylinders for the HCl and thiosulphate will not be mixed up. The amount of HCl will be 5 ml each time, and the amount of sodium thiosulphate will be accurate with the measurement being at the bottom of the meniscus for each quantity chosen. I will not stir the chemicals at all and I will try to move them around as little as possible. This is to prevent any unwanted collisions that may inadvertently speed up the rate of reaction. All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus make the entire investigation more successful.
Preliminary Test
The reason for doing this preliminary experiment is to check that my main experiment is appropriate and is going to work. This now means that when I do my main experiment I can cross-reference those results with those of the preliminary test. The results showed that the rate of reaction was slower for the solution that had less sodium thiosulphate in it. My results were of a large quantity of sodium thiosulphate and a small quantity of sodium thiosulphate. The results do seem to be quite accurate when you look at the time taken although there is one result, the last result on 5ml of sodium thiosulphate that seems a little out. This could be due to human error where too much or too little of a certain chemical were put in. Or the measuring cylinders and beakers could have been a dirty, therefore affecting to the solution.
I have not needed to modify my experiment because the results are what I predicted and seem to have been produced quite successfully. The experiment went very well without any problems and the difficulty factor wasn’t too high.
Method
Set up the apparatus by having a conical flask on a white piece of paper with a cross on the paper directly under the bottom of the flask. You will have to look through the solution to see the cross. Add the 5ml of hydrochloric acid solution to the 50ml of sodium thiosulphate to form sulphur. Using the stopwatch time how long it takes for they’re to be enough sulphur to block the view of the cross, which is drawn on the white piece of paper, then note down the results. Repeat this experiment with the same amount of solution but vary the concentration of the sodium thiosulphate by adding extra water each time, e.g. 45ml of sodium thiosulphate, 5ml of hydrochloric acid and 5ml of water. The next recording will be the same other than 40ml of sodium thiosulphate and 10ml of water. Plot the results in a table showing the time taken for the cross to disappear under different concentrations, the concentrations will be from 45ml of sodium thiosulphate to 20ml of sodium thiosulphate topping up the solution as I mentioned in the example above for each recording. Then plot a graph that will have 1/time for the cross to disappear against the concentration.
Results
My results show that as the quantity of sodium thiosulphate was increased the quicker it took for the cross to disappear. Obviously as the quantity of sodium thiosulphate increased the quantity of water decreased meaning the solution was less concentrated. All the results have been obtained and they seem to be quite good, mainly showing correlation with a few results that were a little out. As was hoped at the start there was three recording of the times for each solution and an average worked out from those figures. The results and the averages were rounded up or down to the nearest whole number and rate of reactions are to two decimal places.
My graph showing time against concentration gives good curves panning outwards from all the three sets of times for each solution. This meant a curve of best fit could be introduced. The graph is plotted from the table of results and is what I expected. (See graph 1) The second graph is of the average rate against concentration that plots the results in a good pattern. A line of best fit could be put through to help show the pattern of results and if there were any results that were anomalous. This graph was also plotted from the table. (See graph 2)
These results were obtained because of the reasons that I stated in the prediction. I said that when the concentration doubled the rate of reaction would also double. As I have a limited number of results there are not that many concentrations that I did double. These are the results of the doubled concentrations:
Concentration of Average time
Sodium thiosulphate
20 104
40 37 (Doubled = 74)
25 81
45 37 (Doubled = 74)
As you can see the results for the 25ml and the 45ml of sodium thiosulphate are quite close and although not exact they seem to go with my prediction. The problem with the 20ml and the 40ml of sodium thiosulphate results is that there are anomalous results compared to the other results. The average time comes out to the same as 45ml time and that is not in correlation with the other results. If the results had been done more accurately then the prediction of the rate of reaction doubling when the concentration doubles would have been more reliable. The rate of reaction is doubled when you double the concentration because the number of particles is increased meaning there is more chance of successful collisions meaning sulphur will be produced obscuring the cross quicker.
Evaluation
When you look at the table of results and especially the graph you can see that thee are some anomalous results. On the graph showing time against concentration there are three main anomalous results. No.1 is at 25ml and 85 seconds and the other anomalous result, No.2 is at 45ml and around 41 seconds, and No.3 is at 40ml and 35 seconds. No. 1 and 2 are both anomalous because they are higher than the other two results that were taken for their specific solution. This is probably mainly because of human error in watching the cross disappear. Also the measurements were probably a little out, which would make all the difference. Contaminants in the beakers and the conical flask may have also altered the results. No. 3 is not either higher or lower it is just very far off the curve of best fit for the second set of results. It is close to the other results at 40ml but not in line with the other results in other solutions.
Another anomalous result is on the second graph where it shows average rate against concentration. The average rate between the 40 and 45ml results is the same. This must be anomalous because all the other results form a nice straight pattern but this one results sticks out of the line. It also does not go along with my prediction that the rate of reaction increases as the concentration increases.
The sources of error in this experiment are human error mainly with the visibility of the cross. Other errors are the inaccurate measuring of quantities and the cleaning and sterilising of the all the equipment that is used. As you do not know what was in the beakers or flaks before or if any dirt has entered them etc. A smaller error could be the stopwatch, whether it stops properly or has a fault so the timing is different than the last stopwatch that you used. Time it takes you to respond to there not being a cross could affect the results a little.
There are ways that the experiment could be improved. Using more accurate measurement devices could be quite critical. If you used a measuring cylinder that increased in 1ml notches instead of 10ml notches then you could be more accurate with the pouring. I think more results could have been taken so that I could have a better look at the doubling of the concentration theory; also a better average could then have been taken to level out any anomalous results. Making sure that all the equipment was clean and rinsed out could help more in the accuracy. I think if I had taken a recording of the temperature of the solution each time the experiment would have improved because then I may have be able to explain some of the anomalous results because the temperature may have been higher, this couldn’t be done during my experiment. The temperature could have been a factor.
To extend my investigation I could easily introduce the other factors that affect rate of reaction. An easy experiment could be to keep the same solution throughout the experiment but change the temperature of the solution. Heating up the solution and taken the recordings from that. You could then prove that temperature does or does not affect rate of reaction. Adding a catalyst would also be interesting to see what catalysts work the fastest if they work at all. All of these different experiments could then be linked together to see which of the different factors affects the rate of reaction the most. When this is done you could then mix the experiments together. E.g. Adding a catalyst and increasing the temperature, would this increase the rate of reaction even more or would they cancel each other out and only one of the factors would be an affecter?