Also a burette is used to make the volumes of solutions used as accurate as possible, and the experiments are repeated to make sure that the results are reliable.
Prediction
In our experiment we will change the concentration of the HCL used to react with a ribbon of mg. I think that the higher the concentration of HCL the faster the rate of reaction. The rate of reaction increases due to the collision theory: increasing the concentration simply means increasing the number of particles, which can collide and react. And the more successful collisions per second mean a faster rate of reaction. The temperature of the experiment will also affect the rate of reaction due to collision theory; at a higher temperature there will be more particles of activation energy, increasing the chances of a successful collision, also there will be enough energy to break the bonds.
Key Variables
Keeping the temperature constant is one of the problems I will encounter during the experiment. This is because the experiment is exothermic (gives off heat). This will greatly affect the rate of reaction as the higher the temp the more energy the particles have to break the bonds and the particles will move faster and more particles will be of activation energy and the percentage of successful reactions will rise. Also going back to temperature, when the temperature of most reactions is increased by 10 degrees Celsius the rate of reaction is doubled, as temperature and rate of reaction are directionally proportional, this emphasises the importance of temperature
Another variable, which will have to be kept constant, is the amount of mg used in the reactions. If there is more mg in one reaction than another then that means that the rate of reactions will happen faster as there are more particles to react with and the chances of a successful collision will rise.
The third variable to be kept constant is the surface area of the mg. Surface area is important as the more of the surface of the mg exposed the higher the number of particles of mg the HCL can react with, increasing the rate of reaction. So all of the mg particles should be coiled the same way so the area of mg exposed is the same in each reaction.
The fourth variable to be kept constant is the volume of solution used, because if the amount of solution is changed the it will be easier for a smaller volume of solution to heat up than a large volume of solution and as I have explained, temperature would alter the results dramatically, ‘when the temperature of most reactions is increased by 10 degrees Celsius the rate of reaction is doubled, as temperature and rate of reaction are directionally proportional’.
Results 1
Results 2
Average Results
Analysis
The results I gained from each experiment were all very accurate and each set of results proved to be reliable by not being more than 10cm3 away from each other. There is a noticeable pattern represented on the graph. The graph shows that the higher the concentration of HCL, the faster the mg reacted with it; on the graph this is shown by the steepness of the lines.
With 1mol/dm3 it takes 10secs to make 28cm3 of hydrogen whereas with 0.4mol/dm3 it takes 100secs to make just 28.5cm3 of hydrogen.
There isn’t an exact pattern but if the experiment was 100% accurate there may have been a pattern as the average results table shows that when the concentration is increased by 0.2mol/dm3 the amount of gas produced in 10secs isn’t far from doubling. Temperature hasn’t been too much of a problem but it may have been temperature, which stopped the pattern from forming. The experiment did heat up quite a bit with the higher concentrations but this is because the experiments are exothermic so its was bound to give off quite a lot of heat, especially when high concentrations wee used. The results I have gained have complemented my predictions perfectly as they fitted with what I had predicted.
Evaluation
During the experiment there were not very many anomalies as the results that I obtained were very accurate. One of the most obvious anomalies was with the concentration of 0.6mol/dm3 at the 80second point, but I would not put this down to human error, as it isn’t really that big of an anomaly, it may have been caused from the apparatus we had to use. One of the problems with the apparatus was when we dropped the mg into flask of HCL as some of the gas escaped, as it was impossible to block the flask with the bung before the reaction started, and at 0.6mol/dm3 the reaction happened quite fast. To get around this problem we could have used flasks with longer necks so that we could block the flask up before the mg and HCL began reacting as the mg would take longer to reach the HCL.
Furthermore the gas syringes were pumped up a bit with air when the bungs were put in the flasks as air was forced into the flask, through the tubing and into the syringe, this is probably why my readings at 0seconds start at 1cm3 of gas, but there is no sensible way of getting around this problem
Another problem with apparatus was that it was not entirely clean as it had been used quite a lot before hand, the flask we used had numerous stains in it that couldn’t be cleaned of and because we don’t know what sort of stains these were there is no way of knowing exactly how the effected the experiment. Also we could not inspect the inner part of the connecting tube, so if it was a little bit clogged up inside it could have slowed the amount of hydrogen rushing into the syringe which would give us inaccurate rates of reactions.
Also the iron wool could have been more effective if it was courser, by the time one part if the mg was cleaned another part needed a second cleaning as it took quite some time to all of the magnesium oxide off.
The final problem was the syringe, it kept on stick and didn’t move as smoothly as I would have liked it, although we twisted it to loosen it I don’t feel this was sufficient and some sort of lubrication should have been used. The best thing that we could have done with the apparatus was to replace it all with new apparatus that had not been used so there was no chemical stains on it and everything that had to move did so very smoothly i.e. the flask and tubing and the syringe.
Before we started this experiment we done another experiment to find out what length of mg to use for this experiment and we used a syringe in this experiment so we were able to find out hat by twisting the inner part of the syringe we could loosen it, this turned out to be very useful for this experiment as we were able to increase the accuracy of the results.
If we were to do another experiment using another variable I would use temperature as the variable and keep the concentration and length of mg constant. In this experiment I know that the rate of reaction would increase with temperature due to collision theory ‘the higher the temp the more energy the particles have to break the bonds and the particles will move faster and more particles will be of activation energy and the percentage of successful reactions will rise’.
The problem with this experiment would be that it is very hard to keep the temp constant and a top of the range water bath would be needed.
Rates of Reaction
Rate of reaction =amount of gas/time (time used must be kept constant)
0.2mol/dm3
Rate=3/30
Rate=0.1
0.4mol/dm3
Rate=12/30
Rate=0.4
0.6mol/dm3
Rate=21/30
Rate=0.7
0.8mol/dm3
Rate=42/30
Rate=1.4
1.0mol/dm3
Rate=73/30
Rate=2.43
