I predicted that rate of reaction would increase with concentration of acid, and my prediction has been proved correct. However, the results could have fitted my prediction better; the difference in rate could have been more pronounced, and not so slight. Evaluation I think my method was fairly good. I can be sure that the results are reliable because I took repeat readings; however, in order to make the results even more reliable I could have taken 2, 3 or even 4 more repeat readings and averaged them out.
Also, in order to make the results more reliable I could have continued timing and recording volume of gas until the reactions were fully complete (I would know this because the volume of gas would stop increasing). However, to do this would have taken too much time; also, the gas syringe didn’t have the capacity to complete the reaction, because the most gas it could hold was 100cm3, and many of the concentrations would have produced more gas than this. In order to prevent this I could have used less liquid overall and less marble chips, but this method would have then taken longer.
The graphs show that my results were accurate enough to allow me to draw a conclusion, but could have been more accurate. Most of the points lie fairly close to the line of best fit, but they could have been closer. Although I measured the reactants very accurately, it was difficult to measure the amount of gas produced; the scale on the gas syringe went up by 1, with no markings between each number, so in a lot of cases it was an approximation if the line fell between two numbers.
When I drew a line of best fit for the graphs, particularly the ones where the concentration is lower, I found that the ideal line of best fit (drawn in dashed lines on the lower concentration graphs) did not go through 0,0. This is because the syringe did not start at 0cm3; whenever I put the stopper into the conical flask, this action caused a slight pressure of gas in the syringe and the syringe immediately jumped about 3cm3. This is why in my results tables we can see that with 0% acid, the volume of gas was consistently 3 rather than 0, although no gas was being produced.
Another factor which may affect the graphs is the fact that some gas, if very little, was lost between adding the marble chips, and putting in the stopper. I could have avoided both these things by employing a slightly different method; instead of adding the chips and then putting in the stopper, I could have had the chips in a small test-tube suspended inside the conical flask by a piece of string, with the stopper in, enabling me to start the reaction by pulling the string and tipping the chips out, thus preventing any loss of gas.
However, this proved too difficult to do because of the size of the marble chips and the fact that they would have just got lodged in the test tube. Another aspect which could have caused some of the anomalies is the fact that occasionally the gas syringe got stuck for a while, and then suddenly jumped. There is nothing I could have done to alter this fact, but it was an unfortunate occurrence. It is this occurrence that caused the anomalous results in the reaction where the concentration was 90% acid, circled on the graph.
The surface area of the marble chips could also slightly have affected the rate of reaction, as I mentioned in my plan; however, I made a point of using two similarly-sized chips weighing in total 1g for each experiment, so that the surface area couldn’t change too much from experiment to experiment. Something else which has to be taken into account is the fact that the gas had to go up the delivery tube to the syringe, against gravity.
On the higher concentrations when a lot of gas was being produced this was probably not a problem, but on the lower concentrations, in some cases there may not have been enough pressure to push the gas up the syringe. I think that I obtained enough evidence in order to allow me to draw a conclusion, because I took my readings over a large range. I also took the readings very close together, every 10 seconds. The fact that I used such a large range, and did so many experiments means that any anomalous results aren’t of such consequence, because there are always other results very close in time or concentration to these.
FURTHER INVESTIGATIONS There are many ways in which I could extend this sort of investigation, for example I could investigate how surface area or temperature effect this same experiment; e. g. keeping the concentration the same but using different sizes of marble chips, or heating the experiment etc. Other reactions I could investigate include the reaction between sodium thiosulphate solution and dilute hydrochloric acid (HCl + sodium thiosulphate –> sodium chloride + sulphur dioxide + sulphur + water), or the decomposition of hydrogen peroxide solution (hydrogen peroxide –> oxygen + water).Using either of these reactions, particularly the second, I could also investigate the effect that a catalyst has on the rate of reaction.
BIBLIOGRAPHY Chemistry For You – Lawrie Ryan Co-ordinated Science: Chemistry – RoseMarie Gallager, Paul Ingram The Usborne Internet-Linked Science Encyclopaedia The Internet (www. gcsechemistry. com ) Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.