Experiment to investigate the rates of reaction between sodium thiosulphate solution and dilute hydrochloric acid.

Method:

1. First of all, the equipment needed will be collected, and the apparatus will be set up as shown.
2. A black cross will be drawn on to the plain piece of paper, and placed under the flask.
3. Next, 50ml of sodium thiosulphate solution will be measured out into the measuring cylinder, because this is the most accurate piece of equipment we can use, which will give the correct amount needed.  Using meniscus will also help to find the most accurate measurement possible.  Meniscus is the curved upper surface of a liquid in a tube.
4. This will then be poured into the flask ready for the hydrochloric acid to be added.
5. Then, 10ml of hydrochloric acid will be measured out into the second measuring cylinder, since the other has been used to hold sodium thiosulphate solution, and therefore would contaminate the hydrochloric acid before it is added to the sodium thiosulphate solution in the flask.  Meniscus will also be used to find the most accurate measurement here for the hydrochloric acid.
6. The hydrochloric acid will be added to the sodium thiosulphate solution in the flask.  The total volume of the solution is therefore 60ml.
7. The moment the hydrochloric acid is added, the stopwatch will be started to time the rate of reaction between the sodium thiosulphate solution and the hydrochloric acid.
8. The black cross will then be watched over the top of the flask, until the black cross cannot be seen anymore.  Once the cross cannot be seen at all, this means that enough of the precipitate (solid sulphur) formed from the chemical reaction has covered the cross.  The stopwatch will be stopped immediately.
9. This time will then be recorded down into the results table, and the experiment will be repeated three times.  Repeating the experiment three times will give more reliable results and an average will then be able to be worked out.
10. Once this has been repeated three times, the experiment will be done again, except that the amount of sodium thiosulphate solution used, will be decreased by 10ml, and the amount of dilute hydrochloric acid will remain the same.  To keep the total volume the same, to keep it a fair experiment, 10ml of water will also be added, which will dilute the sodium thiosulphate solution to make it a weaker solution.  
11. This will also be repeated three times, so that an average can be worked out.  Once this has been completed, the experiment will continue, but the amount of sodium thiosulphate solution will be decreased by 10ml each time, and more water will be added each time to keep the total volume of the solution constant, and to lower the concentration of the sodium thiosulphate solution.  The amount of dilute hydrochloric acid must be kept the same throughout the whole experiment, since we are testing for the difference in the rate of reaction, that sodium thiosulphate solution makes happen when tested at different concentration levels with dilute hydrochloric acid.
12. All together, there should be five slightly different experiments where the concentration of the sodium thiosulphate solution will vary by 10ml each time.  All of these will be repeated three times to give averages and more reliable results.              

Results:

Analysis:

This experiment has showed me that as the volume of sodium thiosulphate solution increases, the time taken decreases.  Also, as the volume of sodium thiosulphate solution increases, the rate of reaction also increases.  This is because as time decreases, speed increases meaning the rate (speed) of reaction will increases if the time taken decreases.

As the volume of sodium thiosulphate solution doubles, the time taken nearly halves.  When the volume of sodium thiosulphate solution is at 20cm3, the time is 84 seconds, and when the concentration gets to 40cm3, the time is 41 seconds, which is practically half.  At 25cm3 the time is 70 seconds, and at 50cm3 the time is 34 seconds.  This is inverse proportion.

On the line graph with a line of best fit, I found that as the volume of sodium thiosulphate solution doubles, the rate of reaction also doubles.  When the volume of sodium thiosulphate solution is at 10cm3, the rate of reaction is 0.006 seconds -1 and at 20cm3, the rate of reaction is 0.012 seconds -1.  At 40cm3 the rate of reaction is 0.024 seconds -1.  This is direct proportion.

The time taken decreases when the volume increases because the particles collide more frequently meaning that the chemical reaction happens quicker, decreasing the time.

As the time decreases, this means that the rate of reaction must increase since the chemical reaction happens quicker, because the particles are closer together so they have a greater chance of colliding.

This also shows that this agrees with my prediction because the higher the volume of sodium thiosulphate solution, the quicker the rate of reaction and the formation of the precipitate of solid sulphur were.  It also shows that there must have been enough energy (activation energy) for collisions between molecules of reactants to start bond-breaking for the reaction to happen in the first place.

Evaluation:

The results obtained seem fairly reliable and accurate, although there might have been slight changes in the amounts of solution used since I didn’t always use meniscus.  This is the curved upper surface of a liquid in a tube, which helps to find an accurate measurement.  Also, a different flask could not be used each time because there wasn’t enough, so anything left in the flask could have contaminated the part of the experiment slightly and therefore affected the results.  There was only one anomalous result, which happened when the volume of sodium thiosulphate solution was at 30cm3.  The average time for this result was 48.3 seconds, which showed to be a bit further out from the other results.  Everything looked like it was supposed to look on the graphs, which proves that my results are quite accurate.

The method used was quite good, but there had to be enough time to do the whole experiment in one go otherwise the experiment would of have to have been done again because the temperature needs to be kept the same, since according to scientists raising the temperature makes particles collide more often in a certain time, and makes it more likely that collisions result in a reaction.  So, if one half of the experiment is done and the other half another time, then the temperature could have changed so to keep things as fair as possible, the whole experiment needed to be done altogether.

The results are good enough to support a firm conclusion because the graph matched another graph for the same experiment that I found in a booklet.  Also, none of my results were more than 2 seconds out.  Everything was kept fair, and the results turned out as what I expected them to turn out as, which proves my prediction and the scientific theory of rates of reaction, but I would suggest that to make absolutely sure that the results are accurate enough, the experiment should be done again.

Improvement wise, I would only really suggest that meniscus is always used to make sure that the exact amount of solution is used each time to keep things fair, and that a different flask is used each time, even in repeating the same part of the experiment just to make sure that nothing left in the flask will change any of the results.  E.g. If any water is left in the flask from washing it up, if a different flask can’t be used then any of the excess water could dilute sodium thiosulphate solution more than it is supposed, therefore affecting the results.

Further work, which could be done to give more evidence for rates of reaction, could be to test the effect of temperature on particles in a solution, since scientists believe that as we increase the temperature, we increase the rate of reaction.

Further improvements, could be to re-do the experiment over again to check if the anomalous result resulted in any thing that went wrong during the first experiment, so to be as accurate as possible, this could be done to check it.