b) Apparatus.
c) Preliminary work.
In the preliminary experiment 20ml of water was mixed with 30ml of sodium
thiosulphate. These were heated in a conical flask in a water bath (see diagram above) up to 30 C and then 5ml of hydrochloric acid was added. The time it took for the cross to disappear was recorded. It took 34s.
The preliminary was useful on two accounts. Firstly it is obvious that the experiment is fast enough and does not need the help of a catalyst. In fact, at 30 C, the reaction was slightly fast. In the proper experiment, 30ml of water will be mixed with 20ml of sodium thiosulphate instead of 20ml of water with 30ml of sodium thiosulphate. This should make the reaction slightly slower, thus easier to record, meaning that at higher temperatures the results should be more accurate.
- Predictions.
Before a chemical change can take place, the reactants must collide with each other. Any factor which increases the number of collisions per second in a given volume of solution will increase the reaction. As the temperature increases the atoms vibrate faster, so there are more collisions per second with the acid. Hence the rate will increase.
- Method.
- Add 20ml of Sodium thiosulphate to 30ml of distilled water in a conical flask.
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Heat a water bath up to 25 C.
- Add 5ml of Hydrochloric acid.
- Start the stopwatch immediately.
- As soon as the cross has disappeared, stop the watch and record the time taken.
- To ensure a fair test, clean out the conical flask thoroughly with distilled water.
- Repeat the experiment again
- Repeat at temperatures of 30C, 35C, 40C, 45C, 50C, 55C and 60C, twice each so an average can be obtained.
f) Results
From this table, a graph can be plotted, showing rate of reaction (x axis) against the temperature of the water bath (y axis). It is shown on graph paper on the next page.
On the graph, one result is circled. This is an anomalous result, a result that is slightly out of alignment with the line of best fit. This could have been caused by a number of things. Firstly, it was very hard to judge when the cross had fully disappeared. Also, the sodium thiosulphate was being used by a whole class and it was very possible that some foolish member had slightly contaminated the sodium thiosulphate. This would have lead to a reaction that would have been too quick which is so in this case (see graph). One cannot rule out an error on our part either, and that we heated the water bath slightly above the required temperature, although this is unlikely because both readings that were taken were very close, which shows that the most likely reason for this anomalous result is that the sodium thiosulphate was contaminated for that temperature.
g) Conclusion
My prediction proved correct. As the temperature increases, the rate of reaction increases. Also, as the temperature increases, the rate of reaction increases more and more each time.
At 25C (just above room temperature) the sodium thiosulphate ions vibrate. This means they occasionally collide with the acid, thus sparking a reaction. When the temperature increases, the sodium thiosulphate ions vibrate faster, so they collide with the acid more frequently than they did before. Also, each time the temperature goes up, the sodium thiosulphate ion vibrations increase more than they did the last time the temperature rose.
Eventually, the reactions would stop, because the temperature would get too high and eventually boil all the water in the water bath. This could not be proved as the reactions got too quick to record accurately.
h) Evaluation
The experiment is not perfect by any means but I would say there is definitely enough evidence to support a firm conclusion. On the graph, there is a very apparent best fit line, and the results backed up the conclusion very well.
However, there are many points to be improved, and thought about. for one, the moment the cross has completely disappeared relies far too much on human judgement and is very dubious. When looking for something very intently, one may imagine something is there when it is not. Human reaction time on the stopwatch when this has registered on the stopwatch is also far from perfect, but this has to do without using severely complicated and expensive equipment.
One problem with doing the experiment was the shortage of time. If we had considerably longer, then we could have used a much weaker concentration of sodium thiosulphate (say 10ml sodium thiosulphate and 40ml distilled water), thus being able to find out the rate of the reaction at higher temperatures, and perhaps discovering how high a temperature the reaction continues up to. Certainly, if further work on this topic was carried out, this would be the aim.
