solution is kept at 50 cubic cm. Otherwise the time for the reaction will be changed and my
anomalies will be caused.
The same cross always needs to be used. If a different cross id used it could be a different
size or shade. This would change when the reaction is over.
It has been discovered that increasing temperature by 10 degrees halves the time for a
reaction to take place. Consequently the temperature needs to be kept the same throughout
the experiment. This could have been done by placing the conical flask into a water bath.
However, this could not be done as there would have been no way of knowing when the
reaction was over because the cross wouldn't of been able to of been used. Instead a
successful attempt was made to complete all of the readings on the same day. This way any
chances of extreme variations in the temperature were stopped.
Obtaining Evidence
In this experiment five readings were taken. The experiment was repeated. This would give
the chance for any anomalies to be identified. The gathered results were then placed into the
following table. The table shows the rate of reaction (1/seconds), as well as concentration of
sodium thiosulphate and time. The concentration was recorded to two decimal places, the
time was recorder to two significant places and the rate of reaction which is recorded to five
decimal places.
Analysing and considering evidence
With use of my results I was able to create two graphs or each experiment in order to see if
there was any correlation between concentration and rate of reaction. By looking at the graphs
I can see that when the concentration of the sodium thiosulphate is decreased the rate of
reaction also decreased. This means that there is a positive correlation between concentration
and rate of reaction.
With use of scientific knowledge this can be explained. With a lower concentration of sodium
thiosulphate there were less particles of that kind. Consequently, there was less chance of
there being successful collisions between the hydrochloric acid particles and sodium
thiosulphate particles which resulted in the rate of reaction lowering. Hence the following
statement can be made; as the concentration is decreases the rate of reaction also
decreases.
These results support my prediction. My prediction was also fairly accurate when I stated that I
also think that by halving the concentration the time will double. Looking at both sets of results
it can been seen that the second prediction I made was quite accurate in that it when the
concentration was halved the resulting time was very close to double the previous time. This
could also be explained using scientific knowledge. When the concentration is halved then
half of the sodium thiosulphate particles are lost. This then means that the chances of
successful collision between the two types of particles have also been halved. This would
result in the time doubling (roughly).
Evaluation of Experiment
I believe that I do have an accurate set of results and that I do have enough results to be sure
about my conclusion. However, I feel that I could have gained even more accurate results,
and therefore an even more accurate conclusion, if I had taken more readings.
Looking at my graphs there do not appear to be any anomalous results because they all follow
the trend well and fit into the pattern. However I feel that as I lowered the concentration and
the time increased the results became more varied. This can be shown in the way that there
was a percentage difference in my two results bigger than 10%. Although I feel that this could
have been caused by human error when deciding when the cross had totally disappeared.
My method of carrying out my experiment was fairly accurate. I made sure that I washed the
beakers clean for each reaction I had and I measured the same amount of solution in each
time. Although I think there is a human error factor involved when you are measuring liquids
and also looking for an end point in the reaction. I feel that this could have been overcome by
of using a light beam instead of a cross. When the light intensity drops below a certain level
this could signify the end of the reaction and the timer stopped.
