fear of it reacting. The water molecules get in between the Na2S2O3 particles and spread them
out but do not react with them. Then I will use the other measuring tube to measure 5cm³ of
HCl. Once it is measured, I will pour it into the conical flask. As soon as the HCl touches the
sodium thiosulphate I will press the start button on the stop clock. When the sodium
thiosulphate and hydrochloric acid have fully reacted the solution will have turned too a colour
that makes me unabe to see the black cross. As soon as that happens, I will press stop on the
stop clock. I will then record the time on my result table. To work out the rate of reaction, I will
inverse (1/time taken) the time it takes for the reaction to finish then I will multiply the time by
1000. This will give me my rate of reaction. I will then repeat the experiment increasing the
concentration of the HCl by 1cm³ and decreasing the H2O concentration by 1cm³. I will
continue till I reach 10cm³ of HCl. I will do this 3 times for each measurement to give a good
result for each average.
Fair Test
To ensure this experiment is a fair test I will make my measurements as accurate as possible by
using the most accurate and clean measuring devices available. I will only change the one
variable, the measurments of HCl and H2O. Also, I will have to pay full attention to the solution
when it is reacting so I don't press stop too late. I will have to make sure the solution is kept still
because movement will increase the rate of reaction. Also, temperature will be a factor, I will
have to try to do the experiment in the same temperature, but unfortunately i cannot control this
so it may affect my experiment slightly.
Prediction
The particle collision theory tells us that the higher the concentraion the greater the chance of
the reaction occouring. When the water is added, it decreases the concentration, so it greatly
decreases the chance of the sodium thiosulphate molecules coliding with the hydrochloric acid
molecules. In contrast to this, when there is less water, there are more molecules to collide so
there is a greater chance of the mollecules coliding in the right orientation, so they bond, this
means the reaction will happen faster. This leads me to predict that as I increase the
concentration of Hydrochloric acid increases the time it takes for the solution to fully react will
decrease, so the rate of reaction will increase.
Results
Analysis
As the concentration of the acid increases, the average time it takes for the reaction to take
place decreases, so the rates are increasing. My results and graph both show that my prediction
was correct, however I think that the whole of the second set of results are anomalous. Even
including the anomalous results, this shows that the particle collision theory is correct. When the
number of acid molecules are increased, there is an increased chance of having molecules that
will collide with the right orientation so they bond. My graph shows a steady line of best fit, this
shows that my prediction was accurate and correct.
Evaluation
On the results table it is very easy to spot the set of anomalous results (second set). Since this
happened to the whole set of results I think that there may have been an increase in
temperature. When temperature is increased, the molecules vibrate faster which leads to more
HCl molecules colliding with Na2S2O3 and bonding, this creates a faster reaction, speeding up
the reaction rate. In future experiments I will use a thermometre to make sure the temperature is
the same when I do each experiment.
I believe the experiment was quite suitable for this investigation because of how easy it is to
repeat each experiment. There are some ways that I could expand this investigation to give me a
different set of results. Instead of using concentration as the variable, I could use temperature or
movement. Temperature affects how the molecules move, and as the temperature rises, the time
for the reaction to take place should decrease. Movement would be an interesting experiment,
as it would be hard to control the movement, so it could probably never be fair.
If I was to repeat this experiment I would want to change the way the results are recorded. My
eyesight is not perfect, so my results will not always be as accurate as possible. To get more
accurate results I could use a colorimeter. A colorimeter measures the intensity of light that
travels through the solution. This would be better than using the cross, because you could just
stop once you reach a certain intenstiy.
I think that my results are vaild and that my experiment was fair. The set of results one and
three are very similar and this shows my results are valid. My investigation was fair, the only
thing that affected the experiment was the temperature, which I have no power over. I could
have perhaps carried out this experiment more times but I know that it would have just showed
the same as what my results already show me. It would have been better though for my
conclusion had I done a couple of more experiments because I would have been able to analyse
my results more. Also, if I was doing an investigation on the particle collision theory and not
reaction rates, then I could have used all the variables, in different experiments, that I have
stated throughout this investigation.
Overall, I think that this investigation went really well as I had no real difficulties, apart from
stopping the stop clock on time. I believe that this experiment is suitable and could be used by
anyone to test the rates of reaction for HCl and Na2S2O3.