The factor I have chosen to alter is the volume of hydrochloric acid. I will start with 15cm³ and add an extra 5cm³ to the solution of sodium thiosulphate each time. I will have seven different volumes and take three sets of results of each different volume. By doing the experiment so many times for each volume I will be able to see if my experiment was accurate and reliable, and also if the rate of reaction does change because of changing the volume of hydrochloric acid used.
Prediction
I expect the rate of reaction to increase in speed ahs the volume of hydrochloric acid added each time increases. This is because if the solution has a higher volume of hydrochloric acid there will be more acid particles for the sodium thiosulphate particles to collide with increasing the speed of reaction.
Method
For my experiment I will need the following apparatus:
- 2 measuring cylinders
- Conical flask
- Stopwatch
- Thermometer
I measure out 10cm³ of thiosulphate in to one of the measuring cylinders then pour it into the conical flask.
I will then measure out Xcm³ of hydrochloric acid in to the second measuring cylinder.
I will place an “X” on a piece of paper and place it under the conical flask before adding the hydrochloric acid I just measured to the sodium thiosulphate.
I will then add the hydrochloric acid to the sodium thiosulphate that is already placed in the conical flask. After finishing adding the whole of the hydrochloric acid to the sodium thiosulphate I will start the stopwatch.
When the chemicals have reacted with each other a cloudy substance should be left, when I can no longer see the cross under the conical flask through the cloudy substance I will stop the stopwatch and record the time down.
I will repeat what I have just done 3 times for each different volume of hydrochloric acid used each time. I will keep the sodium thiosulphate at 10cm³ every time.
Results
Results
I have drawn a line graph using my average results; I have also added a line of best fit to show if my results are accurate. By drawing a line of best fit it also shows me if there are any anomalous results or not.
Conclusion
My line graph goes up steadily with no anomalous results. The change in my line graph changes immediately as I change the volume of hydrochloric acid used changes. It goes up by 2-7 seconds approximately for every 5cm³ added each time. The rate of reaction slows down as more hydrochloric acid is used each time.
Explanation
I think that the pattern of my graph goes up in a steady line because the volume of hydrochloric acid been used each time as increased by the same volume each time and has not changed. This effect as happened because the conditions I did the experiment under each time stayed the same.
Evaluation
I cannot be sure of just how accurate my experiment was because of the following reasons:
- The temperature may have been a little bit higher or lower every time I did the experiment, which could have caused the particles to move that little bit faster so the reaction may have took place a little bit faster.
- Starting the stopwatch at the right time was a difficult choice due to I didn’t know if should start timing as I started to pour the hydrochloric acid into the sodium thiosulphate or wait until I had poured all of it in. Either one the two choices might have meant a few seconds difference in my results.
- Knowing when to stop the stopwatch a tricky as well, because it wasn’t easy to judge exactly when the cross-had disappeared. This may have also meant a few seconds difference in my results.
My results seem to be very reliable, with them having about 1-3 seconds approximately between them for each different volume of hydrochloric acid used. My three results taken for each different volume of acid where all reliable enough to work out an average from to be used in my line graph. There was no anomalous results in my table or what showed up on my line graph.
Improvements
I don’t believe that I could make any major improvements to my experiment due to my results been so reliable. The only real improvements I could make are:
- Deciding on a fixed point when to start my stopwatch, fro example, starting it as soon as I start adding the hydrochloric acid to the sodium thiosulphate or starting after I have finished adding the hydrochloric acid.
- I could also try to stop my stopwatch more accurately on the moment of the “X” disappearing which is hard to determine.
- I could try to keep the solution at a fixed temperature so the particles are of the two chemicals move at the same rate.
