To make the experiment a fair test, the temperature of the room in which the experiment s being carried out will have to be the same all of the time. The timer that is used to record the time it takes for the reaction to occur will have to be started and stopped at the same time for each run of the experiment, e.g. when ½ of the second liquid has been poured in, the timer is started and when the liquid is completely opaque, the timer will be stopped. For each run of the experiment, the liquids will be put into the same containers each time, and the total volume of liquids will be kept constant.
In the experiment, the time it takes for the reaction to happen (the liquid to go completely opaque) will be recorded. To decide when the reaction has finished, a cross will be drawn on a sheet of paper and placed under the conical flask in which the reaction takes place. The same person each time will look down into the flask and stop the timer when they cannot see the cross. Six runs will be done at different concentrations, and each will be done twice.
Apparatus
Conical flask
2x Measuring cylinders
Teat pipette
Beaker
Timer
Method
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Measure out 10cm3 of sodium thiosulphate and 10cm3 of hydrochloric acid into two separate measuring cylinders.
- Pour the hydrochloric acid into the conical flask. Then pour the sodium thiosulphate in, starting the timer when half the liquid has been poured in.
- Swirl the flask three times then place it on the cross.
- Look down into the neck of the flask to see the cross. Stop the timer when enough sulphur has been formed to make the liquid opaque i.e. the cross is no longer visible.
- Repeat with the same values of liquid until the results are consistent.
- Repeat with the following values of sodium thiosulphate and water.
Safety
Safety glasses should be worn at all times during the experiment. Lids should be kept on bottles when they are not being used, and placed away from the edge of the table.
Results
Conclusion
As the concentration of sodium thiosulphate increased, so did the rate of the reaction. This was because there were more particles as the concentration increased, so the activation energy was reached quicker, and the reaction was completed quicker.
The prediction that the rate of reation would increase with higher concentration was correct.
I think that I took the right amount of readings to obtain a suitable range of results. They were all very close to the line of best fit as we carried out the experiment safely and accurately.
The method could have been improved by removing the errors that a human would make by using machines. For example a light and sensor could be used to detect when the solution has turned opaque, and a mass could be used to tell exactly when to start the timer. The experiment could have been carried out in a more controlled environment, e.g. a vacuum to keep the temperature and pressure the same.
