Investigating Rates of Reaction

Investigating Rates of Reaction
The reaction between sodium thiosulphate solution and hydrochloric
acid produces a precipitate of sulphur. When first mixed they form a
colourless solution, but, as the reaction proceeds, a yellow
suspension of sulphur is produced which after some time when
sufficient sulphur has been formed, makes the mixture opaque.
The factors I could investigate that would speed up the rate of the
reaction are temperature, concentration and the use of a catalyst. I
will choose to look at the effects of changing the concentration of a
liquid. I can predict that when the concentration of one of the
reactants is increased and the concentration of the other stays the
same, the rate of the reaction will increase. This is because in a
more concentrated liquid, the particles are closer together and more
numerous, and so have a greater chance of colliding. As there are more
fruitful collisions, the activation energy needed for the reaction to
proceed is lowered. Therefore the rate of reaction will increase.

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.


Conical flask

2x Measuring cylinders

Teat pipette




1) Measure out 10cm3 of sodium thiosulphate and 10cm3 of hydrochloric
acid into two separate measuring cylinders.

2) 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.

3) Swirl the flask three times then place it on the cross.

4) 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.

5) Repeat with the same values of liquid until the results are

6) Repeat with the following values of sodium thiosulphate and water.


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.


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

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