Chemistry

rates of reaction Sodium Thiosulphate + Hydrochloric Acid

Na2S2O3 (aq) + 2HCl (aq)

Sodium Chloride + Water + Sulphur Dioxide + Sulphur

2NaCl (aq) + H2O (l) + SO2 (g) + S (s)

Fair Test

To make this experiment a fair test, we need to make sure we do a

number of things. In this experiment we are trying to find the rate of

reaction using concentration as a factor, so there is a number of

things we need to make sure we do to keep it a fair test.

Firstly, we need to keep a chemical at a constant concentration. So,

in this experiment we have chosen to keep hydrochloric acid at a

constant concentration (5cm3). We could have, however, used Sodium

Thiosulphate as a constant, but we had chosen to use Hydrochloric

acid.

Next we must make sure that the solution is kept at a constant volume

throughout the experiment. If the volume is different, then it could

give different results to if it was at a constant volume. We must also

make sure that we add both the water and the Sodium Thiosulphate at

exactly the same time (into the beaker with the hydrochloric acid in

it), or it could affect the results of the experiment.

We must start the stopwatch at the exact time as we put the water and

the Sodium Thiosulphate into the beaker. To do this it is a lot easier

if there are two people doing the experiment, so one person can put

the two substances in the HCL, and one person can start the stopwatch.

Also The person timing the experiment will look for the disappearance

of the cross, otherwise there would be a time lapse between seeing the

cross disappear and telling the other person to stop the clock and

then eventually stopping the clock.

It is also important