GCSE Chemistry - Sodium Thiosulphate Coursework

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GCSE Chemistry - Sodium Thiosulphate Coursework

We must produce a piece of coursework investigating the rates of reaction, and the effect different changes have on them. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. It is measured by dividing 1 by the time taken for the reaction to take place. There is five factors which affect the rate of a reaction, according to the collision theory of reacting particles: temperature, concentration (of solution), pressure (in gases), surface are (of solid reactants), and catalysts. I have chosen to investigate the effect temperature and concentration have on a reaction. This is because they are the most practical to investigate - it would take longer to prepare a solid in powdered and unpowdered form, and it is difficult to get accurate readings due to the inevitabilities of human errors, and as gas is mostly colourless it is difficult to gauge a reaction changing the pressure, and if a substance is added to give the gas colour, it may influence the outcome of the experiment. Similarly the use of a catalyst complicates things, and if used incorrectly could alter the outcome of the experiment.

Aim: -

To see the effects of a change in temperature and concentration on the rate of a reaction. The reaction that will be used is:

Sodium Thiosulphate + Hydrochloric Acid

Na2S2O3 (aq) + 2HCl (aq)

Sodium Chloride + Water + Sulphur Dioxide +

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


S (s)

Two series of experiments will be carried out - one changing the temperature (while everything else remains constant) and one varying the concentration (while keeping everything else constant). Both the sodium thiosulphate and the Hydrochloric acid are soluble in water, so the concentration of either can be changed. However I have chosen to vary the sodium thiosulphate as it is available in larger amounts, and various concentrations are prepared. When the temperature is constant room temperature will be used as the temperature as it more practical and will not need to be monitored. When the temperature is being varied a water bath will be used to heat up the acid and thiosulphate to the necessary temperature.

I decided which temperatures and concentrations to use during my preliminary series of experiments -

1 mol/dm3 of HCl (acid concentration will be fixed)

10-35g/dm3 of sodium thiosulphate (all of these concentrations will be tested in turn going up in steps of 5g/dm3)

20-70ºC temperature (all of these temperatures will be used going up in steps of 10ºC)

Concentrations of 5, and 40 g/dm3 of thiosulphate were available to me but my preliminary work showed that the 5 g/dm3 and 40g/dm3 were too slow and fast respectively in reacting to be worth testing. Similarly any temperature below 20ºC reacted too slowly, and 80ºC and 90ºC reacted too quickly to be worth including in my final results.

Using my preliminary experiments I decided on using the following apparatus:

1 thermometer

1 beaker

2 measuring cylinders

1 conical flask

1 tripod

1 gauze

1 heatproof mat

1 stopwatch

1 Bunsen burner

X board

1 pair of tongs

1 pair of goggles

1 apron

Method: -

Experiment 1 - Changing the concentration

5 cm3 of HCl (at concentration 1 mol./dm3) and 15 cm3 of sodium thiosulphate (at varying concentrations - 10 to 35 g/dm3) are poured out into two measuring cylinders and then poured into a conical flask, which is placed on top of a board marked with letter X. The stopwatch will now be started. When the mixture has turned sufficiently cloudy so that the letter X can no longer be seen the stopwatch will be stopped and the time will be recorded. The experiment is repeated with all the concentrations. The whole procedure is then repeated.

Experiment 2 - Changing the temperature

5 cm of HCl (at concentration 1 mol./dm3) and 15 cm of sodium thiosulphate (at varying...
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