The Effect of Concentration on the Reaction of Sodium Thiosulphate with acid

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The Effect of Concentration on the Reaction of Sodium Thiosulphate with acid

Harry Weeks

This is an investigation to find the effect that altered concentration has on the rate of a reaction. The reaction in question here will be that of sodium thiosulphate with hydrochloric acid. The equation for this reaction is thus:

Na2S2O3 (aq) + 2HCl (aq) ==> 2NaCl (aq) + S (s) + H2SO3 (aq)

Sodium Thiosulphate + Hydrochloric Acid --> Sodium Chloride + Sulphur + Sulphuric Acid

This reaction begins as a colourless solution of sodium thiosulphate and hydrochloric acid, but gradually turns cloudy and yellow with the presence of a sulphur precipitate. This precipitate is what we aim to exploit in order to show what effect concentration has on a reaction.

Apparatus: Conical Flask

Stop Watch

Black Cross

Measuring Cylinder x2

Method:

The two reactants are mixed in a conical flask. The concentration of the hydrochloric acid is kept constant at 1Mdm-3 (bench HCl) and 10cm3 is added each time giving 0.01M in each experiment. The concentration of sodium thiosulphate is altered each time but its volume will always be 30cm3. The concentration is altered from the 0.1Mdm-3 by adding water. So that when there is 30cm3 of sodium thiosulphate, there is no water and with 20cm3 of sodium thiosulphate, 10cm3 of water was added to make the 30cm3 of dilute sodium thiosulphate. This means that, although the concentration is being altered, the depth of the reactants stays the same. This allows the black spot to be seen with the same ease in each experiment.

As the two solutions react, the yellow precipitate begins to become apparent. This obscures a piece of paper with a black cross written in indelible ink, which is placed underneath the flask. The clock is started when the two substances first interacted and then stopped when the cross beneath is completely obscured.
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This set up is repeated for five different concentrations of sodium thiosulphate, three times each. Therefore a total of fifteen measurements will be recorded.

Apparatus Diagram

The concentration chosen were selected due to calculations made using moles allowing an excess of hydrochloric acid. When there is 30cm3 of sodium thiosulphate at 1Mdm-3, there are 0.003 moles. As there is twice that amount of hydrochloric acid in the formula, there needs to be at least 0.006 moles of hydrochloric acid. To make an excess we chose 10cm3 which gives 0.01 moles.

Either of the two ...

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