Rates of reaction of Sodium Thiosulphate with Hydrochloric Acid

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Chemistry Coursework

Rates of reaction of

Sodium Thiosulphate with Hydrochloric Acid

Michela Nardini


Sodium thiosulphate and hydrochloric acid reaction rates.


To investigate how altering the concentration of sodium thiosulphate affects the time taken for the reaction with hydrochloric acid.


In a reaction of hydrochloric acid and sodium thiosulphate my prediction would be as follows: with the concentration of hydrochloric acid remaining constant, as the volume of sodium thiosulphate decreases and the volume of water increases the reaction time will increase. One would not expect the halving of the concentration of sodium thiosulphate to result in a doubling of the reaction time as volume is three dimensional and not linear. I also predict that there will be a precipitation of solid sulphur.

From my research done, sodium thiosulpate and hydrochloric acid will react as in the equation below:

Na2S2O3(aq) + 2HCl(aq)                       2NaCl(aq) + S(s) + SO2(g) +H2O(l)


This chemical balance equation explains that sodium thiosulphate, in an aqueous solution, when mixed and allowed to react with hydrochloric acid, in an aqueous solution, will form sodium chloride in an aqueous solution, solid sulphur together with sulphur dioxide gas and water. The solid sulphur which forms in the solution causes the water to look cloudy. It is interesting to note that sodium chloride is ordinary table salt. The sulphur dioxide gas, which is formed, smells like rotten eggs.

 To explain the reaction rates I will use collision theory:

This theory says that the rate of reaction simply depends upon how often and how hard the reacting particles collide with each other. The basic idea is that particles have to collide in order to react, and they have to collide hard enough. More collisions increase the rate of reaction. Four methods of increasing the rate of reaction can be explained in terms of increasing the number of collisions between the reacting particles: 1) Temperature increases will cause the particles to move more quickly which in turn will cause more collisions. 2) Increasing concentration of reactants means that there are more particles of reactant knocking about between the water molecules, which makes collisions between the important particles more likely. 3) Smaller size of solid particles of a reactant will increase the surface area, which means that particles will have a larger area to work on and there will be more useful collisions. 4) The presence of a catalyst is used to increase the number of collisions.

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In the reaction of sodium thiosulphate and hydrochloric acid temperature should have been kept constant, no catalyst was added to the solution, no solid particles were present and only the concentration of the solution was changed.

In the reaction of sodium thiosulphate and hydrochloric acid, as the concentration of sodium thiosulphate is decreased, by adding more water to form 50cm3 of solution, there are fewer particles of reactant to collide and react with each other. Collisions will happen less often and they would have less energy because they have further to travel. Some particles will collide and will bounce ...

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