An Investigation into the effect that the concentration of Hydrochloric Acid has on a reaction with Sodium Thiosulphate.

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Stephen Pritchard                Chemistry Coursework                        

An Investigation into the effect that the concentration of Hydrochloric Acid has on a reaction with Sodium Thiosulphate

In this investigation I aim to see how the change in concentration of hydrochloric acid affects its rate of reaction with Sodium Thiosulphate. I will do this by measuring the time it takes for a cross to placed under a conical flask to ‘disappear’ due to a precipitate being formed by the sulphur.

The chemical equation of the reaction we are doing is:

Sodium Thiosulphate + Hydrochloric Acid → Sodium Chloride + Water + Sulphur Dioxide + Sulphur

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

Variables

Just for a reaction to take place molecules have to collide with each other with sufficient energy to take place (collision theory), there are many different things that can make the rate of reaction between chemical substances alter.

The following variables can alter the rate of reaction and so I shall have to control them.

These are my Control Variables

  • Amount and concentration of Sodium Thiosulphate – because I am varying the concentration of hydrochloric acid, if I also change the concentration of sodium thiosulphate I will not know what is causing the change in the rate of reaction.
  • The temperature of the chemicals - By heating the chemicals, you will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster because they have more energy (kinetic theory).
  • Pressure – although I will not be able to keep this the same it should not change enough to alter the rate of reaction. By changing the pressure, the molecules have more (lower pressure) or less (higher pressure) space; the frequency of collisions between the molecules will change, this is collision theory again. 

My independent variable (the variable I choose to change) is the concentration of hydrochloric acid. I have chosen to vary hydrochloric acid rather than sodium thiosulphate because the ratio of the concentration shall make it easier to draw graphs and make direct comparisons.

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The dependant variable will be the rate of reaction; this should be a direct result of the independent variable.

Safety

While doing the experiments it is important to be cautious against possible dangers. At all times a lab coat and goggles should be worn to prevent unnecessary accidents.

The sodium thiosulphate is at a very low concentration and so isn’t very dangerous but the hydrochloric acid is an irritant. If either of these reactants were spilt on skin, the affected area should be washed and the teacher should be notified immediately. If any were to ...

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