An experiment to investigate the factors controlling the rate of the sodium thiosulphate / acid reaction.

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An experiment to investigate the factors controlling the rate of the sodium thiosulphate / acid reaction.

Planning

I am investigating the factors controlling the rate of thiosulphate / acid reaction. The reaction that will be taking place will follow the rate at which sulphur is formed in the reaction of sodium thiosulphate with dilute hydrochloric acid. In the experiment the sulphur will appear as an extremely fine precipitate. This will slowly be followed by a milky appearance in the reaction mixture in the conical flask. As the amount of the sulphur in the mixture increases, the precipitate will become milkier. Gradually the liquid will become more and more opaque. To investigate the rate of the reaction, we will use the cross idea. A cross will be placed under the conical flask and when it cannot be seen anymore, because the liquid is opaque, I will know that the reaction is completed. In the experiment I will change the thiosulphate concentration but I will keep the temperature and concentration of hydrochloric acid constant at all times.

        The equation for the experiment is as simple as the experiment:

Na2S2O3(aq) + 2HCL(aq)                                                S(s) + S02(g) + 2NaCl(aq) + H20(I)

        There are five factors that will affect the rate of a given reaction, however, only two of these needs to be considered in this experiment. These five factors are temperature, concentration, surface area, pressure and catalysts. Only the effect of a change in concentration and temperature will change the rate. The surface area would only be a factor in an experiment when a reagent is a solid and so the reaction had taken place between a gas/solid or a solution/solid and the two reagents are both liquids. Pressure, as a factor in an experiment would only occur if a gas were present. As the reagents used are solutions, gas pressure will not make any difference. Finally a catalyst (a substance which increases the rate of chemical reaction, but is not used up in the process) as a factor can only be considered if there is a catalyst for the reaction. As there are no catalysts for this reaction it cannot be a factor. This leaves concentration and temperature. These are the two factors that we are able to talk about.

        Firstly we must consider the effect of concentration as a variable. If I increase the number of particles in a fixed volume that are able to react in a reaction, then the probability of molecular collisions occurring will increase. As a result, any increase in concentration should also increase the rate of reaction. However the collisions that actually occur have to be effective collisions. One must realise that not every collision leads to a reaction. As a rule of thumb, if I double the concentration I will double the rate of reaction.

        

                Low Concentration                                High Concentration

Now we must investigate into the effect of temperature. At a higher temperature molecules in the reaction have a higher kinetic energy and so they can move more quickly. Therefore, the molecules must collide more frequently and the rate of reaction increases. More to the point they have greater energy. As a rule, if I increase the temperature by 10 degrees the rate of reaction will double. However this relationship will only apply to reactions with particular value of the activation energy. The activation energy is the minimum kinetic energy. However this rule does give a person a good idea of how rates change temperature. If one lowers the temperature from room temperature to 10 degrees the rate of reaction frequently halves. Then if I reduce the rate again by 10 degrees to 0 it halves the rate again. It is a reduction to one quarter of the rate at the beginning.

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                No. of particles

with given KE

                                                        Hotter substance

                                                                Higher temperature

                                Activation energy

                                Energy                        Min K.E for reaction

No of molecules with K.E > The minimum energy needed to react is directly proportional to                                                    the shaded area

By looking at the graph one can say that by looking at the shape of the curve the area surely must increase with increased temperature.

 

The Preliminary Experiment

The aim of the preliminary experiment was to find out suitable concentrations of thiosulphate one ...

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