To investigate the effect on the rate of reaction when changing the concentration of sodium thiosulphate in the reaction between hydrochloric acid and sodium thiosulphate.

The concentrations I will use will be:

1. 50cm³ of pure sodium thiosulphate
2. 40cm³:10cm³ water
3. 30cm³:20cm³ water
4. 20cm³:30cm³ water
5. 10cm³:40cm³ water

Diagram

                           5cm³ Hcl

        Conical flask containing a concentration of
                                                sodium thiosulphate.  The flask is directly
                                                above an ‘X’ on a piece of paper.

Fair Test

I will to make the experiment as much of a fair test as possible by:

• Making sure that I use exactly 5cm³ Hcl each time
• The same volume of dilute sodium thiosulphate is used each time
• The right concentration of sodium thiosulphate is measured out
• Trying to do all of the experiments within a short period of time so that they are done at the same temperature, because temperature could also affect the rate of reaction.  It is necessary that all other variables are kept the same.

Safety

To ensure that the experiment is kept as safe as possible I will:

• Wear goggles

• Wash my hands after each experiment because I will be working with an acid.
• Put the flask on a flat surface and keep away from the edge.

Results

The results prove my prediction right that as the concentration of sodium thiosulphate increased, so did the rate of chemical reaction between the two substances.

Analysis of graph

The curve on the graph shows that the lower concentrations of sodium thiosulphate were slower reactions and therefore it took longer for the ‘X’ to disappear. This again proves my prediction.

Conclusion

The evidence from the results and graph prove that my prediction was correct.  The results showed a pattern of the time taken increasing as the concentration of sodium thiosulphate decreased.  The sodium chloride was produced slower which is what made the ‘X’ disappear in a larger amount of time.  The graph also proved this correct.

Evaluation

Although the experiment gave me the right results, there were certain problems that I encountered.  The one major problem was keeping all of the other variables constant.  I managed to keep the volume of dilute sodium thiosulphate and hydrochloric acid virtually the same each time but temperature was the hardest to control.  It was almost impossible to do all of the experiments at the same temperature, so I had to do them within the shortest time possible and hope that the temperature didn’t change too much.  If I was to do this experiment again I would do more tries to get a more accurate average and use more concentrations of sodium thiosulphate to get more results on the graph, but this would take more time.