An experiment to investigate the effect of changing the concentration of Sodium Thiosulphate in the reaction between Hydrochloric Acid and Sodium Thiosulphate.

                                1 small beaker (for Hydrochloric Acid)

                                2 small measuring cylinders

                                1 stop clock

Prediction

• I predict that the higher the concentration of Sodium Thiosulphate in the Sodium Thiosulphate – water solution, the faster the reaction will be.

• A chemical reaction can only occur between particles
  when they collide (hit each other).
  (Particles may be atoms, ions or molecules.)

• If you have a situation involving two molecules (or ions e.t.c) they can only react together if they come into contact with each other. They first have to collide, and then they may react.

• However, It isn't enough for the two molecules to collide - they have to collide the right way around, and they have to collide with enough energy for bonds to break (activation energy).

• Even if the molecules are orientated properly, you still won't get a reaction unless the particles collide with a certain minimum energy called the activation energy of the reaction.

• Activation energy is the minimum energy required before a reaction can occur. You can show this on a graph for the reaction. For example it could look like this:

• Obviously, the more of each substance you have the more likely it is a collision will occur. Therefore if there is more of each substance you will have more collisions in a certain amount of time, which will give you an improved rate of reaction.

Fair Test

I will need to control:

1. Temperature – at room temperature, check with a thermometer.
2. Volume of solution – Total volume (water and Sodium Thiosulphate) =30ml
3. Volume of Acid – Volume of Hydrochloric Acid = 20ml
4. Stirring – no

If any of these changes to a greater or lower quantity they could speed up, or slow down the reaction, invalidating the results.

Number and range of values

Number:        six different values

Range:        change concentrations by 5ml each time

e.g.:                1)         5ml Sodium Thiosulphate, 25ml water

                2)        10ml Sodium Thiosulphate, 20ml water

Accurate Evidence

I will use the smallest plausible measuring devices.

1. Small scale measuring cylinders,
2. Pipettes,
3. Distilled water bottles,
4. Stop clock,
5. Burette.

Reliable Evidence

To obtain reliable evidence I will repeat my investigation after having obtained my results.  When I have more than one set of results I cam compare them.  If they are very similar or not will allow me to decide if my experiment was reliable or not, and if not, how to improve it.

Trial Results

                

I used my trial results to help me decide what values to use.  Part one contains results that I decided not to use.  5 ml sodium and 15 ml water was an acceptable result but 7ml sodium and 13 ml water was not different enough to use those two as the range.  10 ml sodium and 10 ml water was different enough, but with 5 ml difference each time a total solution of 20 ml would not give me enough variables, e.g. only four. I then decided (see part two) that I would have 30 ml of solution with a range of 5 ml.  This gave me a high enough range as well as enough variables (six).                                                          

Results.

                                                                      Time for soln. to go cloudy           

Conclusion

In conclusion, I can see that the higher the concentration of Sodium Thiosulphate, the lower the time the reaction will take.

This can also be expressed as, the higher the concentration of Sodium Thiosulphate, the higher the rate of reaction.

This shows that my hypothesis was correct, as when the Concentration of Sodium Thiosulphate increases, the reaction is quicker/rate is longer.

We can see proof of this in the fact that on the graphs when the Concentration increases the average reaction time decreases.

In addition, as the Concentration increases the rate of reaction increases.

Another thing that these results show is that the time the reaction takes is in inverse proportion to the rate of reaction.

Evaluation

Anomalous results

I have only one anomalous result. It is the same on both of my graphs( and is circled on them ).This is the 2nd result that I took, but all of my other results are accurate.

Accuracy  

As I only have one anomalous result I believe that my results are fairly accurate. I think I got accurate results because I used very accurate measuring devices, e.g.:

• Burette for Sodium Thiosulphate,

• Burette for Hydrochloric Acid,

• 10 ml measuring cylinders for water,

• Stop clock for timing,

• Calculator for calculating results.

However, I did get one anomalous result that shows that there is room for improvement. This could have been caused by something simple like mistakes with measuring.

Alternatively, it could also have been caused by other factors. These could include:

• At a point in the investigation the supply of Hydrochloric acid ran out. While another batch was quickly prepared, this meant that the new supply would not be of exactly the same concentration. This is unlikely to upset the balance of the experiment in a measurable way, but it could.

• Another possibility exists. During the experiment a white precipitate built up in the conical flask used for the reaction. Despite that the flasks were cleaned after each attempt, normal washing did not remove this. Instead, scouring was necessary. At first I did not notice this, as I had not been looking for it. When I did notice it I was able to remove the precipitate but it might have influenced the end result.  

On my results table it can be seen that I did 2nd attempts on only half of my results. This is because I did not have enough remaining time or materials to repeat all of the results, and was unable to identify anomalous results from my graph. I estimated that I would only have enough left for half of my repeats. Therefore, instead of only doing the 1st half I thought it would be more useful to repeat every other result to get a more accurate overall image.

Appropriateness of equipment and procedure.

I think that the equipment and procedure are both appropriate to the investigation, as they are safe, reliable and accurate.

Improving the investigation.

If I were to do the experiment again there are only two things I think I would change:

• I would use a smaller volume of Hydrochloric Acid throughout, as this would allow me to repeat all of my results again.

• I would also attempt to work slightly faster as this would give me time to finish all repeats.

Both of these improvements aim not to improve the actual experiment , but just it’s accuracy.

Expanding the investigation

How.

I can only think of one way to improve the investigation and that is too do a similar experiment with just one change. Instead of concentration of Sodium Thiosulphate the controlled variable would be the temperature of the solution.

Why.

This would allow us to measure how temperature affects reaction rate, with the hypothesis being that the higher the temperature the higher the rate of reaction.

If compared with the results of our investigation we could also see which affects the rate more out of these variables.

Bibliography

Key science (Chemistry) - by Eileen Ramsden.

Internet-Yahoo search engine.