Disappearing Cross Experiment

To plan and carry out an experiment to discover how to change the rate of reaction. The reaction being studied is between sodium thiosulphate (Na2S203) and hydrochloric acid (HCl). The reaction I am using is:

Sodium Thiosulphate + Hydrochloric acid ->

Sodium Chloride + water + sulphur dioxide + sulphur

Na2S203 (aq) + 2HCl (aq) -> 2NaCl (aq) + H20 (l) + SO2 (g) + S (s)

* Equipment

*

Diagram

* Method

) With a piece of paper put on a bold cross, which is easy to see.

2) Put the conical flask on top of the cross with 20ml³ of hydrochloric acid.

3) Add 20ml³ of sodium thiosulphate into another flask and heat to specific temperatures with the Bunsen burner (and using a thermometer).

4) Once it is at the correct temperature, add it to the hydrochloric acid and start the stopwatch.

5) As soon as you can no longer see the cross, stop the stopwatch.

6) Record the temperature of the sodium thiosulphate and how long it took to make the cross 'disappear'.

7) Go back to step two until you have collected enough results.

* Safety

Goggles should be worn during the experiment to protect your eyes from any chemicals that may splash. When using hot beakers, tongs will be used, as so not to burn hands/fingers. The hot beaker will also be placed on a heatproof mat to protect the bench.

* Fair Test

So m test is as valid as possible, I must try to make the test fair. I will try my hardest to be at the same angle each time (to view the cross) and stop the stopwatch at the same point each time (when the cross 'disappears'). I will make sure that the hydrochloric acid and sodium thiosulphate will not mix before they should be (e.g. mixing in measuring cylinders). I will make sure that the conical flask is washed out each time, as so not to prevent a reaction to take place. I will make sure I use the same amount of sodium thiosulphate and hydrochloric acid each time (20ml³).

* Prediction

I predict that as the temperature of sodium thiosulphate increases, the rate of the reaction will be faster. I predict this because of the collision theory. This is where at one temperature, the particles move at one speed and collide, but at a higher temperature, they collide more because they are moving faster.

From the above we can see that when the temperature is higher, the sodium thiosulphate (oo) is moving faster which causes it to collide with the hydrochloric acid (o) more often, causing a chemical reaction faster.

* Results

Temperature of Sodium Thiosulphate

Time taken

(Results 1)

(secs)

Time taken

(Results 2)

(secs)

Time taken

(Average)

(secs)

20°C

51.87

48.24

50.08

30°C

39.65

40.12

39.885

40°C

29.18

27.93

28.09

50°C

6.65

7.45

7.05

* Conclusion

From the results and graph above I can see that as the temperature rose, the faster the solution went cloudy. This means that the rate of reaction increased as the temperature rose as it took less time for the chemical reaction to take place. Each time I went up 10 degrees, the time went down between 10 and 15 seconds each time.

This shows that the collision theory must be taking place as said in my prediction. As the hotter the sodium thiosulphate got, the quicker the reaction, showing that collision must be happening more rapidly.

* Evaluation

My experiment went well and according to plan but there were a few errors in it like all experiments have. For example, I only obtained two sets of results and then worked out an average. I could have done more tests to gain a more reliable average.

There may have also been an error from when I stopped the stopwatch after the 'x' had disappeared. I may of pressed it different times each time causing inaccuracy. Another human era may have been when I measured the liquids. I may have been a ml³ out or two every so often.

If I was to do this again, then perhaps I could use a computer that could read light would of been more use, and more accurate. I would also make sure my measurements are much more accurate, and do the experiment more times.

Steven Alder 12/02/03

Homework / Science / Chemistry / Coursework