The sodium thiosulphate will be made to a constant concentration before the experiment takes place, as will the hydrochloric acid. I will not be diluting the solution of HCl, as it will already be quite diluted so as not to be dangerous and also to control it as a variable.
For the hydrochloric acid I will use 5cm³ each time, and the volume of sodium thiosulphate will begin with 50cm³. Before each further reading, I will reduce the amount of sodium thiosulphate by 5cm³, and replace it with the same amount of water. For example, the second reading will involve the standard 5cm³ of HCl, plus a solution containing 45cm³ of sodium thiosulphate and 5cm³ of water. The third reading will involve 40cm³ of sodium thiosulphate and 10cm³ of water, and so on, until there is 30cm³ of sodium thiosulphate solution and 20cm³.
The ratios between HCl and Na2S3O2 vary at each reading, beginning with 1:10, then 1:9, then 1:8, then 1:7 and finally 1:6. These variables are incorporated into the varying of concentration of sodium thiosulphate.
At each reading I will take 3 measurements, to work out fair average results at the end. I will take a range of 5 readings, as explained before, beginning with 50cm³ of sodium thiosulphate solution and ending with a solution of 30cm³ and 20cm³ water.
I estimate that any change in temperature will have a very small effect on the results. Therefore I will disregard the matter of temperature control in this experiment as it is of little importance to my investigation.
Apparatus:
Glass flask
- Safety goggles
- 100cm³ measuring cylinder
- 10cm³ measuring cylinder
- Paper with cross X marked on it
- Sodium thiosulphate solution
- Dilute hydrochloric acid solution
- Distilled water
- Stopwatch
- Beakers
Experimental Procedure
I will put on my safety goggles before beginning measurements. I will place the flask over the paper with a black cross X marked on it in the middle. I will take the solution of sodium thiosulphate and measure out 50cm³. I will then take the solution of hydrochloric acid and measure out 5cm³. I will pour the sodium thiosulphate into the flask, and pick up the stopwatch. Upon pouring in the 5cm³ of hydrochloric acid I will begin timing the reaction between the sodium thiosulphate and the hydrochloric acid solutions. Looking down upon the flask and the solution in it, I will stop the watch when the solution turns opaque to the point when the cross X is no longer visible. I will record the time taken for the solution to turn murky, and repeat the reading twice, being sure to clean out the flask container before the next measurement.
I will then repeat the experiment, being sure to rid the flask of any trace of solution before pouring any sodium thiosulphate or hydrochloric acid into the flask, only this time reducing the amount of sodium thiosulphate by 5cm³ each time. Again I will take 3 measurements at each different reading of concentration of sodium thiosulphate. The experiment will end when I take the third reading of 30cm³ sodium thiosulphate solution, 20cm³ water.
The safety goggles are the most important aspect of the experiment, as it is crucial to maintain a high standard of safety when working with corrosive chemicals.
Diagram:
Graph to show predicted appearance of results
Method and Results
I carried the experimental investigation as stated in the Plan section, under Experimental Procedure. Having gained satisfactory results, I have arranged them in a table.
Table of Results
To draw my graph, I used the reciprocal function on my calculator on each of the averages, to gain a number that can be used to draw up a graph to show direct proportionality between 1/Average Time taken and concentration of sodium thiosulphate.
Conclusion
In terms of drawing a conclusion from my results, I can say that they compliment my prediction, as shown by the similarity in my prediction graph (a straight declining line), and also generally in the description of inverse proportionality. As anticipated, inverse proportionality is shown between the concentration of sodium thiosulphate and the time taken for the solution to turn opaque.
Na2S2O3 + 2HCl ---> 2NaCl + S + H20 + S02
Scientific Explanation
As the hydrochloric acid particles and the sodium thiosulphate particles collide into each other solid sulphur is given off and gives the solution a yellowish tinge. After a while this sulphur builds up and clouds the solution to the point when the cross beneath becomes cloaked. The particle collision theory states that the more of a particular chemical there is present in a solution (in my case sodium thiosulphate) then the more it will collide with the other particles (the particles that concern my experiment are the hydrochloric acids). Therefore, if a 100% concentrated solution of sodium thiosulphate is added to a solution of hydrochloric acid, all the particles will collide more often, and the reaction will occur quicker than a 60% sodium thiosulphate 40% water solution, because the 40% water has replaced what used to be sodium thiosulphate, thus reducing the speed of the reaction. Another point to make is the time of the reaction increased rapidly towards the end of my results.
Evaluation
The results I have obtained were of quite a high general standard, however there were a few minor anomalies. One anomaly was the second result for 30cm³ of sodium thiosulphate solution. Here is the extract from my table of results, with the anomaly shown in italics.
It stood out as being an abnormally long time taken, in comparison with the other two results, and also seen in the effect on the graph of results it has. This could be for a number of reasons. Firstly, going back to the planning section, there is the issue of temperature control, which was left aside despite being recognised as an influencing variable. If left uncontrolled then chances are that anomalies like the above will occur. I have decided not to ignore this result because the unconformity is only slight
Also, the slight inconsistencies between each group of three results are also down to the fluctuating room temperature.
These errors however, are only slight and therefore are not to be worried about in terms of being insufficient for drawing solid conclusions.
I still believe my results to be reliable, as they average out quite impressively. There are minor differences in each of the sets of three readings that were bound to occur due to uncontrolled temperature. The differences are merely fractional, and the averages worked out give a very clear picture of ideal results. There were bound to be the odd fluctuation in the length of time for the solution to turn opaque because of human error. Every time it is myself that judges when the solution has turned sufficiently opaque and I stop the watch systematically. Because my reflexes are not perfect it may seem like one measurement was longer than another, when it was only my slow reactions that distorted the result slightly. The particle collisions theory may also be to blame because if the region of the solution directly above the cross X had a larger amount of collisions, just by chance, then the reaction time would be quicker than average.
I would suggest that in further experiments, the variable of heat be included, and the pair who carries out the investigation work as a team and play different roles in the experiment. One of them should time the reaction as I did, and the other should keep the temperature as constant as possible. That way anomalous results will be almost entirely eradicated. The other two issues of human reflexes and of chance of areas of collision are unfortunately uncontrollable. Some kind of computer will surely be able to judge when a solution has turned opaque better than a human being.
To test my conclusion, I would suggest doing a similar investigation, but with magnesium strips instead of sodium thiosulphate. It will be easier to judge reaction time because one would only have to stop the watch upon seeing the magnesium completely dissolve. Also using magnesium in an experiment is a lot simpler and cheaper than using sodium thiosulphate.