If the concentration of Sodium Thiosulphate is diluted, less Sodium Thiosulphate ions are present, causing fewer reactant particles to be present and therefore reducing the number of collisions with Hydrochloric Acid ions. In addition, there will be an increase in the number water ions. Water ions will have no effect on the formation of the precipitate. The result will be a slower time of reaction for precipitate formation allowing the pencilled ‘X’ to stay visible for a longer period of time.
The charges on the ions are given below:
Na2 1+ S2O3 2- H1+ Cl 1-
Therefore, the balanced equation for the reaction is:
Na2S2O3 + 2HCl → S + 2NaCl + SO2 + H2O
Sodium Thiosulphate plus Hydrochloric Acid gives Sulphur, plus Sodium Chloride, plus Sulphur Dioxide, plus water.
Equipment
3 small beakers (100ml)
3 measuring cylinders
3 conical flasks
Stop watch
White piece of paper with an ‘X’ drawn on it
2 labels saying ‘HCl’
2 labels saying ‘Na2S2O3’
2 labels saying ‘H2O’
Solutions of HCl
Solutions of Na2S2O3
De-ionised water
Method
- Assemble equipment and prepare a plan to conduct the experiment in a safe fashion –
- Ensure equipment is safely placed on the work table
- Ensure safety and personal equipment is used
- Ensure proper procedures are followed to form the concentrations of Sodium Thiosulphate.
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Ensure that during the experiment that the Hydrochloric Acid is added to the water – not the other way.
- Rinse well with de-ionised water, all the beakers, measuring cylinders and the conical flask.
- Ensure the conical flask is dry before the start of the experiment.
- Fix labels to the measuring cylinders and beakers.
(the 3 measuring cylinders and the 3 beakers will contain the different solutions – Sodium Thiosulphate, Hydrochloric Acid and de-ionised water)
- Rinse the measuring cylinders and beakers with their proposed solution.
- Take approximately 100ml of each solution and place in the correct beaker.
- Draw an ‘X’ in pencil on a white piece of paper.
- Accurately measure the following concentrations of solutions in the measuring cylinder ensuring the meniscus curve is on the correct mark. (Put the measuring cylinder on a steady, smooth surface at eye-level to guarantee accurate measurements.)
- Put the conical flask on top of the white piece of paper with the pencilled ‘X’.
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The experiment can now be carried out:
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Put the water into the conical flask, followed by the Hydrochloric Acid, then the Na2S2O3 and start the stopwatch. Mix the solution by ‘swirling’ the conical flask in a uniform manner for a set number of times or a set duration. Ensure that the flask is set on the marked paper immediately after mixing. A precipitate will begin to form in the flask.
- Measure the time taken for the precipitate to be produced so that the ‘X’ can no longer be seen.
- Record the time taken in a table. (An example of the table is shown below.)
- Repeat the experiment 3 times.
- Take an average of the time taken and plot a graph of best fit.
Diagram
Graph
To obtain a graph going through the origin, the reciprocal of time (1/time) must be plotted on the Y-axis. (Plotting time will give a curve of best fit.) The percentage of concentration of Sodium Thiosulphate should be plotted on the X-axis.
Results (Physical Results)
Results (Processed Results)
Percentage Na2S2O3 = concentration of Na2S2O3
Total of water and Na2S2O3
1/time = 1
Average time taken
Conclusion
It is concluded that:
The greater the concentration of Sodium Thiosulphate the faster the reaction time.
100% Na2S2O3 took 14.4 x10 –3 seconds to obscure the ‘X’ ;while
30% Na2S2O3 took 2.7 x10 –3 seconds.
The speed of the reaction is inversely proportional to the time taken for the reaction to finish.
The graph clearly illustrates that as the concentration of Sodium Thiosulphate is increased, the time taken for the ‘X’ to be obscured, ie the precipitate to be formed, reduces.
This conclusion concurs with the prediction that a faster reaction will be obtained using greater concentrations of Sodium Thiosulphate. In addition the experiment also follows particle theory in that as more ions are present in a solution, the probability of collision with particles of another solution also increases, thus aiding the chemical reaction and speeding up the process. The greater the concentration of Sodium Thiosulphate, the greater the number of reactive ions are present in the solution to react with the Hydrochloric Acid ions.
The precipitate is formed much faster which reduces visibility through the flask and reduces the time for the ‘X’ to be obscured from sight.
The experiment is robust and the conclusions are valid. The concentrations of Sodium Thiosulphate are sufficiently accurate to represent a reasonable demonstration of the reaction with Hydrochloric Acid. The range of concentrations chosen allow for a broad experimentation method and to draw valid conclusions.
Although there were two anomalies in the results – two points on the graph are outside the ‘line of correlation’ - they are not significant and follow the general pattern of the graph. The anomalies may be explained as two results found in the lower concentrations of Sodium Thiosulphate likely to produce fewer ions for the reaction and hence reduce the probability of collision with the Hydrochloric Acid ions, producing a slightly slower time for reaction.
The graph of the experiment confirms that the ‘X’ was obscured from vision faster by the precipitate at the greater concentrations of Sodium Thiosulphate.
Thus is it concluded that:
The greater the concentration of Sodium Thiosulphate, the faster the reaction with Hydrochloric Acid.
Evaluation
The experiment was a very reasonable demonstration of the reaction between Sodium Thiosulphate and Hydrochloric acid, and the conclusions are valid.
However, there are a number of points that may affect the accuracy of the results and that could be changed in a repetition of the experiment:
- A burette could be used instead of a measuring cylinder for the containing the solutions. A burette would enable a more accurate reading to be obtained.
- A light intensity instrument could be used to determine the time taken for the ‘X’ to be obscured. In that way, the subjective assessment by eye would be removed and the results made more accurate.
- Whilst the experiment was carried out at room temperature a minor fluctuation may have occurred that could affect the accuracy of the results. Repetition at a known fixed temperature would ensure more accurate results and remove any temperature variables.
- Mixing of the solution was carried out by hand simply swirling the conical flask. This may be effective, but it is largely incapable of being consistent and measurable. A magnetic stirrer, allowing only a fixed and consistent mixing operation to take place would allow more accurate results and remove any subjectivity.