This experiment's title is: "investigate how the concentration of Sodium Thiosulphate affects the rate of reaction between Sodium Thiosulphate and Hydrochloric Acid".

3.3.2   Theoretical Explanation

• The gradual clouding of the mixture, which causes the “X” to disappear, is actually caused by the release of sulphur as a product of the reaction.  

• Since, when the X disappears it is because a certain amount of Sulphur has been produced it means that if the X takes longer to disappear, the reaction is slower. Therefore, we can use the following formulas to describe this:

                                                        R = rate of reaction

                                                       X = amount of sulphur

• The above formulas help to prove my prediction, which is: the time taken for the X to disappear will decrease as the concentration of the Sodium Thiosulphate increases accordingly.  This means that the rate of the reaction will increase as the concentration increases.  Therefore, I believe that the rate of the reaction is proportional to the reciprocal of the time taken for the X to disappear.  

• This can be explained by using the ‘Collision Theory’.  The collision theory states that: if there are more particles of a fixed amount of substance (a larger concentration), the particles are more likely to collide with each other than they would be in a more dilute (less concentrated) solution. This is shown in the below diagram:

In the first box, there are less particles of Sodium Thiosulphate to collide with the particles of Hydrochloric Acid.  In the second box, there are more particles, so the chance of collision is increased therefore the rate of reaction also increases.

3.4   Experimental Procedure

3.4.1   Apparatus

The apparatus required for this investigation is listed below:

• Sodium Thiosulphate
• Hydrochloric Acid
• Sheet of Paper with  letter ‘X’ on it
• 4 Conical Flasks / Beakers
• Graduated Cylinders
• Stopwatch

The below diagram shows all of this apparatus set-up:

3.4.2   Method

• Measure out 50cm3 of 80g/dm3 concentrated Sodium Thiosulphate and  pour into one of the conical flasks, and place it over the sheet of paper with  “X” on it.

• Add 10cm3 of Hydrochloric Acid.

• Time how long it takes for the X to disappear whilst looking down at it through the solution.  

• Repeat above steps at least twice in order to obtain accurate results.

• Repeat above four steps for each of the following concentrations of Sodium Thiosulphate: 5g/dm3 , 10 g/dm3, 20 g/dm3, 40 g/dm3, 80 g/dm3.

3.4.3   Fair Testing

• Numerous results should be taken for each concentration in order to obtain the most accurate results possible.

• In addition to this all of the controlled variables mentioned earlier must be kept constant in order to obtain fair results each time the experiment is preformed.

3.5   Results

2. 3.5.1   Range of Readings

• We performed a preliminary experiment, in order to decide upon a range of results. The range of readings that we decided upon are shown in the following table:

• In order to make the Sodium Thiosulphate more dilute measured amounts of water will be added to it (the amounts are shown in the table above) before the HCl is added.

• For smaller volumes of Sodium Thiosulphate, we will double the amount measured out.  So, instead of 50cm3 of Sodium Thiosulphate and water solution being measured, 100cm3 will be measured out. This is so that we can measure the Sodium Thiosulphate accurately.

3.5.2   Repeat Readings

• To ensure fair and accurate results, I will take two separate readings for each concentration of Sodium Thiosulphate, an average will then be calculated and placed in the final result table..

• If the two readings vary by a substantial amount of time (possibly due to human error) . I will take a third reading and discard the most inaccurate result.

3.5.3   Processing Results

• The table into which I will place my results is shown below:

• I will also plot a scatter graph with a best – fit line of the concentrations of the Sodium Thiosulphate against the average times for the X to ‘disappear’.  Below, is an example of what the graph may look like:

3. Obtaining Evidence

1. 3.1   Result Table

3. 3.2   Graphs

• On the next two pages are the graphs I created on “Microsoft Excel”.

• On the second graph , the best-fit line should go through the origin, because if there is no concentration of Sodium Thiosulphate  the X will take infinite time to disappear and the reciprocal of infinite time is no time

4.   Interpretation and Evaluation

4. 4.1   Result Processing

• The table in 3.1 is where my raw data was placed. It is from here that all of my other results where calculated and this data was also used to plot my two graphs.

• The graphs were created to give a visual representation of my results:

1. Concentration of sodium thiosulphate against average time for the X to disappear. This graph shows that as the concentration increased the time decreased.

2. Concentration of sodium thiosulphate against the reciprocal of the average time for the X to disappear. This graph proves the formula I stated in my theoretical explanation, ‘rate of reaction is proportional to the reciprocal of time’.

5. 4.2   Conclusion

• I can now draw the conclusion, that the time taken for the ‘X’ to disappear decreases as the concentration of the Sodium Thiosulphate increases.  Therefore, the rate of the reaction increases as the concentration of the sodium thiosulphate increases.

• Therefore, this means that the rate of the reaction is proportional to the reciprocal of the time taken for the X to disappear:

This is graphically represented by the graph: “Concentration Against Reciprocal Of Time”

7. 4.3   Hypothesis and Results

• My hypothesis was “as the concentration of the Sodium Thiosulphate (Na2S203) increases, the amount of time taken for the ‘X’ to vanish behind the cloudy solution will decrease. Therefore, this means that the rate of the reaction between the two chemicals will have increased”.

• From my result table and from graph 1, I can see that the results confirm my hypothesis.  The points follow a negative curved shape; this shows that time taken for the ‘X’ to disappear does decrease as the concentration of the sodium thiosulphate increases.

8. 4.4   Theoretical Explanation of Conclusion

• The rate of reaction increases as the concentration increases because as the concentration increases, there is more of a chance of the particles of the Sodium Thiosulphate and the Hydrochloric Acid colliding, this is also know as the collision theory. It states that: “if there are more particles of a fixed amount of substance (a larger concentration), the particles are more likely to collide with each other than they would be in a more dilute (less concentrated) solution. This is shown in the below diagram:

In the first box, there are less particles of Sodium Thiosulphate to collide with the particles of Hydrochloric Acid.  In the second box, there are more particles, so the chance of collision is increased therefore the rate of reaction also increases.

9. 4.5   Anomalous and Inaccurate Results

• In graph 1, all the results appear to be quite close or on the best-fit line, so there are no real anomalous or inaccurate results.

• In graph 2, all of the results seem reasonably close to the trend –line apart from the result for 40g/dm3 of Sodium Thiosulphate. This result is significantly above the trend line. This is probably due to human error, possibly the time was recorded incorrectly or the measurements were not very accurate.

• The line in graph 2 should go through the origin. However it does not appear to do this. This is probably due to the inaccurate 40g/dm3 result, which, could of altered the line of best fit by a significant enough margin to make it miss the origin.

10. 4.6   Future Improvements

1. 4.6.1   Experimental Procedure

• Throughout the experiment, we found it extremely difficult to measure the smaller volumes of the liquids. This meant that the measurements were not entirely accurate, hence the final results were not as accurate as they could have been. In order to improve this, we could have used more sophisticated apparatus to measure the volumes and to time how long the “ x” takes to disappear. For example: a syringe, or other more accurate apparatus to measure the smaller  volumes.

• We could also have chosen an easier dependent variable to measure. This is because it was nearly impossible to determine exactly when the “x” disappeared using the human eye. Either this or a more sophisticated system of determining when the “x” disappeared such as a camcorder fitted to a pc could have been used.

4.6.2   Execution of Experimental Procedure

• In my opinion, I feel that under the circumstances and restraints of the simple equipment used the experiment was preformed extremely well. Using a larger volume of the solution than required as we did made it far easier to measure out the smaller volumes of Sodium Thiosulphate therefore enabling us to obtain more accurate results. Overall, as there was only one major anomaly over both of the graphs I feel that our experiment was a success.