PAG 10.2: Rates – Thiosulfate & Acid
The Disappearing Cross Experiment
- A simple experiment which can be done to determine how the rate of reaction is affected by concentration is the disappearing cross experiment
- This experiment can be done for a number of different reactions, but the following reaction is commonly used:
Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + SO2 (g) + S (s)
- In this reaction, sodium thiosulphate reacts with hydrochloric acid
- The key product in this experiment is the solid sulfur which causes the solution to become opaque
- There are two factors which can be easily investigated using the disappearing cross reaction:
- Changing the concentration - this can be for the hydrochloric acid or the sodium thiosulfate solution
- Changing the temperature (OCR tend to use this one to calculate the activation energy of a reaction)
Examiner Tip
- The disappearing cross experiment can be used for any reaction where a solid is produced as one of the products, as this will cause the solution to become cloudy.
- It does not have to be done in a conical flask, it could be done in a test tube with the cross placed underneath / behind the test tube.
- Careful: This is a very simple experiment, but exam questions can take it in many different directions from a relatively straight forward rate calculation to the more complex Arrhenius plot and equation to determine activation energy
Method
- Measure 50.0 cm3 of sodium thiosulfate solution into a conical flask
- Place the conical flask on the black cross
- Measure 5.0 cm3 of dilute hydrochloric into a measuring cylinder
- Add the hydrochloric acid to the conical flask and immediately start the stopwatch
- Swirl the mixture
- Stop the stopwatch when the cross disappears and record the time
- Repeat the experiment for a minimum of four more / different concentrations
Dilution of sodium thiosulfate solution table
| Volume of sodium thiosulfate (cm3) | Volume of water (cm3) | Relative dilution |
| 10 | 40 | 0.2 |
| 20 | 30 | 0.4 |
| 30 | 20 | 0.6 |
| 40 | 10 | 0.8 |
| 50 | 0 | 1.0 |
- It is more likely that you would be given a known concentration of sodium thiosulfate solution, typically 0.25 mol dm3, and asked to create different concentrations by diluting that solution
- Careful: The process is the same as outlined in the table above where the total volume must remain constant when diluting
Diagram showing the apparatus needed to investigate reaction rate in the disappearing cross experiment
Specimen Results
- Results could be expressed in terms of relative dilution of the sodium thiosulfate solution (less common) or in terms of concentration of the sodium thiosulfate solution
| Relative dilution of sodium thiosulfate solution | Time for cross to disappear (s) |
| 0.2 | 115.2 |
| 0.4 | 57.6 |
| 0.6 | 30.0 |
| 0.8 | 15.6 |
| 1.0 | 7.2 |
| Concentration of sodium thiosulfate solution (mol dm–3) | Time for cross to disappear (s) |
| 0.05 | 115.2 |
| 0.10 | 57.6 |
| 0.15 | 30.0 |
| 0.20 | 15.6 |
| 0.25 | 7.2 |
Analysis
- These results could be used in a number of different ways:
-
- Plot a graph of the results with the concentration of sodium thiosulfate on the x-axis and time on the y-axis
- Comment on the effect of changing concentration (this is more GCSE level)
- Increasing the concentration of the sodium thiosulfate solution increases the rate of reaction
- This means that the time for the cross to disappear decreases
- Use the graph to determine the order of the reaction with respect to sodium thiosulfate
- The shape of the plotted graph will indicate if the reaction is 0, 1 or 2 order
- If this reaction was performed at different temperatures instead of with different concentrations, then the results could be manipulated, plotted and used to calculate the activation energy for this reaction
