Rates - Thiosulfate (OCR A Level Chemistry A): Revision Note

PAG 10.2: Rates – Thiosulfate & acid

The disappearing cross experiment

• The disappearing cross experiment is a simple way to investigate how concentration affects the rate of reaction

• It can be used with any reaction that produces a solid, as this makes the solution cloudy

  • A common example is:

Na₂S₂O₃ (aq) + 2HCl (aq)  → 2NaCl (aq) + H₂O (l) + SO₂ (g) + S (s)

• In this reaction, solid sulfur forms and turns the solution opaque

• You can investigate two key factors:

  • Concentration of either the acid or sodium thiosulfate

  • Temperature (OCR often use this version to calculate activation energy)

Method

1. Measure 50.0 cm³ of sodium thiosulfate solution into a conical flask

2. Place the conical flask on the black cross

3. Measure 5.0 cm³ of dilute hydrochloric into a measuring cylinder

4. Add the hydrochloric acid to the conical flask and immediately start the stopwatch

5. Swirl the mixture

6. Stop the stopwatch when the cross disappears and record the time

7. Repeat the experiment for a minimum of four more / different concentrations

Dilution of sodium thiosulfate solution table

• It is likely that you would be given a known concentration of sodium thiosulfate solution

  • Typically 0.25 mol dm³

• From this you will be 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

Specimen results

• Results may be shown using either:

  • Relative dilution of the sodium thiosulfate solution

  • Actual concentrations (mol dm⁻³) 

Analysis

• These results can be analysed in 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

Practical skills reminder

• This practical develops key skills in monitoring rate of reaction using a disappearing visual cue.

• It also supports:

  • Measuring fixed volumes and mixing reagents accurately using a conical flask and measuring cylinders

  • Timing a visible endpoint using a stopwatch

  • Drawing and interpreting graphs to identify reaction order

• Understanding the link between concentration, temperature, and rate