Rate of reaction

• Disappearing cross with sodium thiosulphate and HCl


  The reaction is S​​2O3​ + 2H+ → SO​2(g)​ + S(s) + H​2O (l)

  1. Set up a Bunsen burner, tripod, gauze and heatproof mat
  2. Label two boiling tubes A and B and place them in a beaker of water
  3. Mark an x on the side of a beaker with a marker
  4. A would be the aqueous sodium thiosulphate and B is the HCl
  5. Place a thermometer in the water and in tube A
  6. Heat temperature of water using a Bunsen burner to 20°C
  7. Pour B into A and measure the temperature once you cant see the cross and record results
  8. Repeat 2-7 but increase temp by 10°C
• What are the two methods to measure the rate of reaction mentioned?

  • Initial rates method (iodine clock reaction)
  • Continuous monitoring method (gas volume measurement)
• What is a potential issue with judging the disappeared cross method?

  • It can be unreliable due to human judgment e.g. eyes are subjective or too slow when stopping the clock watch
  • Reaction may be too slow at low temperatures
  • There may not be enough precipitate of sulfur forming to make the cross disappear