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
• Iodine clock reaction

  Equations are

  • H2O2(aq) + 2H+ (aq) + 2I- (aq) → I2(aq) + 2H2O(l)
  • 2S2O3 2– (aq) + I2(aq) → 2I- (aq) + S4O6 2– (aq)
  • The I2 produced reacts with all of the thiosulfate ions present. Excess I2 remains in solution which then reacts with starch to form a blue-black solution
• Iodine clock reaction method

  1. Rinse and fill a burette with potassium iodide solution.
  2. Transfer 10cm^3 hydrogen peroxide solution from a burette to beaker
  3. Use measuring cylinder to add 25 cm3 of sulfuric acid to a different beaker.
  4. Use measuring cylinder to add 20 cm3 of distilled water to the same beaker.
  5. Use a dropping pipette to add about 1 cm3 of starch solution to the same beaker
  6. Use the burette to add 5.0 cm3 of potassium iodide solution to the mixture in the beaker.
• Iodine clock reaction method

  1. Finally, add 5.0 cm3 of sodium thiosulfate solution from a burette to the mixture in the beaker
  2. Stir the mixture in the beaker. Pour the hydrogen peroxide solution beaker into beaker and immediately start the timer.
  3. Stop the timer when the mixture in beaker turns blue-black
  4. Rinse the 250 cm3 beaker with distilled water and dry it with apaper towel.
  5. Repeat steps in four further experiments changing the concentration of potassium iodide.
  6. Plot a graph of initial rate (y) versus concentration (x) to determine the order.