practical 3

• describe a method to measure the rate of reaction of sodium thiosulfate with hydrochloric acid (disappearing cross)

  1. add sodium thiosulfate to the tube with the X underneath
  2. place a thermometer in the tube and record the initial temperature
  3. add HCl and start the timer
  4. look down the tube and stop the timer when the cross is no longer visible
  5. record the final temperature and calculate average temperature using the initial and final values
  6. repeat at four other temperatures
  7. calculate rate using 1/t and plot a grph of rate against average temperature
• what temperature must the reaction mixture not exceed?

  55C
• how would you minimise the escape of sulfur dioxide?

  use a lid (with holes to hold the thermometer and stirrer) or carry out in a fume cupboard, ensuring the room is well-ventilated
• what is a stop bath and why is it used?

  contains sodium carbonate solution and phenolphthalein to add product mixture to in order to neutralise the sulfur dioxide

  once the colour changes, the sodium carbonate has been used up and will need to be replenished
• what is the equation for the reaction between hydrochloric acid and sodium thiosulfate?

  hydrochloric acid + sodium thiosulfate -> sodium chloride + sulfur dioxide + sulfur + water

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

  = white ppt of sulfur is responsible for the cloudy solution that causes the cross to disappear