Investigating rate of reaction

• Clock reaction

  more convenient way of obtaining the initial rate of reaction by taking a single measurement
• assumption of clock reaction

  no significant change in the rate of reaction between the start of the reaction and the time when the measurement is taken

  • initial rate is proportional to 1/ t
• clock reaction is repeated several times with different concentrations
• aqueous iodine is an orange brown colour
• iodine clock reaction

  use potassium iodide to obtain the iodine which is colourless
• Iodine clock reaction

  the solution is colourless at the start and the time t is measured for the blue-black colour of the starch-iodine to appear
• iodine clock reaction

  plot graph of 1/t against concentration
• iodine clock


  • add hydrogen peroxide to form iodine from the potassium iodide
  • colour change is then delayed by adding sodium thiosulfate to the reaction as it uses and removes the iodine (acts as reaction clock)
  • once the chemical is all used up the blue-black colour appears
• iodine clock

  the iodine produced is in excess to all the other reagents
• iodine clock


  H2O2 (aq) + 2I- (aq) + 2H+ (aq) —> I2 (aq) + 2H2O (l)
• iodine clock reaction

  plot a table of concentration of H2O2 with time for colour to appear and 1/t
• 1/t = s-1
• iodine clock reaction

  first order for H2O2

  first order for I-

  zero order for H+
• measuring rate

  measuring the loss of mass of a reactant over time

  measuring the volume of gas formed over time
• colour change

  monitoring rate with a colorimeter
• colorimeter


  wavelength of light passes through a coloured solution, controlled using a filter

  • amount of light absorbed by the solution is measured
• iodination of propanone

  rate of reaction can be measured throughout the reaction by using a colorimeter

  • iodine decolourises
• iodination of propanone


  CH3COCH3 (aq) + I2 (aq) —> CH3COCH2I (aq) + H+ (aq) + I- (aq)
• iodination of propanone

  -necessary to measure absorbance of a set of known concentration (standard solution) of iodine and obtain a calibration curve

  -carry reaction and take absorbance readings of the reaction mixture at measure time intervals

  -use calibration curve to measure the concentration of iodine at each absorbance

  -plot graph of concentration of iodine against time
• iodination of propanone

  select a filter that gives the strongest absorbance for the solution you are using

  • complementary colour of the solution under investigation