7 - MEASURING RATE OF REACTION

• REASONS TO MEASURE RATE OF REACTION?

  • find orders with respect to each reagent and rate equation
  • calculate activation energy
• TWO METHODS?

  initial rate method, continuous rate method
• INITIAL RATE METHOD?

  ex. H2SO4 + S2O3 (2-) -> S (s) + SO2 + SO4 (2-) + H2O

  1. Measure 50 cm^3 Na2S2O3 into conical flask
  2. Place on laminated cross
  3. Measure 5 cm^3 H2SO4 into measuring cylinder
  4. Add acid to beaker and immediately start timer
  5. Swirl flask, stop timer when cross completely disappears
  6. Repeat with different concentrations of Na2S2O3 and H2SO4
  7. For each reagent, plot graph of initial rate (y-axis) against concentration (x-axis) to determine the order
• IMPROVEMENT TO INITIAL RATE METHOD?

  colorimeter to reduce human error
• CONTINUOUS RATE METHOD?

  1. Colorimeter: measuring time vs absorbance, absorbance is proportional to concentration of iodine
  2. Quenching: aliquots of reaction mixture sampled at different times, quenched to stop reaction (cooling, diluting, neutralising) then titrated against
  3. Gas syringe: volume of gas produced
• CONCENTRATION-TIME GRAPHS?

  0th order: straight line

  1st order: slight curve, almost straight

  2nd order: curve
• RATE-CONCENTRATION GRAPHS?

  0th order: straight, horizontal line

  1st order: straight line from origin (concentration on x-axis)

  2nd order: straight line from origin (concentration^2 on x-axis)