RP7

• Initial rate method


  1. Measure 50 cm3 of sodium thiosulfate into a conical flask and place on a laminated cross

  2. Measure 5 cm3 of sulfuric acid into small measuring cylinder

  3. Add the acid to the beaker and start the timer, swirl the flask and stop the timer when the cross completely disappears

  4. Repeat the experiment varying the concentration of sodium thiosulfate and then the concentration of sulfuric acid

  5. For each reagent plot a graph of initial rate (y) versus concentration (x) to determine the order with respect to each reagent
• The reasons for practically measuring the rate of a reaction are: a. to find the orders with respect to each reagent and hence the rate equation, b. to calculate the activation energy of a reaction
• Initial rate method


  Only the very beginning of a reaction is measured
• Example reaction of initial rates

  • H2SO4 (aq) + S2O3 2- (aq) S (s) + SO2 (g) + SO4 2- (aq) +H2O (l)
  • Sulphur ppt which obscures cross over white tile (time taken to do so recorded)
• Continuous rate method


  1. The reaction is followed for a significant length of time

  2. A concentration (of reactant) time graph may be plotted

  3. A tangent can be drawn at any point during the reaction, and the gradient is equal to the rate of reaction (with respect to the reactant being monitored)

  4. A rate-concentration graph may then be plotted to identify the order with respect to any reagent
• How the continuous rate method reaction can be followed

  • Colorimeter: if any of the reagents or products are coloured (usually iodine) can follow time vs absorbance- abs proportional to conc. of iodine but calibration graph is required to determine exact conc.
  • Quenching: Aliquots of a reaction mixture can be sampled at different times- they are quenched by either diluting, cooling or neutralising the acid/base catalyst. The aliquots are titrated against to work out the concentration of the reagent present.
  • Gas syringe: The volume of gas produced
• Important graphs to interpret

  • Concentration-time graphs:
  • 0th order: negative gradient, straight line
  • 1st order: negative curve
  • 2nd order: negative curve, steeper than 1st order
  • Rate-concentration graphs:
  • 0th: straight line where x=0
  • 1st order: straight line through the ORIGIN
  • 2nd order: Curve from origin
• Exam question: Measuring rate of reaction by continuous method

  1. Display the results in a suitable graph

  2. Circle an additional point

  3. Draw a suitable line of best fit

  4. Calculate the rate of reaction at 30 seconds
• Exam question: Using results to calculate activation energy

  1. Work out the rate equation

  2. Calculate k for all experiments with different temperatures

  3. Calculate 1/T and lnk for use with the rearranged Arrhenius equation

  4. Plot a graph of lnk versus 1/T and use the gradient to calculate activation energy