RP7: Rate of reaction

• The order with respect to each reactant can only be determined through experiments
• Two experiments to find orders:

  • Continuous monitoring (1 experiment, multiple readings)
  • Initial rates (1 reading, multiple experiments)
• Continuous monitoring measures:

  • By taking samples at regular intervals
  • By using visible indicator (pH, gas volume etc)
• When taking a sample and the mixture is still reacting, quench the sample with cold water
• Quenching - Adding large volumes of distilled water to dilute and slow down a reaction
• Problem when using a physical property in CM:

  • Records the amount of product produced so volumes need to be converted to reactant concentration
• Iodine clock:

  1. Measure known volumes of reactants (keep separate)
  2. Measure a known volume of sodium thiosulfate solution
  3. Start the stop clock when the last of the reactants is added to the conical flask
  4. Stop the timer when the mixture turns blue/black and record time
  5. Repeat experiment but change potassium iodide volume
  6. Rate = 1/time
  7. Plot graph of rate against volume of potassium iodide
• Initial rates features:

  • Concentration does not need to be known
  • Total volume must be constant
  • Concentration of reactants is proportional to volume
  • Rate proportional to 1/time
• Monitoring of gas production:

  1. Measure volume of reagent and add to conical flask
  2. Set up gas syringe in the stand
  3. Weigh reagent needed to be added
  4. Add reagent to conical flask and place bung and start stopwatch
  5. Record volume of gas collected
  6. Plot volume of gas produced against time
• Continuous monitoring:

  1. Add volume of 50cm3 of each reagent together and start timer
  2. At every 30 second interval, transfer a sample from mixture to a clean conical flask
  3. Quench the reaction by adding 20cm3 of cold water
  4. Titrate sample with reagent in burette to determine concentration
  5. Plot concentration v time and draw tangents at two places
  6. Determine rate by calculating gradient
  7. Compare change in concentration to change in rate
  8. Explain orders
• Effect of temperature on an inital rate of reaction(6):

  • Use cross method
  • Put acid in beaker ontop of cross
  • Add second reactant and start timing
  • Repeat at different temperatures
  • Record time for cross to disappear
  • Volumes and concentrations kept constant
  • RATE = 1/T
  • Plot graph of rate against temperature
  • Draw graph to show the order
• Series of experiments to determine order (6):

  • Measure known volumes of reagents
  • Measure known amount of reagent to be added
  • Put reagents in separate containers
  • Start time at point of mixing
  • Time recorded for appearance of colour change
  • Use same concentration of reagents
  • Same temperature
  • Repeat with different concentrations of reagent to be tested
  • RATE = 1/time
  • Plot rate against concentration
  • Interpret graph to find order