Rate equations

• Rate of reaction - change in concentration per unit of time
• Activation energy - The minimum amount of energy needed for a reaction to take place.
• High temperature:

  • Increases kinetic energy of molecules
  • Increased frequency of collisions
  • Energy is greater than or equal to activation energy
• High concentration/pressure:

  • More particles in a given space
  • Higher chance of successful collisions
  • Energy is greater than or equal to activation energy
• Higher surface area:

  • Larger surface area
  • Higher rate of successful collisions
• Catalyst - A substance that increases the rate of a chemical reaction without being used up in the reaction
• Catalyst:

  • Provides an alternate reaction route
  • Lower activation energy
• High rate of reaction at start but decreases:

  • More reactants at the start
  • Higher chance of successful collisions
  • Energy is greater than or equal to activation energy
  • Reaction continues until all reactants used (RoR =0)
• Zero order - Changing the concentration has no effect on the rate of reaction
• First order - the rate of reaction is directly proportional to the concentration of the reactants.
• Second order - rate is proportional to the square of the change in concentration
• Using a large excess of a reactant will mean the order of reaction is zero with respect to the reactant as concentration is effectively constant
• Orders with respect to each reactant can only be determined by experiments
• Two experiments to determine order:

  • Continuous monitoring
  • Initial rates
• Continuous monitoring method:

  • Take samples at regular intervals
  • Use a visible indicator or physical property
  • Reaction still occurs in sample so need to quench the sample
• Quenching - Add large volumes of distilled water to the sample to cool it down and dilute it to stop the reaction
• Monitoring physical properties:

  • Amount of product produced needs converting to reactant concentration
  • Colour change = uses colorimeter
• Graphs for continuous monitoring:

  • Concentration/time
• Zero order - line is constant
• Orders from continuous monitoring graphs:

  • Two tangents and find concentrations
  • Compare factors of change in concentration and rates
• Graphs for initial rates method:

  • Rate/volume
  • Rate/concentration
• Initial rates:

  • Total volume must be constant
  • Concentration of reactants proportional to volume
  • Rate is proportional to 1/time
• Rate of reaction in initial rates:

  • Rate = change in concentration/time
• Iodine clock:

  • Measure known volumes of reactants (keep separate)
  • Measure a known volume of sodium thiosulfate
  • Start timer when last reactants added to conical flask
  • Stop timer when the mixture turns blue/black (record time)
  • Repeat experiments but change potassium iodide volume
  • Rate = 1/time
  • Plot graph 1/time against volume of potassium iodide
• Do not add sodium thiosulfate last in the iodine clock
• Relationship between temperature and rate constant is arrhenius equation
• If temperature is higher than 10K then rate has approximately doubled
• -Ea/RT is unitless
• Units for K = units for A
• Activation energy is always positive
• To identify RDS:

  • Species in the rate determining step must match the species in the rate equation
• Alkaline solution can be used to neutralise acids in experiments
• Why the order of a reaction with respect to a reactant could be ignored:

  • Concentration of reactant is much large than the other reactant
  • Concentration of the reactant is effectively constant
• Straight line graphs:

  • Gradient is constant
  • Rate of reaction does not change as concentration changes
  • Zero order
• Why reactions are fast at the start:

  • Higher concentration of reactants
  • Increased frequency of successful collisions
• First order on a rate concentration graph has a constant gradient and is a straight line.
• When measuring rate and iodine reacts with another molecule and I2 is not formed the visible change is that the brown colour is removed
• Why doubling the temperature has a much greater affect than doubling the concentration of a first order reactant:

  • More molecules with energy greater than activation energy
  • Doubling temperature causes more molecules with energy greater than activation energy than doubling the concentration
• Alkali can be used to neutralise a solution (quench) so stops the reaction
• State and explain what must be done to a sample before titration:

  • Quench
  • By dilution to stop the rate of reaction