3.5 Chemical Kinetics

• Methods for measuring rates of chemical reactions

  • Measuring gas volume (at constant pressure) using a gas syringe
  • Measuring gas pressure (at constant volume)
  • Measurement of change in mass
  • Colorimetry
  • Sampling and quenching
• Measuring gas volume (at constant pressure) using a gas syringe

  • Can only be used when a gas is produced in a reaction
• Measuring gas pressure (at constant volume)


  • Used when both reactants and products are gases and the pressure changes as the number of gas molecules changes
• Measurement of change in mass

  • Used when a dense gas is produced during a reaction
• Colorimetry


  • Used when something with colour is produced or used up during a reaction
• Sampling and quenching


  • Used when all other methods aren't suitable
  • Involves removing small samples of the reaction mixture at regular time intervals
  • Samples are placed in ice water to lower the concentration of the reactants and to stop the reaction continuing (quenching)
  • Samples can then be tested by other techniques like titration
• Rate of reaction


  Change in concentration/time
• Calculating rate of reaction from a curve

  1. Draw a tangent to the curve

  2. Calculate the gradient of the tangent
• Zero order

  Concentration of the particular reactant will not affect the rate of reaction
• First order


  Whatever you do to the concentration of this reactant, exactly the same thing will happen to the rate
• Second order


  Whatever you do to the concentration of this reactant, the rate will be the square of that
• Rate constant


  Constant for a given reaction at a given temperature, but is not changed by changing the concentration of reactants
• Order of reaction


  The power to which the concentration is raised in the rate equation
• Rate-determining step


  The slowest step in a reaction mechanism that limits the overall rate of reaction
• Using rate equations to determine rate-determining step

  1. If first order, one particle in rate-determining step

  2. If second order, two particles must collide

  3. If third order, three particles must collide
• Arrhenius equation


  • 𝑘 = Ae^(-Ea/RT)
  • k = rate constant
  • A = frequency factor
  • e = mathematical constant
  • Ea = activation energy
  • R = gas constant
  • T = temperature in Kelvin
• Arrhenius equation in log form

  ln ( k ) = - Ea RT + ln ( A )
• What gives a straight line with gradient -Ea/RT?

  1/T gives a straight line with gradient –Eα RT [remember y = mx + c]