Rate equations

• Rate of a reaction

  Shows how fast reactants are converted into products
• Rate constant


  A constant that, along with the concentrations of reactants, determines the rate of a reaction
• Order of the reaction


  The constants m and n that show the effect of the concentration of each reactant on the rate of the reaction
• Total order of reaction


  The sum of the separate orders
• Orders of reaction


  • Zero order
  • First order
  • Second order
• Zero order


  • The concentration of this species has no impact on rate
  • Shown graphically as a horizontal line
  • Rate = k
• First order


  • The concentration of the species and rate are directly proportional
  • Doubling concentration doubles the rate
  • Rate = k[A]
• Second order


  • The rate is proportional to the concentration squared
  • Doubling the concentration will increase the rate by four
  • Rate = k[A]^2
• Rate constant (k)


  Constant when the reaction temperature is constant, can be found by rearranging the rate equation
• Arrhenius equation


  Shows how the rate constant k and temperature are related exponentially
• The logged form of the Arrhenius equation can be used in the form 'y = mx + c' to show the relationship graphically
• On a graph of lnk against 1/T, the gradient is negative and constant and the y-intercept is lnA
• Rate determining step


  The slowest step of the reaction that determines the overall rate
• The rate equation contains all the species involved in the stages up to and including the rate determining step
• Determining rate equations experimentally

  1. Monitor concentration of reaction mixture over time

  2. Use concentration-time graph to find rate by drawing tangent at t=0

  3. Repeat at varying concentrations to get data for concentration and rate

  4. Plot graph of rate against concentration to determine order of reaction