kinetics

• Order of reaction


  Explains how different species can have more of an effect on the reaction than others
• Orders of reaction


  • Range from zero to second order
• Zero order


  The concentration of this species has no impact on rate
• Zero order reactions are shown graphically as a horizontal line
• First order


  The concentration of the species and rate are directly proportional
• Doubling the concentration of a first order reactant doubles the rate
• Second order


  The rate is proportional to the concentration squared

  
• Doubling the concentration of a second order reactant will increase the rate by four
• General rate equation


  Rate = k[A]^m[B]^n, where m and n are the orders of the reaction with respect to reactant A and reactant B
• If [A] is doubled but the rate stays the same, then the order with respect to A is 0
• If [A] is doubled and the rate also doubles, then the order with respect to A is 1
• If [A] is doubled and the rate changes to be four times faster, then the order with respect to A is 2
• Total order of reaction

  The sum of the separate orders: m + n
• Rate constant (k)


  Constant when the reaction temperature is constant, can be found by rearranging the rate equation
• The rate constant has varying units depending on the number of species and their orders of reaction
• Determining rate equations


  Monitor the concentration of a reaction mixture over time, draw a tangent to the curve at t=0 to find the rate, repeat at varying concentrations to determine the order of reaction
• Initial rate experiments


  Collect data on initial rates with varying concentrations of reactants, use this to determine the orders of reaction
• The rate equation can be used to calculate the rate constant
• Rate determining step


  The slowest step of the reaction, which determines the overall rate
• Zero order reactants have no influence on the rate of a reaction so they cannot be included in the rate-determining step
• The powers in the rate equation indicate the number of molecules of each substance involved in the slowest step
• Any intermediates generated in the slowest step must be reactants in another step as they are not present in the balanced overall equation
• Arrhenius equation


  Shows how the rate constant k and temperature are related exponentially: k = Ae^(-Ea/RT)
• On a graph of lnk against 1/T, the gradient is negative and constant and the y-intercept is lnA