1.9 Rate equations

• • The rate of a chemical reaction is related to the concentration of reactants by a rate equation of the form: Rate = k[A]m [B]n
  • where m and n are the orders of reaction with respect to reactants A and B and k is the rate constant.
  • The orders m and n are restricted to the values 0, 1, and 2
• The rate constant k varies with temperature as shown by the equation:

  k = Ae^–Ea/RT

  where A is a constant, known as the Arrhenius constant, Ea is the activation energy and T is the temperature in K.
• Define the term overall order of reaction. [1 mark] - 2020 Paper 2

  Before markscheme: (the power to which the concentration of that reactant is raised in the rate equation)

  • The sum of powers/indices (to which the concentrations are raised in the rate equation)
• Define rate constant

  • A number that connects the concentration of reactants in a reaction to the rate of that reaction.
  • The larger the value of k, the faster the rate of reaction.
• • The rate equation is an experimentally determined relationship.
  • The orders with respect to reactants can provide information about the mechanism of a reaction.
• Use the orders with respect to reactants to provide information about the rate determining/limiting step of a reaction!!!

  • Suggest why the order with respect to iodine is zero.
  • Iodine is not involved in (or before) the rate determining / slow(est) / limiting step (in the mechanism)
• A series of experiments was carried out using concentrations of propanone approximately 100 times the concentrations of iodine and hydrochloric acid. Suggest the rate equation under these conditions. Explain your answer.

  • rate = k [H+] LARGE EXCESS OF PROPANONE
  • so [CH3COCH3] is effectively CONSTANT
• How to work out INTERIM rates (harder example)

  • Given order of reaction for the first reactant
  • THEN use that to work out INTERIM rate (what would happen if there was no change in concentration
  • Compare ACTUAL initial rate with the INTERIM rates
  • ACTUAL initial rate divided by INTERIM rate
  • This is the ratio
  • Find the ratio of concentration you increased by
  • Find order of reaction with respect to that concentration