Reactivity 2.2 How fast? The rate of chemical change (HL)
Reactivity 2.2.6
​Understandings:
  • Many reactions occur in a series of elementary steps. The slowest step determines the rate of the reaction.
Learning outcomes:
  • Evaluate proposed reaction mechanisms and recognise intermediates.​
  • Distinguish between intermediates and transition states, and recognise both in energy profiles of reactions.
Additional notes:
  • Include examples where the rate-determining step is not the first step.
  • Proposed reaction mechanisms must be consistent with kinetic and stoichiometric data.
Linking questions:
  • Reactivity 3.4 Which mechanism in the hydrolysis of halogenoalkanes involves an intermediate?
This video covers rate equations. 
This video covers transition states and catalysts. 
Reactivity 2.2.7
​Understandings:
  • Energy profiles can be used to show the activation energy and transition state of the rate-determining step in a multistep reaction.
Learning outcomes:
  • Construct and interpret energy profiles from kinetic data.
This video covers energy level profiles for multi-step reactions. 
Reactivity 2.2.8
​Understandings:
  • The molecularity of an elementary step is the number of reacting particles taking part in that step.
Learning outcomes:
  • Interpret the terms “unimolecular”, “bimolecular” and “termolecular”.​
This video covers molecularity. 
Reactivity 2.2.9
​Understandings:
  • Rate equations depend on the mechanism of the reaction and can only be determined experimentally.
Learning outcomes:
  • Deduce the rate equation for a reaction from experimental data.
This video covers reaction mechanisms. 
Reactivity 2.2.10
​Understandings:
  • The order of a reaction with respect to a reactant is the exponent to which the concentration of the reactant is raised in the rate equation.
  • The order with respect to a reactant can describe the number of particles taking part in the rate- determining step.
  • The overall reaction order is the sum of the orders with respect to each reactant.
Learning outcomes:
  • Sketch, identify and analyse graphical representations of zero, first and second order reactions.
Additional notes:
  • Concentration–time and rate–concentration graphs should be included.
  • Only integer values for order of reaction will be assessed.
Linking questions:
  • Tool 1, 3, Inquiry 2 What measurements are needed to deduce the order of reaction for a specific reactant?
  • Nature of science Why are reaction mechanisms only considered as “possible mechanisms”?
This video covers orders of reaction. 
This video covers graphical representations of zero, first and second-order reactions.
Reactivity 2.2.11 and 2.2.12
​Understandings:
  • The rate constant, k, is temperature dependent and its units are determined from the overall order of the reaction.
  • ​The Arrhenius equation uses the temperature dependence of the rate constant to determine the activation energy.
Learning outcomes:
  • Solve problems involving the rate equation, including the units of k.
  • Describe the qualitative relationship between temperature and the rate constant.
  • Analyse graphical representations of the Arrhenius equation, including its linear form.
Additional notes:
  • The Arrhenius equation and its linear form are given in the data booklet.
Linking questions:
  • Reactivity 3.4 What are the rate equations and units of k for the reactions of primary and tertiary halogenoalkanes with aqueous alkali?
This video coves the effect of temperature on the rate constant k. 
Reactivity 2.2.12
​Understandings:
  • The Arrhenius equation uses the temperature dependence of the rate constant to determine the activation energy.
Learning outcomes:
  • Describe the qualitative relationship between temperature and the rate constant.
  • Analyse graphical representations of the Arrhenius equation, including its linear form.
Additional notes:
  • The Arrhenius equation and its linear form are given in the data booklet.
This video covers the Arrhenius equation. 
Reactivity 2.2.13
​Understandings:
  • The Arrhenius factor, A, takes into account the frequency of collisions with proper orientations.
Learning outcomes:
  • Determine the activation energy and the Arrhenius factor from experimental data.
This video covers how to determine the activation energy for a reaction.