Colourful Solutions > The rate of chemical change

Introduction

The study of kinetics, or rates of reaction, attempts to answer one of the big questions in chemistry, "how do chemical reactions occur"?

Syllabus reference R2.2.1
Syllabus reference R2.2.2
Syllabus reference R2.2.3
Syllabus reference R2.2.4
Syllabus reference R2.2.5
Syllabus reference R2.2.6
Syllabus reference R2.2.7
Syllabus reference R2.2.8
Syllabus reference R2.2.9
Syllabus reference R2.2.10
Syllabus reference R2.2.11
Syllabus reference R2.2.12
Syllabus reference R2.2.13

Reactivity 2.2.1 - The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

  • Determine rates of reaction.

Guidance

  • Calculation of reaction rates from tangents of graphs of concentration, volume or mass against time should be covered.

Tools and links

  • Tool 1, 3, Inquiry 2 - Concentration changes in reactions are not usually measured directly. What methods are used to provide data to determine the rate of reactions?
  • Tool 1 - What experiments measuring reaction rates might use time as i) a dependent variable ii) an independent variable?

Reactivity 2.2.2 - Species react as a result of collisions of sufficient energy and proper orientation.

  • Explain the relationship between the kinetic energy of the particles and the temperature in kelvin, and the role of collision geometry.

Guidance

Tools and links

  • Structure 1.1 - What is the relationship between the kinetic molecular theory and collision theory?

Reactivity 2.2.3 - Factors that influence the rate of a reaction include pressure, concentration, surface area, temperature and the presence of a catalyst.

  • Predict and explain the effects of changing conditions on the rate of a reaction.

Guidance

Tools and links

  • Tool 1 - What variables must be controlled in studying the effect of a factor on the rate of a reaction?
  • Nature of science, Tool 3, Inquiry 3 - How can graphs provide evidence of systematic and random error?

Reactivity 2.2.4 - Activation energy, Ea, is the minimum energy that colliding particles need for a successful collision leading to a reaction.

  • Construct Maxwell–Boltzmann energy distribution curves to explain the effect of temperature on the probability of successful collisions.

Guidance

Tools and links

Reactivity 2.2.5 - Catalysts increase the rate of reaction by providing an alternative reaction pathway with lower Ea.

  • Sketch and explain energy profiles with and without catalysts for endothermic and exothermic reactions.
  • Construct Maxwell–Boltzmann energy distribution curves to explain the effect of different values for Ea on the probability of successful collisions.

Guidance

  • Biological catalysts are called enzymes.
  • The different mechanisms of homogeneous and heterogeneous catalysts will not be assessed.

Tools and links

  • Reactivity 2.3 - What is the relative effect of a catalyst on the rate of the forward and backward reactions?
  • AHL Structure 3.1 - What are the features of transition elements that make them useful as catalysts?

Reactivity 2.2.6 - Many reactions occur in a series of elementary steps. The slowest step determines the rate of the reaction. (HL)

  • Evaluate proposed reaction mechanisms and recognize reaction intermediates.
  • Distinguish between intermediates and transition states, and recognize both in energy profiles of reactions.

Guidance

  • Include examples where the rate-determining step is not the first step.
  • Proposed reaction mechanisms must be consistent with kinetic and stoichiometric data.

Tools and links

  • Reactivity 3.4 - Which mechanism in the hydrolysis of halogenoalkanes involves an intermediate?

Reactivity 2.2.7 - Energy profiles can be used to show the activation energy and transition state of the rate-determining step in a multistep reaction. (HL)

  • Construct and interpret energy profiles from kinetic data.

Guidance

Tools and links

Reactivity 2.2.8 - The molecularity of an elementary step is the number of reacting particles taking part in that step. (HL)

  • Interpret the terms “unimolecular”, “bimolecular” and “termolecular”.

Guidance

Tools and links

Reactivity 2.2.9 - Rate equations depend on the mechanism of the reaction and can only be determined experimentally. (HL)

  • Deduce the rate equation for a reaction from experimental data.

Guidance

Tools and links

Reactivity 2.2.10 - 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. (HL)

  • 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.
  • Sketch, identify and analyse graphical representations of zero, first and second order reactions.

Guidance

  • Concentration–time and rate–concentration graphs should be included.
  • Only integer values for order of reaction will be assessed.

Tools and links

  • Tool 1, Tool 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”?

Reactivity 2.2.11 - The rate constant, k, is temperature dependent and its units are determined from the overall order of the reaction. (HL)

  • Solve problems involving the rate equation, including the units of k.

Guidance

Tools and links

  • Reactivity 3.4 - What are the rate equations and units of k for the reactions of primary and tertiary halogenoalkanes with aqueous alkali?

Reactivity 2.2.12 - The Arrhenius equation uses the temperature dependence of the rate constant to determine the activation energy. (HL)

  • Describe the qualitative relationship between temperature and the rate constant.
  • Analyse graphical representations of the Arrhenius equation, including its linear form.

Guidance

  • The Arrhenius equation and its linear form are given in the data booklet.

Tools and links

Reactivity 2.2.13 - The Arrhenius factor, A, takes into account the frequency of collisions with proper orientations. (HL)

  • Determine the activation energy and the Arrhenius factor from experimental data.

Guidance

Tools and links