Unit 8. Rate Determining Step

This website provides learning and teaching tools for a first course on kinetics and reaction engineering. The course is divided into four parts (I through IV). Here, in Part II of the course, the focus is on chemical reaction kinetics, and more specifically, on rate expressions, which are mathematical models of reaction rates. As you progress through Part II, you will learn how rate expressions are generated from experimental kinetics data.

This first section of Part II of the course focuses upon the selection of an equation to be tested as a rate expression. The equation to be tested can be chosen simply for its mathematical convenience. Alternatively, theory can be used to select the mathematical form of the equation to be tested. For some reactions, theory can be applied directly. In other cases the reaction must be described in terms of a group of reactions that comprise what is known as a reaction mechanism. In the latter case theory can be applied to the reactions in the mechanism which are then combined to get the mathematical form of the equation to be tested.

An apparent rate expression for a macroscopically observed, non-elementary reaction can be generated if the reaction mechanism is known, as described in Unit 6. The resulting rate expression is of limited utility because it will include concentrations or partial pressures of reactive intermediates, which are very small and challenging to measure. Unit 7 showed how to use the Bodenstein steady state approximation to eliminate the concentrations of reactive intermediates from mechanistic rate expressions. Unit 8 introduces the concept of a rate determining step and shows how the presence of a rate determining step in a reaction mechanism leads to an alternative approach to the generation of a mechanistic rate expression.

Learning Resources

• Archive (.zip) - Contains all learning resources listed below for this unit
• Documents to Read:
  • Unit 08 Informational Reading (.pdf)
  • Example 8.1 (.pdf)
• Videos to Watch (please right-click and save, then play back locally on your computer):
  • Unit 08 Informational Video (.mov)
  • Example 8.1 Video (.mov)
• Reference Files:
  • Unit 8 Definitions, Nomenclature and Equations (.pdf)

Teaching Resources

• Archive (.zip) - Contains all teaching resources listed below for this unit
• Sample Class
  • Online Pre-class Quiz (.pdf file)
  • Redacted slides to make available to students before class
    • Keynote 09
    • PowerPoint
  • Class Lesson Plan (.pdf)
  • Complete slides to make available to students after class
    • Keynote 09
    • PowerPoint

Practice Problems

1. The macroscopically observed reaction between iodine and methyl formate is given in equation (1a) below. Suppose the spectroscopic and other evidence indicates that the reaction is non-elementary and actually occurs via the mechanism given in equations (1b) Through (1e). First derive four rate expressions based upon this mechanism by assuming that each of the four steps is rate-determining. Then derive a fifth rate expression using the Bodenstein steady state approximation with the additional assumptions that steps (1d) and (1e) are effective irreversible. Comment upon the results.

(Problem Statement as .pdf file)

2. Suppose that iodopropane reacts to produce iodine according to the apparent reaction (2a), but that the reaction is not elementary. By separately assuming each of the three mechanistic steps proposed in equations (2b) through (2d) to be rate-determining, generate three possible rate expressions for reaction (2).

(Problem Statement as .pdf file)

3. Suppose that the macroscopically observed reaction (3a) actually occurs according to the mechanism given in reactions (3b) through (3e). Derive a rate expression for reaction (3a) assuming step (3e) to be rate-determining.

(Problem Statement as .pdf file)

4. Phosgene formation, reaction (4a), is non-elementary; it has been proposed to occur via the mechanism consisting of reactions (4b) through (4d). Generate a rate expression for the non-elementary reaction (4a) assuming mechanistic step (4d) to be rate-determining.

(Problem Statement as .pdf file)