Unit 6. Reaction Mechanisms

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.

Most of the time, the balanced equation for a chemical reaction does not reveal the actual process that takes place at the molecular level. Instead, it simply conveys the net effect of a set of different elementary reactions that actually occur at the molecular level. This group of elementary reactions is collectively referred to as the reaction mechanism. In other words, when a reaction mechanism occurs at the molecular level, it appears, from a macroscopic perspective, that a single (non-elementary) reaction is taking place. As a consequence, it is possible to generate a rate expression for the single reaction that appears to be taking place, even though it never actually occurs at the molecular level. If the underlying reaction mechanism is known, it can be used to generate the mathematical form of the rate expression for the apparent, macroscopically observed reaction. Unit 6 describes how to do this.

Learning Resources

• Archive (.zip) - Contains all learning resources listed below for this unit
• Documents to Read:
  • Unit 06 Informational Reading (.pdf)
  • Example 6.1 (.pdf)
  • Example 6.2 (.pdf)
• Videos to Watch (please right-click and save, then play back locally on your computer):
  • Unit 06 Informational Video (.mov)
  • Example 6.1 Video (.mov)
  • Example 6.2 Video (.mov)
• Reference Files:
  • Unit 6 Definitions, Nomenclature and Equations (.pdf)
• Other Files:
  • Example_6_1.m - MATLAB file used in the solution of Example 6.1

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
  • Learning activity files to make available to students after class
    • Activity_6_1.m - MATLAB file used in the solution of Activity 6.1
• Alternative Questions (.pdf) that could be used in a pre-class quiz

Practice Problems

(Problem Statement as .pdf file)

2*. Repeat Example 6.2, but instead of generating the generalized rate expression from the rate of formation of NO, generate it from the rate with respect to either O₂ or N₂. Upon doing so, you will find that you get a different rate expression than that obtained in Example 6.2. A reaction can't have two different rate expressions that both accurately describe how the rate varies with the environmental variables. Discuss the validity of these rate expressions, explaining how they are consistent with the definitions of the generalized rate and the rate with respect to a participant species.

(Problem Statement as .pdf file)

(Problem Statement as .pdf file)