In the previous chapter, we discussed the rates of chemical reactions. In this chapter, we will expand on the concepts of chemical reaction rates by exploring what the rate law implies about the mechanistic pathways that reactions actually follow to proceed from reactants to products. Typically, one determines a rate law that describes a chemical reaction, and then suggests a mechanism that can be (or might not be!) consistent with the observed kinetics. This chapter will be concerned with reconciling reaction mechanisms with predicted rate laws.
- 12.1: Reaction Mechanisms
- A reaction mechanism is a set of elementary reactions steps, that when taken in aggregate define a chemical pathway that connects reactants to products. An elementary reaction is one that proceeds by a single process, such a molecular (or atomic) decomposition or a molecular collision.
- 12.2: Concentration Profiles for Some Simple Mechanisms
- To illustrate how mechanisms may affect the concentration profile for a reaction, we can examine some simple mechanisms
- 12.3: The Connection between Reaction Mechanisms and Reaction Rate Laws
- The great value of chemical kinetics is that it can give us insights into the actual reaction pathways (mechanisms) that reactants take to form the products of reactions. Analyzing a reaction mechanism to determine the type of rate law that is consistent (or not consistent) with the specific mechanism can give us significant insight.
- 12.4: The Rate Determining Step Approximation
- The rate determining step approximation is one of the simplest approximations one can make to analyze a proposed mechanism to deduce the rate law it predicts. Simply stated, the rate determining step approximation says that a mechanism can proceed no faster than its slowest step.
- 12.5: The Steady-State Approximation
- One of the most commonly used and most attractive approximations is the steady state approximation. This approximation can be applied to the rate of change of concentration of a highly reactive (short lived) intermediate that holds a constant value over a long period of time.
- 12.6: The Equilibrium Approximation
- In many cases, the formation of a reactive intermediate (or even a longer lived intermediate) involves a reversible step. This is the case if the intermediate can decompose to reform reactants with a significant probability as well as moving on to form products. In many cases, this will lead to a pre-equilibrium condition in which the equilibrium approximation can be applied.
- 12.7: The Lindemann Mechanism
- The Lindemann mechanism (Lindemann, Arrhenius, Langmuir, Dhar, Perrin, & Lewis, 1922) is a useful one to demonstrate some of the techniques we use for relating chemical mechanisms to rate laws. In this mechanism, a reactant is collisionally activated to a highly energetic form that can then go on to react to form products.
- 12.8: The Michaelis-Menten Mechanism
- The Michaelis-Menten mechanism (Michaelis & Menten, 1913) is one which many enzyme mitigated reactions follow. The basic mechanism involves an enzyme (E, a biological catalyst) and a substrate (S) which must connect to form an enzyme-substrate complex (ES) in order for the substrate to be degraded (or augmented) to form a product (P).
- 12.9: Chain Reactions
- A large number of reactions proceed through a series of steps that can collectively be classified as a chain reaction. The reactions contain steps that can be classified as initiation step – a step that creates the intermediates from stable species propagation step – a step that consumes an intermediate, but creates a new one termination step – a step that consumes intermediates without creating new ones
- 12.10: Catalysis
- There are many examples of reactions that involve catalysis. One that is of current importance to the chemistry of the environment is the catalytic decomposition of ozone
- 12.11: Oscillating Reactions
- In most cases, the conversion of reactants into products is a fairly smooth process, in that the concentrations of the reactants decrease in a regular manner, and those of the products increase in a similar regular manner. However, some reactions can show irregular behavior in this regard. One particularly peculiar (but interesting!) phenomenon is that of oscillating reactions, in which reactant concentrations can rise and fall as the reaction progresses.
- 12.E: Chemical Kinetics II (Exercises)
- Exercises for Chapter 12 "Chemical Kinetics II" in Fleming's Physical Chemistry Textmap.
- 12.S: Chemical Kinetics II (Summary)
- Summary for Chapter 12 "Chemical Kinetics II" in Fleming's Physical Chemistry Textmap.
Contributors and Attributions
Patrick E. Fleming (Department of Chemistry and Biochemistry; California State University, East Bay)