Module 8.4: Reaction Mechanisms

8.4.1 Elementary reactions

I Elementary reactions: reactions that occur at a molecular level, illustrating the mechanism of the overall reaction

1. An elementary reaction always deals with molecules, not bulk matter, and therefore do not have units attached to them

2. While the coefficients of ordinary chemical reactions tell nothing about the overall mechanism, elementary reactions do

II Molecularity: the number of molecules coming together to react in an elementary reaction

1. If the elementary step occurs by n molecules colliding, molecularity is n

   1. 1 molecule would be unimolecular, 2 molecules would be bimolecular, etc.

2. Molecularity refers to an individual reaction, order refers to an empirical quantity.

III Kinetic information (rate law) can only support a proposed mechanism; it can never confirm it.

8.4.2 Formulation of rate laws

IV Intermediate: species that is formed in one elementary reaction and is destroyed in the other

1. An intermediate species must not be present in the overall chemical equation

2. An intermediate species is more likely to be plausible if it is an analogue of a known compound

3. An acceptable rate law must not include intermediates in its final rate law

   1. This can be avoided by calculating the net rate of formation of the intermediate and solving for the concentration of the intermediate

V Rate-determining step (RDS): the elementary reaction step that determines the overall rate law of the reaction

1. The RDS is always the slowest step of the reaction that has the highest activation energy

   1. If a molecule has enough kinetic energy to overcome the highest potential energy barrier, all other (smaller) barriers will be easy to overcome

2. Side-stepping occurs when a different elementary step is preferred (that leads to the same product) as it is faster than the slow step

VI It is common to see consecutive reactions, where a reactant produces an intermediate, that subsequently decays into the product

1. Very common in nuclear decay and biochemical processes

VII Approximations are very useful for consecutive reactions

1. Pre-equilibrium: The assumption that the intermediate is in rapid equilibrium with its reactants

   1. Condition: the decay of the intermediate must be slow enough that the intermediate can be approximated to reach equilibrium with its reactants

2. Steady-state approximation: The assumption that the concentrations of all intermediates remain constant and small throughout the reaction

   1. As the concentrations remain constant, the net rate formation of intermediate = 0

   2. In some cases, it may be wise to also approximate that some rate laws are much more significant than others and negate those that aren't as significant

VIII unimolecular reactions occur according to the Lindemann mechanism

1. According to collision theory, it seems unimolecular reactions are impossible as one molecule cannot collide by itself

2. Instead, two molecules collide together to form an energetically excited reactant molecule

   1. This excited molecule can either go on to form products or to go back into its ground state