COMPOSITE REACTIONS AND THE STEADY STATE APPROXIMATION

Types of Composite Reactions

Composite (or Multistep, or Non-Elementary) Reactions can be divided into the following categories:

Type	Definition	Example
Competing (or Parrallel) Reactions	Where the reactant is consumed by two different reactions to form different products.	Chlorine atoms generated by the ultraviolet photolysis of chlorofluorocarbons in the stratosphere, can react with ozone or methane: Cl + O3 → ClO + O2 Cl + CH4 → HCl + CH3
Consecutive (or Series) Reactions	Where the reactant forms an intermediate product, which reacts further to form another product.	The mother-daughter radioactive decay of polonium-218: 218Po →214Pb → 214Bi
Reversible (or Opposing) Reactions	Where the reaction can proceed in either direction.	The combination of two benzene molecules to form one molecule of hydrogen and one of diphenyl:

A composite reaction can be any one, or any combination of the types listed above.

An example of a combination of competing and consecutive reactions is the formation of butadiene from ethanol:

C₂H₅OH → C₂H₄ + H₂O

C₂H₅OH → CH₃CHO + H₂

C₂H₄ + CH₃CHO → C₄H₆ + H₂O

Review Question

A → B

B + C → D

D → B + C

Competing and Consecutive Reactions

Competing and Reversible Reactions

Consecutive and Reversible Reactions

Consecutive and Series Reactions