The chemical reactions can be simple or complex. All the single-step reactions are simple and the reactions involving more than one step are complex. As chemical kinetics deals with the study of the rates and mechanisms of chemical reactions, it defines the prevailing simplicity and complexity as well as the level of complexity if applicable.
In a quest to define the simplicity or complexity level, the arrow number rule is readily applied. The arrow number rule states that;
“The level of complexity can be defined by the number of arrows associated
with the chemical reaction/reaction mechanism”
A simple reaction lies at the zero level of complexity and is associated with only one arrow. The first level of complexity is the two-arrow system. Three arrow systems are at the level second level of complexity and so on.
The rule is more elaborated in the table below:
Complexities and Arrows relative to Reactions
Examples of 4 Arrow systems
An example of a 4-arrow system is the pyrolysis reaction of acetaldehyde.
The kinetics of this reaction is simplified by applying steady-state kinetics to the free radical species and the order is found to be fractional (1.5) as;
ν = ki[CH3CHO]3/2
Examples of 5 Arrow systems
Similarly, an example of 5 or more arrow chain reactions is the chain reaction between H2 and Br2.
Levels of Complexity
The complexity levels in any chemical reaction can be elaborated with the help of the following diagram:
How do you identify a complex reaction?
First of all, look at the number of arrows associated with chemical reactions, taking all elementary steps into account.
- If the number of arrows is one, it is a simple reaction i.e. all single-step reactions are simple reactions.
- If the reaction inclusive of all elementary steps has two or more arrows, it would be declared as a complex reaction.
What kinds of reactions are called complex?
Two or more step reactions are complex reactions. In other words, the reactions for which molecularity and order are not same, are called complex reactions.
What is the significance of an activated complex in a chemical reaction?
An activated complex is something that actually governs the thermodynamics as well as kinetics of a chemical reaction. If the activated complex does not form, the reaction won’t take place.
The energy of an activated complex decides the speed of a chemical reaction. Moreover, If more than one activated complexes are possible, the reaction will follow the lowest energy pathway, thus governing the reaction.
Why molecularity has no meaning for complex reactions?
Molecularity has no meaning for complex reactions because of the dependency of the whole reaction speed on the slowest/the rate-determining step. The molecularity of the rate-determining step is called order. Therefore, order is more significant than molecularity for complex reactions.