Write about elementary and complex reactions.

(N/A) balanced chemical equation does not provide a complete picture of how a reaction proceeds.
$(a)$ Elementary reaction: Reactions that occur in a single step are known as elementary reactions. For these reactions,the order is equal to the molecularity.
$(b)$ Complex reaction: Reactions that proceed through a sequence of elementary steps (collectively called the reaction mechanism) to form products are known as complex reactions.
Complex reactions involving more than three molecules in the stoichiometric equation must occur in multiple steps.
For complex reactions,the overall order is determined by the slowest step,and the molecularity of this slowest step corresponds to the order of the overall reaction.
Examples: $(i)$ The oxidation of ethane to $CO_2$ and $H_2O$ involves a series of intermediate steps forming alcohol,aldehyde,and acid. $(ii)$ Reverse reactions and side reactions (e.g.,the nitration of phenol yields both $o$-nitrophenol and $p$-nitrophenol).