Question

Differentiate between Complex reaction and elementary reaction?

Answer 1

In a complex chemical reaction, the reactant molecules (intermediates) pass through different transition states due to their bond breaking and energy cleavage. Elementary reaction means single reaction.
This means that there are more molecules taking part, and hence many variables will be affected to a larger degree than in the case of elementary reaction.

Complete step by step answer:

Complex reaction	elementary reaction
Occurs in multi (or) many steps	Occurs in single step
Overall order values are large	Overall order values are small.
Sometimes fractional orders such as $\dfrac{1}{2}$,$\dfrac{1}{3}$, $\dfrac{3}{2}$	Total and pseudo order values lie between $0,1,2$
Many side reactions are present.	No side reactions
Series of transition states	One transition state
In some complex reactions products are not formed in steps directly involving the reactants	Products are formed directly from the reactants
Experimental overall rate constant value, differ from the calculated values. Theories of reaction rates do not agree with complex reactions.	Experimental rate constant values accept with the calculated values. Theories of reaction rates apply fine on simple reactions.
Examples are Reaction between ${H_2}$ AND $B{r_2}$	Examples are cis-trans isomerization

Additional Information:
Molecularity term in chemistry is that the amount of molecules that near react in an elementary reaction and is up to the sum of stoichiometric coefficients of reactants during this elementary reaction.

Note:
Most organic electrochemical reactions are complex, involving large numbers of electrons in the overall reaction. Molecularity has no meaning for complex reactions. The minimum number of molecules, atoms of reactants required for an elementary reaction to occur is indicated by the sum of the stoichiometric coefficients of The reactant within the chemical equation is called molecularity of the reaction. So molecularity has no meaning for complex reactions.