2. • Chemical Reaction rarely proceed in one step.
• In a conventional single step writing of the stoichiometric
equation of a chemical reaction, only initial and final states
of the reaction system are indicated.
• In case of multistep reactions the rate of overall reaction is
determined primarily by the slowest step.
• It is customary in chemical kinetics to class all the
reactions as the elementary and Non-elementary reactions
3. Elementary reaction
The individual reaction steps which
contribute to the overall reaction are termed as elementary
reactions.
(OR)
The reaction in which the molecules react
exactly as the stoichiometric equation written for a reaction, is
called as an elementary reaction.
4. Non-Elementary Reaction
The reactions in which formation of product
from the original reactants occur through a series of steps are
called as non- elementary reactions.
5. • For non-elementary reactions there is difference between
order and stoichiometric coefficient .
• We observe Non-Elementary reaction as a single reaction
rather than elementary reactions in series because the
amount of the intermediates formed is negligibly small and
cannot be detected.
6. For the irreversible elementary reaction
aA+bB rR
The rate equation is
-rA = kCa
A.Cb
B
For the non elementary reaction
aA+bB rR
The rate equation (rate law),by analogy with a true simple (or elementary)
reaction may be written as
-rA = k Cα
A . Cβ
B
Where α and β are the orders with respect to A and B and are found by
experimental observations.
NOTE: α ≠ a and β ≠ b
7. Difference between elementary and non-
elementary reactions
•Elementary reactions are single step reactions whereas non
elementary reactions are multistep reactions , i.e , such
reactions occur through a series of reaction steps.
•Elementary reactions are simple in nature whereas the non
elementary reactions are complex in nature.
8. •For elementary reactions the order agrees with the
stoichiometry whereas for non elementary reaction ,the
order does not agrees with the stoichiometry(i.e., there is
a difference between order and stoichiometric
coefficient)
•For an elementary reaction, the order of reaction must be
an integer whereas for a non elementary reaction, the
order of reaction may be an integer or have a fractional
value
9. • For an elementary reaction 2A R, the rate law is –rA=kC2
A
whereas for a non elementary reaction 2B S, the rate law may
be –rB=k Cα
B
where α ≠ 2.
• An example of an elementary reaction:
C2H5OH+CH3COOH CH3COOC2H5+H2O
• Examples of non elementary reaction:
H2+Br2 2HBr
CH3CHO CH4+CO
10. Representation of Elementary reaction
While writing a rate equation we may use any measure
equivalent to concentration, e.g., partial pressure of the reacting
materials in case of gas phase reaction in which case the rate is
given by
–rA = kpn1
A pn2
B
Remember that the order with respect to a reacting
component remains same/unchanged though we use any measure
equivalent to concentration only thing is that it will change the
numerical value and units of k.
11. Elementary reactions are represented by an equation
showing the molecularity as well as the rate constant.
For example, 2A 2S
represents a bio molecular irreversible reaction
with second-order rate constant k , implying that the
rate of reaction is
-rA = rS = k C2
A
12. Representation of a non elementary reaction
:
A non-elementary reaction is the one whose
stoichiometry does not match with its kinetics(i.e., for
such a reaction there is no direct correspondence
between reaction order and stoichiometry).
For example,
Stoichiometry : H2+Br2 2HBr
Rate : rHBr = k1CH2.C½
Br2
k2+(CHBr/CBr2)
