Elementary and non elementary reaction(no-18) - copy

BCRE

1. Elementary and Non-Elementary Reactions

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 -r A = kC a A .C b 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 -r A = k C α A . C β 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 – r A =kC 2 A whereas for a non elementary reaction 2B S , the rate law may be – r B =k C α B where α ≠ 2 . An example of an elementary reaction : C 2 H 5 OH+CH 3 COOH CH 3 COOC 2 H 5 +H 2 O Examples of non elementary reaction : H 2 +Br 2 2HBr CH 3 CHO CH 4 +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 –r A = kp n1 A p n2 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 - r A = r S = k C 2 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 : H 2 +Br 2 2HBr Rate : r HBr = k 1 C H2 .C ½ Br2 k 2 +(C HBr /C Br2 )

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