When more than one stoichiometric equation is chosen to represent the observed changes,then more than one kinetic expression is needed to follow the changing composition of all the reaction components, and we have multiple reactions. Multiple reactions may be classified as: series reactions, A→R→S 25
25 • When more than one stoichiometric equation is chosen to represent the observed changes, then more than one kinetic expression is needed to follow the changing composition of all the reaction components, and we have multiple reactions
parallel reactions,which are of two types A→R and S B→S) competitive side by side and more complicated schemes,an example of which is A+B→R R+B→S Here,reaction proceeds in parallel with respect to B,but in series with respect to A,R,and S. 26
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Elementary and Non-elementary Reactions A+B→R If we postulate that the rate-controlling mechanism involves the collision or interaction of a single molecule of A with a single molecule of B,then the number of collisions of molecules A with B is proportional to the rate of reaction. 27
27 • Elementary and Non-elementary Reactions • If we postulate that the rate-controlling mechanism involves the collision or interaction of a single molecule of A with a single molecule of B, then the number of collisions of molecules A with B is proportional to the rate of reaction. A + B→R
But at given temperature the number of collisions is proportional to the concentration of reactants in the mixture; hence the rate of disappearance of A is given by: -ra =kC CB Such reaction in which the rate equation corresponds to a stoichiometric equation are called elementary reaction. 28
28 • But at given temperature the number of collisions is proportional to the concentration of reactants in the mixture; hence the rate of disappearance of A is given by: • Such reaction in which the rate equation corresponds to a stoichiometric equation are called elementary reaction. A A CB − r = kC
When there is no direct correspondence between stoichiometry and rate,then we have non-elementary reaction. A very famous example of non-elementary reaction is that between hydrogen and bromine. H,+Br2→2HBr k,LH2TBra2 Ta,k+HBrVBr; 29
29 • When there is no direct correspondence between stoichiometry and rate, then we have non-elementary reaction. • A very famous example of non-elementary reaction is that between hydrogen and bromine. 2 2 1 2 1 2 2 HBr 2 2 k HBr Br k H Br r H Br 2HBr / / + = + →