Kinetics of Chain reaction

Chain recations - Characteristics - Mechanism - Steady-state approximation - Kinetics of thermal decomposition of acetaldehyde

1. KINETICS OF CHAIN REACTIONS Dr.P.GOVINDARAJ  Associate Professor & Head , Department of Chemistry SAIVA BHANU KSHATRIYA COLLEGE ARUPPUKOTTAI - 626101 Virudhunagar District, Tamil Nadu, India

2. KINETICS OF CHAIN REACTIONS Chain reactions Chemical reactions in which highly reactive short-lived species like atoms (or) free radicals are produced as intermediates which carry out the reaction at a rapid rate for a long time are called Chain reactions. Mechanism of Chain reaction Chain reaction occur mainly by the following three steps Chain initiation step In this step active intermediates are produced 2. Chain propagation steps In these steps active intermediates reacts with reactant gives product and another active intermediate and this is repeated as a chain until the reactants (or) active intermediate disappear.

3. Example H 2 + Br 2 2HBr Mechanism i ) Chain initiation step Br 2 2Br ii) Chain Propagation steps Br + H 2 HBr + H H + Br 2 HBr + Br k 1 k 2 k 3 iii) Chain termination step Br + Br Br 2 Chain termination step In this step intermediate reacts with each other to form stable molecular species so that the chain is ended with this step k -1 Mechanism of Chain reaction

4. Characteristics of chain reactions The probability factor P of chain reactions is generally greater than unity The chain reactions, sometimes possess excessive speeds which may lead to explosion The chain reactions begin at zero rate and then rises to maximum and then falls of with time shown in the diagram iv) As the chain reaction begin at zero rate , it therefore requires enough time so that the rate of the reaction could be detected experimentally v) The speed of the chain reactions is retarded or accelerated by traces of other substances

5. vi) The rate of chain reactions is influenced by the changes in shape of the containing vessel. vii)The chain reactions are rarely of simple orders but have integral orders which depends upon the vessel shape and other experimental conditions. Steady state approximation states that the rate of formation of active intermediate is equal t o the rate of disappearance of active intermediate in the chain reactions Steady state approximation i.e., the rate of formation of intermediate (R) and the rate of disappearance of intermediate (R) must be equal to zero =   Characteristics of chain reactions

6. Example H 2 + Br 2 2HBr i ) Chain initiation step Br 2 2Br ii) Chain Propagation steps a) Br + H 2 HBr + H b) H + Br 2 HBr + Br k 1 k 2 k 3 iii) Chain inhibition step H + HBr H 2 + Br k -2 iv) Chain termination step Br + Br Br 2 k 4 Steady state approximation

7. On applying steady state approximation to the intermediate H Rate of formation of [H] – Rate of disappearance of [H] = 0 ------(1)   Rate of formation of [H] = k 2 [Br][H 2 ] ------(2) Rate of disappearance of [H] = k 3 [H][Br 2 ] + k -2 [H][HBr] ------( 3 ) Substitute (2) and (3) in (1) we get - k 3 [H][Br 2 ] - k -2 [H][HBr] = 0   Steady state approximation

8. Kinetics of thermal decomposition of Acetaldehyde CH 3 CHO CH 4 + CO Mechanism i ) Chain initiation step CH 3 CHO CH 3 + CHO ii) Chain Propagation steps CH 3 + CH 3 CHO CH 4 + CH 3 CO CH 3 CO CH 3 + CO k 1 k 2 k 3 iii) Chain termination step CH 3 + CH 3 C 2 H 6 k 4 The radical CHO undergoes further reactions, but for simplicity they are ignored here The Thermal decomposition of Acetaldehyde is

9. i.e., Rate of formation of methyl radical = Rate of disappearance of methyl radical ------( 1)   Applying steady state approximation on methyl radicals The rate of formation of Methane is k 1 [CH 3 CHO] + k 3 [CH 3 CO] = k 2 [CH 3 ][CH 3 CHO] + k 4 [CH 3 ][CH 3 ] k 1 [CH 3 CHO] + k 3 [CH 3 CO] - k 2 [CH 3 ][CH 3 CHO] - k 4 [CH 3 ][CH 3 ] = 0 ------( 2) Kinetics of thermal decomposition of Acetaldehyde

10. Applying steady state approximation on CH 3 CO radicals i.e., Rate of formation of CH 3 CO = Rate of disappearance of CH 3 CO k 2 [CH 3 ][CH 3 CHO] = k 3 [CH 3 CO] k 2 [CH 3 ][CH 3 CHO] - k 3 [CH 3 CO]= 0 ------(3) Adding equation (2) and (3) we get the concentration of methyl radicals k 1 [CH 3 CHO] + k 3 [CH 3 CO] - k 2 [CH 3 ][CH 3 CHO] - k 4 [CH 3 ][CH 3 ] + k 2 [CH 3 ][CH 3 CHO] - k 3 [CH 3 CO ] = 0 k 1 [CH 3 CHO] - k 4 [CH 3 ] 2 = 0 k 1 [CH 3 CHO] = k 4 [CH 3 ] 2 Kinetics of thermal decomposition of Acetaldehyde

11. k 1 [CH 3 CHO] = k 4 [CH 3 ] 2 [CH 3 ] 2 = [CH 3 CHO]   k 4 [CH 3 ] 2 = k 1 [CH 3 CHO] [CH 3 ] = ( ) 1/2 [CH 3 CHO] 1/2 -------(4)   Substitute (4) in (1) we get     This is the rate equation for thermal decomposition of CH3CHO and the overall order of this reaction is   Kinetics of thermal decomposition of Acetaldehyde

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