Reaction Mechanism and Pathway By Puan Azduwin Khasri 19 NOVEMBER 2012

Non-elementary Rate Laws Usually we have seen that rate can be expressed as: where α = 1, 2, 3, 0 However, a large no. of rxns, the orders are non-integers Rate laws may also involve a no. of elementary rxns and at least one active intermediate.

Active intermediate is a high energy molecule that reacts virtually as fast as it is formed. It is found in small amounts and shown as A*. The high E is stored in the chemical bonds where high- amplitude oscillations lead to bond ruptures, molecular rearrangement and decomposition. Types of active intermediates;  Collision  Free radicals, unpaired e-  Ionic intermediates  Enzyme-substrat complexes ACTIVE INTERMEDIATES

Pseudo-steady-state Hypothesis (PSSH) The PSSH assumes that the net rate of species A* is zero. r A* ≈ 0 If the active intermediate appears in n reactions, then;

Experimental observations; 1 st order at High concentration of AZO 2 nd order at Low concentration of AZO AZO ETHANE + NITROGEN

Three elementary reaction; The rate law for the active intermediate AZO*; The conc of AZO* not measurable

By applying PSSH to obtain measurable concentration; Rate of formation of product

At low AZO concentrations, At high concentrations The reaction is apparent first order at high AZO concentrations and apparent second order at low AZO concentrations. 2 ND ORDER 1 st ORDER

SymbolicaIly this reaction will be represented as A going to product P, The PSSH in explaining observation of so many first order reaction; in example;

M= inert molecule

The mechanism consists of three elementary reactions: Writing the rate of formation of product Using the PSSH to find the concentrations of A*

SEARCHING FOR A MECHANISM  Species having conc’ns appearing in the denominator of the rate law probably collide with the active intermediate  If a constant appears in the denominator, one of the rxn steps is probably the spontaneous decomposition of the active intermediate  Species having conc’ns in the numerator probably produce the active intermediate

STEPS TO DEDUCE A RATE LAW 1. Assume an active intermediate 2. Postulate a mechanism 3. Model each rxn as an elementary rxn 4. Write the rate laws for active intermediates 5. Use PSSH 6. Eliminate intermediate species 7. If the law obeys the experiment, OK. 1. Assume an active intermediate 2. Postulate a mechanism 3. Model each rxn as an elementary rxn 4. Write the rate laws for active intermediates 5. Use PSSH 6. Eliminate intermediate species 7. If the law obeys the experiment, OK.

CHAIN REACTIONS A chain reaction consists of the following sequence: 1. Initiation : Formation of active intermediate 2. Propagation or chain transfer : Interaction of A* with reactant 3. Termination : deactivation of A* A chain reaction consists of the following sequence: 1. Initiation : Formation of active intermediate 2. Propagation or chain transfer : Interaction of A* with reactant 3. Termination : deactivation of A*

EXAMPLE :CHAIN REACTION Use the PSSH to derive a rate law for the rate of formation of ethylene.

SOLUTION; The rate of formation of ethylene; APPLYING PSSH;

Substituting the concentrations into the elementary Eqn; Solving for the concentration of the free radical The rate of formation of ethylene;

REACTION PATHWAY Reaction pathways help see the connection of all interacting species for multiple reactions. EXAMPLE:ETHANE CRACKING