Reaction Mechanism. l Process by which a reaction occurs l Reaction occurring in a single event or step its called an elementary reaction l Total reaction.

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Presentation transcript:

Reaction Mechanism

l Process by which a reaction occurs l Reaction occurring in a single event or step its called an elementary reaction l Total reaction may by a sequence of elementary reactions l The elementary reactions in a multi-step mechanism must always add to give the chemical equation of the overall reaction

l Substances that are formed in one elementary step and consumed in the next are called intermediates l Number of molecules that participate as reactants in an elementary reaction define the molecularity l Unimolecular – one molecule l Bimolecular – two molecules colliding l Termolecular – three molecules colliding – very rare

Example l It has been proposed that ozone is converted into oxygen gas through the mechanism below a) write the equation for the overall reaction b) identify any intermediates c) describe the molecularity of each step O 3  O 2 + O O 3 + O  2O 2

Rate l It is not the overall reaction that determines the rate law rather the elementary reactions l Rate law of an elementary reaction is based on its molecularity l In general the order for each reactant in an elementary reaction is equal to its coefficient in the elementary reaction

Example l If the following reaction occurs in a single elementary step, predict the rate law: O 3 + NO  NO 2 + O 2

Rate-Determining Step l If reaction occurs in a series of elementary steps, one step is often much slower than the others l The slow step limits the speed of the reaction – called the rate determining step

Example l A reaction occurs according to the mechanism below, write the rate law NO 2 + NO 2  NO 3 + NO (slow) NO 3 + CO  NO 2 + CO 2 (fast)

+ This reaction takes place in three steps

+ EaEa First step is fast Low activation energy

Second step is slow High activation energy + EaEa

+ EaEa Third step is fast Low activation energy

Second step is rate determining

Intermediates are present

Activated Complexes or Transition States

Mechanisms and rates l There is an activation energy for each elementary step. l Slowest step (rate determining) must have the highest activation energy.