Chemical reactions often involve complex processes. Reaction mechanisms break these down into simpler steps, showing how molecules transform. Understanding these steps helps us predict reaction rates and outcomes, crucial for designing efficient chemical processes.
Elementary steps are the building blocks of reaction mechanisms. They represent the simplest possible molecular events, like collisions or bond breaks. By studying these steps, we can uncover the and derive overall reaction rates.
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A reaction mechanism provides a detailed, step-by-step description of how reactants are converted into products at the molecular level
Includes all species and transition states involved in the reaction
Describes the individual molecular events that occur during the reaction, such as the breaking and forming of bonds and the of atoms
The overall reaction is the sum of all the elementary steps in the reaction mechanism
Intermediate species cancel out in the overall reaction equation
The stoichiometry of the overall reaction is determined by the stoichiometry of the elementary steps
Rate-Determining Step and Overall Reaction Rate
The slowest step in a reaction mechanism is called the rate-determining step
Controls the rate of the overall reaction because it has the highest among all the elementary steps
Represents the bottleneck in the reaction pathway
The rate law for the overall reaction can be derived from the rate-determining step in the reaction mechanism
The rate law for the rate-determining step is identical to the rate law for the overall reaction
If the concentration of an intermediate species appears in the rate law for the rate-determining step, the steady-state approximation can be applied to express its concentration in terms of the reactants
Elementary Steps and Rate Laws
Characteristics of Elementary Steps
Elementary steps are the individual molecular events that occur during a reaction
Involve the collision and rearrangement of molecules or ions
Cannot be broken down into simpler substeps
Occur in a single collision event
Examples of elementary steps include:
dissociation: AB→A+B
collision: A+B→AB
Rate Laws for Elementary Steps
The rate law for an elementary step is determined by the molecularity of the reaction
Molecularity is the number of molecules or ions that participate in the step as reactants
For an elementary step, the reaction order for each reactant is equal to its stoichiometric coefficient in the balanced chemical equation for that step
Example: For the elementary step 2A+B→C, the rate law is Rate=k[A]2[B], where k is the
The overall reaction order for an elementary step is the sum of the stoichiometric coefficients of the reactants in the balanced chemical equation for that step
Rate-Determining Step in Mechanisms
Identifying the Rate-Determining Step
The rate-determining step is the slowest step in a multi-step reaction mechanism
Controls the overall rate of the reaction because it has the highest activation energy among all the elementary steps
Represents the bottleneck in the reaction pathway
To identify the rate-determining step, compare the rate constants or activation energies of the individual elementary steps
The step with the smallest rate constant or the highest activation energy is typically the rate-determining step
Steady-State Approximation for Intermediates
If the concentration of an intermediate species appears in the rate law for the rate-determining step, the steady-state approximation can be applied
Assumes that the concentration of the intermediate remains constant during the reaction because its rate of formation is equal to its rate of consumption
Allows the concentration of the intermediate to be expressed in terms of the concentrations of the reactants
Example: Consider the reaction mechanism:
Step 1: A+B⇌C (fast equilibrium)
Step 2: C+D→E (slow, rate-determining)
Applying the steady-state approximation to intermediate C, its concentration can be expressed as: [C]=k−1+k2[D]k1[A][B]
Molecularity vs Reaction Order
Molecularity of Elementary Steps
The molecularity of an elementary step is the number of molecules or ions that participate in the step as reactants
Determines the reaction order for that step
Unimolecular elementary steps have a molecularity of one
Involve a single reactant molecule or ion
Example: A→B+C
Bimolecular elementary steps have a molecularity of two
Involve the collision of two reactant molecules or ions
Example: A+B→C
elementary steps have a molecularity of three
Involve the simultaneous collision of three reactant molecules or ions
Example: A+B+C→D
Termolecular steps are rare because the probability of three particles colliding simultaneously is low
Reaction Order and Rate Laws
The reaction order for an elementary step is determined by the molecularity of the step
Unimolecular steps have a first-order rate law: Rate=k[A]
Bimolecular steps have a second-order rate law: Rate=k[A][B] or Rate=k[A]2 (if both reactants are the same species)
Termolecular steps have a third-order rate law: Rate=k[A][B][C], Rate=k[A]2[B], or Rate=k[A]3 (depending on the reactant species)
The overall reaction order is the sum of the reaction orders for each reactant in the rate-determining step
Example: For the rate-determining step 2A+B→C, the overall reaction order is 3 (2nd order in A, 1st order in B)