Understanding Reaction Mechanisms and Kinetics

reaction mechanism

A step-by-step sequence of elementary reactions that show how the overall reaction occurs.

elementary step

A single molecular event in a mechanism; can't be broken down further.

intermediate

A species that is formed in one step and consumed in a later step; it does not appear in the overall equation.

transition state

A high-energy, unstable configuration that exists at the peak of the energy diagram during a reaction step.

activated complex

The temporary, unstable species at the transition state; it represents the arrangement of atoms as bonds are breaking/forming.

activation energy (Ea)

The minimum energy required for reactants to reach the transition state and react.

activation energy on an energy diagram

It's the vertical distance between the reactants and the peak (transition state) of the curve.

ΔH from an energy diagram

ΔH = energy of products − energy of reactants.

negative ΔH

The reaction is exothermic (releases heat); products are lower in energy than reactants.

rate-determining step

The slowest step in a multi-step mechanism; it controls the overall reaction rate.

rate-determining step on a multi-step energy diagram

The step with the highest activation energy (tallest peak).

collision theory

A theory stating that particles must collide with sufficient energy and correct orientation to react.

factors affecting reaction rate (collision theory)

1) Collision frequency (Z), 2) Energy factor (f), 3) Orientation factor (p)

orientation factor (p)

The fraction of collisions that occur with the correct alignment to lead to a reaction.

collisions below activation energy

No reaction occurs; they just bounce off each other.