Reaction mechanism is a sequence of single reaction steps that sum to the overall equation. For example, a possible mechanism for the overall reaction
2A + B → E + F
might involve these three simpler steps:
(1) A + B → C
(2) C + A → D
(3) D → E + F
Adding the steps and canceling common substances gives the overall equation:
A + B + C + A + D → C + D + E + F or 2A + B → E + F
A mechanism is a hypothesis about how a reaction occurs; chemists propose a mechanism and then test to see that it fits with the observed rate law.
The reaction mechanism describes the sequence of elementary reactions that must occur to go from reactants to products.
Reaction intermediates are formed in one step and then consumed in a later step of the reaction mechanism.
The slowest step in the mechanism is called the rate determining or rate-limiting step.
The overall reaction rate is determined by the rates of the steps up to (and including) the rate determining step.
The rapid evolution of oxygen gas is produced by the following reaction:
- 2 H2O2 (aq) = 2 H2O (l) + O2 (g) + heat
The decomposition of hydrogen peroxide in the presence of iodide ion occurs in two steps:
- H2O2 (aq) + I- (aq) = H2O (l) + OI- (aq)
- H2O2 (aq) + OI- (aq) = H2O (l) + O2 (g) + I- (aq)