3.8. Reaction mechanism

A reaction mechanism is a set of elementary reactions by which overall chemical change occurs (Turányi, 2010). Let’s have an example; the formation of the ozone in stratosphere can be described by the Chapman mechanism, where ozone formation and destruction from oxygen species occur. The set of elementary reactions which describes the ozone formation is the following:

,

(R3.12)

.

(R3.13)

We can consider reactions (R3.12) and (R3.13) as a chemical mechanism for ozone formation in the stratosphere, where the overall (net) reaction is . The net reaction is just shows the transformation of reagents to products, and it reflects the mass conservation. However, equations (R3.12) and (R3.13) – a chemical mechanism – present a sequent of “real” (elementary) steps, which contributes to products formation. The ozone destruction can be written in a set of reactions:

,

(R3.14)

.

(R3.15)

Similarly, the net process is . The rate constants for reaction (R.312) and (R3.14) depend on light intensity, which in this case is the light intensity of the sun.

A reaction intermediate, an intermediate or intermediate species is a molecular entity that is formed from the reactants and reacts further to give the directly observed products of a chemical reaction. If the reaction mechanism includes these elementary steps:

A + B → X*,

(R3.16)

X* → C + D.

(R3.17)

The chemical species X* is an intermediate or intermediate species.