知识阅读 How Ozone is Formed
Ozone （臭氧）is formed in the stratosphere （平流层） by the action of ultraviolet light （紫外光） on oxygen molecules：
3O2
2O3
Experimental evidence suggests that this reaction proceeds in a two-step mechanism：
O22O· step 1
O·+O2
O3 step 2
In the first step an oxygen molecule absorbs a photon （hν） （光子）of high-energy ultraviolet light （λ<280nm）. The energy of the photon breaks the bond of the O2 molecule， which fragments into two oxygen atoms.Oxygen atoms are reactive species that can add to O2 molecule to produce ozone.The ozone molecule formed in this step contains excess energy，and unless that energy is transferred to some other species， the ozone molecule breaks apart to regenerate O2 and an oxygen atom.In the stratosphere， however， high energy ozone molecules usually collide with nitrogen molecules and give up their excess energy before they can break apart.
As written， steps 1 and 2 do not add together to give the balanced chemical equation. However， a valid mechanism must lead to the correct reaction stoichiometry. In the ozone process，two oxygen atoms react with two O2 molecules to give two ozone molecules. In the complete mechanism，the second step occurs twice each time the first time step occurs：
O·+O2O3 O·+O2O3
Chemists find it convenient to write this elementary process just once，but they understand that step 2 must occur twice to write this elementary process just once，but they understand that step 2 must occur twice to consume both the oxygen atoms produce in step 1.Writing all three steps shows that the mechanism does lead to the correct stoichiometry：
O2
2O·
O·+O2O3
O·+O2O3
3O22O3
The oxygen atoms produced in the initial fragmentation step are extremely reactive，so as soon as oxygen atom forms， it is “snapped up” by the nearest available O2 molecule. Consequently， the cleavage of an O2 molecule by a photon is the rate-determining step （定速步骤） in the formation of ozone.