Catalytic destruction of ozone
Definition and meaning of Catalytic destruction of ozone in chemistry.
Catalytic destruction of ozone is the chemical breakdown of stratospheric ozone (O3) into diatomic oxygen (O2) through a repeating two-step cycle in which a trace catalyst, such as a chlorine or bromine atom, reacts with ozone and is then regenerated so it can destroy many more ozone molecules.
In more detail
The general cycle is X + O3 → XO + O2, followed by XO + O → X + O2, where X is a catalytic species (commonly Cl, Br, NO, or OH) released from anthropogenic or natural precursors. Because the catalyst X is regenerated at the end of the cycle, it is not consumed overall, allowing a single chlorine atom to destroy roughly 100,000 ozone molecules before being removed by a termination reaction. This process, driven mainly by chlorine and bromine atoms from human-made chlorofluorocarbons (CFCs) and halons, is responsible for stratospheric ozone depletion, including the Antarctic ozone hole.
Key facts
| Formula | O3 → (3/2) O2, net; catalyst X unchanged |
|---|---|
| Field | Inorganic Chemistry |
| Key catalysts | Cl, Br, NO, OH radicals |
| Catalytic cycle length | ~10^4–10^5 O3 molecules destroyed per Cl atom |
Chlorine atoms released from CFC photolysis destroy ozone via: Cl + O3 → ClO + O2, then ClO + O → Cl + O2; the net reaction is O3 + O → 2 O2, with Cl acting purely as a catalyst.
Frequently asked questions
Why is chlorine such an effective ozone-destroying catalyst?
Because it is regenerated at the end of each cycle rather than consumed, a single chlorine atom can participate in tens of thousands of ozone-destroying cycles before a termination reaction (forming a reservoir species like HCl or ClONO2) removes it.
What stops the catalytic cycle?
Termination reactions convert the reactive catalyst into stable 'reservoir' species (e.g., HCl, ClONO2) that temporarily hold the chlorine or bromine out of the ozone-destroying cycle until they are later photolyzed or reactivated.