Ozone (O₃) Chemistry

What is ozone?

Ozone is an unstable, strong oxidizing gaseous molecule comprised of 3 oxygen atoms

In nature, ozone (O₃ ) is formed by the action of UV radiation on oxygen (O₂) - which causes the 2 atoms in an oxygen molecule (O₂) to split up, after which each single oxygen atom joins another O₂ molecule to make ozone (O₃).

3 O₂ + UV Radiation → 2 O₃

Ozone can also be created when an electrical “spark” acts on oxygen.   E.g. that “clean” smell you detect after a lightning storm is ozone.

Ozone formation

In nature, ozone is created by Near UV radiation

3 ³O₂ → 2 O₃ Exothermic reaction (light energy to heat)

Molecular Oxygen + 240nm< UV< 310nm   → Ozone (O₃) Absorbs UV radiation

Achieved in 2 Steps:

Ozone (Lewis Dot Diagrams)

Some important ozone reactions

Ozone is a strong oxidant.   Oxidation occurs when any atom loses one or more electrons, giving it the ability  to combine with other substances. E.g. an oxygen atom can combine with other substances to form water and gases.

-   Oxidation potential of ozone is 2.7 -  meaning it is much more reactive than oxygen, which has an oxidation potential of 1.23

Ozone strongly absorbs UV radiation to produce singlet oxygen MOLECULES and active oxygen ATOMS

O₃ + hv (λ <= 313nm) → O₂ (¹ Δ_g ) + O(¹D )

O₃+ hv (λ <= 268nm) → O₂  (¹ Σ⁺ or ¹ Δ_g ) + O(¹D )

Ozone reacts with biological molecules to produce the reactive oxidant agent singlet oxygen ¹O₂^*  .     Singlet oxygen is a name given to several higher-energy species of molecular O₂ in which all the electron spins are paired.

 Kanofsky JR, Sima P Singlet oxygen production from the reactions of ozone with biological molecules. J Biol Chem. 1991 May 15;266(14):9039-42. PubMed

Ozone reacts with the unsaturated fatty acids of the lipid layer in cellular membranes, forming hydro-peroxides.    Lipid peroxidation products include alkoxyl RO^▪and peroxyl radicals ROO^▪, singlet oxygen ¹O₂^*, ozonides, carbonides, carbonyls, alkanes and alkenes.

•  If targeted cell membrane does not have sufficient protective antioxidant and antioxidant enzymes (SOD, CAT, GPx and reductase), it will be damaged and destroyed by reactive hydro-peroxide products - such as ¹O₂^* , RO^▪ and ROO^▪.   Healthy cell membranes contain protective antioxidant enzymes, many bacterial cell walls lack them.

E.g. Glutathione catalyzed by glutathione peroxidase reduces/neutralizes lipid hydroperoxides

Ozone is destroyed by hydroxyl radicals.    The reaction converts ozone into oxygen.

O₃ + OH^• → O₂ + HO₂

HO₂ + O₃ → OH^• + 2O₂

EXCITED OXYGEN ATOM oxidizes water to hydrogen peroxide.

O₁^* + H₂O→ H₂O₂

Ozone will oxidize most metals (except gold, platinum and iridium) to oxides of their metals in their highest oxidation state

E.g. 2 Cu⁺ + 2 H₃O⁺ + O₃ →2 Cu²⁺ + 3 H₂O + O₂

Ozone oxidizes nitric oxide to nitrogen dioxide:

NO + O₃ → NO₂ + O₂

Ozone oxidizes ammonia to ammonium nitrate-but does not react with ammonium salts

2 NH₃ + 4 O₃ → NH₄NO₃ + 4 O₂ + H₂O

Ozone reacts with carbon to form carbon dioxide:

C + 2 O₃ → CO₂ + 2 O₂

Ozone oxidizes sulfides to sulfates

PbS + 4 O₃ → PbSO₄ + 4 O₂

Ozone gas reacts with hydrogen sulfide to form sulfur dioxide:

H₂S + O₃ → SO₂ + H₂O

Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide:

S + H₂O + O₃ → H₂SO₄

3 SO₂ + 3 H₂O + O₃ → 3 H₂SO₄

Ozone Decay

Ozone is unstable at high concentrations and decays to oxygen if it is does not contact something oxidizable - ozone has half-life of ~ 30 mins. in atmospheric conditions, but proceeds more rapidly with increasing temperature and pressure

2 O₃ → 3³O₂

In 2steps:

O₃ → ³O₂ + O^*

O^*+ O^*→ ³O₂

Ozone layer depletion

Ozone depletion is caused by two non toxic, non flammable, normally very slowly or non reactive groups of gases: chlorofluorocarbons (CFCs) and halons.   Used as refrigerant, propellants in aerosol spray cans, cleaning solvents in the manufacture of electronic circuit boards and as a blowing agent in the manufacture of foam insulation and furniture foam.

The CFC or halon molecule makes its way to the upper atmosphere where UV light can separate chlorine atoms from the CFC molecule and bromine from the halon. Once released, the chlorine or bromine molecule is capable of destroying ozone molecules in a continuous, repeating chemical reaction that allows a single chlorine or bromine atom to react with as many as 100,000 ozone molecules before finally settling out below the ozone layer

The process by which catalysts such as chlorine (E.g. in the CFC molecule shown above), bromine, nitrogen oxide and hydrogen destroy atmospheric ozone (O3).
Source : Adapted from Hengeveld, 1991.