The Chemistry of Ozone

Scientists were observing rotten egg smell when an electric discharge was passed through air. It was later attributed that the rotten egg smell was due to the formation of a gas and it was named ozone. The name ozone derives from ozein (ὄζειν), the Greek word for smell.

Preparation of Ozone

Ozone is prepared by passing an electric discharge through dry oxygen. The device in which the preparation is done is known as the ozonizer. The following reactions take place.

O₂ + Energy ® O + O

O₂ + O ® O₃

The procedure usually invoves passing of dry oxygen from one end of the apparatus and obtaining ozonized oxygen from the other end. This mixture of ozone and oxygen is then passed into a zone of cool temperature where the temperature is around -115°C. At this temperature ozone condenses into a liquid and thus can be separated from oxygen.

Chemical Reactions of Ozone

Ozone acts as a good oxidising agent and its standard oxidation potential is +2.07V.

The preparation of ozone from oxygen is endothermic implying that the reverse reaction should be exothermic. Ozone easily decomposes to give dioxygen (O₂) and nascent oxygen (O).

Following are some of the reactions illustrating the oxidising property of ozone;

2FeSO₄ + H₂SO₄ + O₃ ® Fe₂(SO₄)₃ + H₂O + O₂

PbS + 4O₃ ® PbSO₄ + 4O₂

NaNO₂ + O₃ ® NaNO₃ + O₂

Na₂SO₃ + O₃ ® Na₂SO₄ + O₂

S + H₂O + O₃ ® H₂SO₄ + O₂

2As + 3H₂O + 5O₃ ® 2H₃AsO₄ + 5 O₂

2P + 3H₂O + 5O₃ ® 2H₃PO₄ + 5 O₂

Neutral KI is converted into I₂ and alkaline KI is converted into KIO₃ and KIO₄ according to the following reactions;

2KI + H₂O + O₃ ® 2KOH + I₂ + O₂

KI + 3O₃ ® KIO₃ + 3O₂

KI + 4O₃ ® KIO₄ + 4O₂

Moist I₂ and dry I₂ are converted into HIO₃ and I₄O₉ respectively according to the following reactions;

5O₃ + H₂O + I₂ ® 5O₂ + HIO₃

9O₃ + I₂ ® 9O₂ + I₄O₉

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide and in aqueous phase produced H₂SO₄ as well.

H₂S + O₃ ® SO₂ + H₂O

H₂S + O₃ ® S + O₂ + H₂O

3H₂S + 4O₃ ® 3H₂SO₄

When mercury reacts with O₃ it forms Hg₂O and starts sticking to the sides of the test tube. Because of this mercury loses its meniscus. This is called as the Tailing of Mercury.

2Hg + O₃ ® Hg₂O + O₂

Another major use of ozone is in the ozonolysis of unsaturated organic compounds. The reaction can be shown as below;

The structure of Ozone is given below. It exists as resonating structures.