Question

Describe the important oxidation reactions of ozone.

Answer 1

Oxidation of metals: Ozone oxidises certain metals like Ag and Hg to their respective oxides.

\[\ce{2Ag + O3 -> \underset{Silver oxide}{Ag2O} + O2}\]

\[\ce{2Hg + O3 -> \underset{Mercurous oxide}{Hg2O} + O2}\]

The mercury oxide produced in the reaction dissolves in mercury when ozone oxidises mercury. Mercury loses its meniscus as a result, and it begins to adhere to the glass's edges. This is known as mercury tailing.

Oxidation of non-metals: When a number of non-metals, including phosphorus, sulphur, iodine, and others, react with ozone in the presence of water, they are oxidised into their oxoacids.

(i) Moist phosphorus is oxidised to phosphoric acid.

\[\ce{P4 + 10O3 + 6H2O -> 4H3PO4 + 10O2}\]

(ii) Moist sulphur is oxidised to sulphuric acid.

\[\ce{S + 3O3 + H2O -> H2SO4 + 3O2}\]

(iii) Moist iodine is oxidised to iodic acid.

\[\ce{I2 + 5O3 + H2O -> 2HIO + 5O2}\]

Dry iodine, however, is converted to a yellow powder, I₄O₉.

Oxidation of metalloids: When water is present, metalloids such as arsenic and antimony undergo oxidation to produce their oxoacids.

(i) Moist arsenic is oxidised to arsenic acid.

\[\ce{2As + 5O3 + 3H2O -> 2H3AsO4 + 5O2}\]

(ii) Moist antimony is oxidised to antimonic acid.

\[\ce{2Sb + 5O3 + 3H2O -> 2H3SbO4 + 5O2}\]

Oxidation of compounds: Many substances undergo oxidation when exposed to ozone. Here are a few typical examples.

(i) Lead sulphide (black) is oxidised to lead sulphate (white).

\[\ce{PbS + 4O3 -> PbSO4 + 4O2}\]

(ii) Acidified ferrous sulphate is oxidised to ferric sulphate.

\[\ce{2FeSO4 + H2SO4  O3 -> Fe2(SO4)3 + H2O + O2}\]

(iii) Potassium iodide is oxidised to iodine.

\[\ce{2KI + H2O + O3 -> 2KOH + I2 + O2}\]