13.1 Ozone

  p Block Elements

Laboratory Preparation:

Ozone is obtained by passing silent electric discharge through dry oxygen. It is believed that some of the oxygen molecules dissociate and then atomic oxygen combines with oxygen molecules to form ozone.

\[\begin{gathered} {O_2}\xrightarrow{{Energy}}O + O \hspace{1em} \\ {O_2} + O \to {O_3} \hspace{1em} \\ 3{O_2} \rightleftarrows 2{O_3} - energy \hspace{1em} \\ \end{gathered} \]

The mixture obtained consists of $5-10%$ ozone by volume and the mixture is called ozonized oxygen.

The apparatus used for this purpose is known as 0zoniser. The commonly used ozonisers are:

(i) Siemen's ozoniser.

(ii) Brodie's ozoniser.

Manufacture: For the manufacture of ozonized air Seimen and Halske's ozoniser is employed.

Recovery of Pure Ozone from Ozonised Oxygen

The ozonised oxygen is passed through a spiral cooled by liquid air. Ozone condenses as its condensation temperature is $ - 112.4^\circ C$.
The liquid ozone thus obtained may contain some dissolved oxygen. This liquid is fractionally distilled to get pure ozone.

Some Reactions Forming Ozone

(i) Fluorine reacts with water at very low temperature when a mixture of ozonised oxygen is formed.

(ii) By action of ultra-violet rays, oxygen can be converted into ozone.

\[\begin{gathered} {O_2}\xrightarrow{{UV}}O + O \hspace{1em} \\ {O_2} + O \to {O_3} \hspace{1em} \\ \end{gathered} \]

(iii) The electrolysis of acidified water with high current density and platinum anode yields ozone on the anode. Nascent oxygen discharged at anode combines with molecular oxygen to form ozone.

Physical Properties:

(a) It is a pale blue gas with a characteristic of a strong smell.

(b) It is slightly soluble in water but more soluble in turpentine oil, glacial acetic acid, or carbon tetrachloride.

Chemical Properties:

(a) Decomposition:

It is unstable and decomposes completely into oxygen at $300^\circ C$.

\[2{O_3}\xrightarrow{{Heat}}3{O_2} + 68KCal\]

$MnO_2$, platinum black, silver, lead dioxide, etc. decomposes ozone at ordinary temperature, ie., they catalyze its decomposition.

(b) Oxidising nature:

Due to the ease with which it can liberate nascent oxygen, it acts as a powerful oxidising agent. The potential equation is:${O_3} \to {O_2} + O$.

The oxidation potential in acid medium is $ + 2.07V$.

$$\begin{equation} \begin{aligned} {O_3} + 2{H^ + } + 2{e^ - } \to {O_2} + {H_2}O\;\quad E^\circ = + 2.07V \\ 2HCl + {O_3} \to {H_2}O + C{l_2} + {O_2} \\\end{aligned} \end{equation} $$

(ii) Acidified ferrous sulphate into ferric sulphate.

\[2FeS{O_4} + {H_2}S{O_4} + {O_3} \to Fe{(S{O_4})_3} + {H_2}O + {O_2}\]

(iii) It liberates iodine from neutral $KI$ solution.

\[2KI + {H_2}O + {O_3} \to 2KOH + {I_2} + {O_2}\]

(iv) Lead sulphide (black) is oxidised to lead sulphate (colourless).

\[PbS + 4{O_3} \to PbS{O_4} + 4{O_2}\]

similarly, $CuS,ZnS$ and $CdS$ are oxidised to $CuS{O_4},ZnS{O_4}$ and $CdSO_4$, respectively.

(v) It oxidises nitrites into nitrates, sulphites into sulphates, arsenite to arsenates, manganate to permanganate and ferroxyanide to ferricyanide.

(vi) Moist iodine is oxidized to iodic acid,

Similarly, moist sulphur, phosphorus and arsenic are oxidized to their corresponding oxyacids.

(vii) $H_2S$ is oxidized to sulphur.

(viii) Alkaline $KI$ is oxidized to potassium iodate and periodate.

(ix) Dry iodine is oxidized to yellow powder, $I_4O_9$

(x) Silver metal is blackened due to alternate oxidation of the metal and reduction of oxide.

(xi) Mercury in presence of ozone is oxidized to suboxide which dissolves in mercury. It starts sticking to the glass and loses mobility. Hence, mercury loses its meniscus in contact with ozone. This is termed as the tailing of mercury.

(xii) Ozone reacts with $KOH$ and forms potassium ozonide, $KO_3$, which is an orange coloured solid contains paramagnetic ${O^{3 - }}$ ion.

In all above reaction, oxygen is evolved.

There are few reactions in which whole of the oxygen is used up in the process of oxidation.

1. Oxidises $SO_2$ to $SO_3$

2. Acidified stannous chloride is oxidized to stannic chloride.

(c) Bleaching Property:

It is a good bleaching agent. The bleaching action is due to its oxidizing action on the organic matter.$${\text{colored}}\;{\text{substance}} + O \to {\text{colorless}}$$

It bleaches oil, ivory, flour, starch, waxes, wood pulp, etc.

(d) Formation of ozonides:

Ozone reacts with unsaturated organic compounds containing double bonds. The products formed are known as ozonides. The ozonides break up when treated with water to form carbonyl compounds. The process is called ozonolysis, $H_2O_2$ is evolved in most of the cases.

(e) Reaction with peroxide:

Reaction of ozone with peroxide results in their mutual reduction with the liberation of oxygen.

Uses:

(i) As a germicide and disinfectant for sterlizing water and improving the atmosphere of crowded places like tube railways mines and cinema halls.

(ii) As a bleaching agent for oils, flour, ivory, wax and delicate fabrics,

Structure of Ozone: