Ozone

Ozone

Ozone is an allotropic form of oxygen. It is present in the upper atmosphere (about 20 km above the surface of the earth). It is believed to be formed in the upper atmosphere by the action of ultraviolet rays on oxygen as

 

3O₂ + Ultraviolet rays → 2O₃
 

ΔH298K = 142.7 kJ mol^-1

 

Ultraviolet rays, which are harmful to human beings, are absorbed by oxygen to form ozone. The layer of ozone thus formed also prevents the remaining ultraviolet rays to reach the earth’s surface.

 

Preparation of Ozone

 

Ozone is prepared by passing silent electric discharge through pure, cold and dry oxygen in a specially designed apparatus called ozoniser. The formation of ozone from oxygen is an endothermic reaction.

 

3O₂ + Ultraviolet rays → 2O₃

ΔH = 142.7 kJ mol^-1

 

Since the formation of ozone is an endothermic process, therefore, it is necessary to use a silent electric discharge (a sparkless electric discharge).The purpose of silent electric discharge (a sparkless electric discharge) is to produce less heat. If concentration of ozone greater than 10% are required, a battery of ozonisers can be used and the pure ozone (b.p. 385 K) can be condensed in a vessel surrounded by liquid oxygen.

 

Ozone is prepared in the laboratory by the following two types of ozonisers:

(i) Siemen’s ozoniser 

(ii) Brodie’s ozoniser

 

Preparation of pure Ozone

In order to get pure ozone, the ozonised oxygen is cooled by liquid air when ozone (b.p. 3855 K) condenses in preference to oxygen (b.p. 90 K). The liquid thus formed still contains some dissolved oxygen which can be removed by carrying out fractional distillation.

 

Properties of Ozone

Physical Properties of Ozone

(i) It is pale blue gas having pungent odour.

 

(ii) It is heavier than air. Its vapour density is 24 while that of air is 14.4. Therefore, it is about 1.5 times heavier than air.

 

(iii) Its boiling point is 385 K and melting point is 24 K.

 

(iv) It is slightly soluble in water but more soluble in organic solvents like turpentine oil, carbon tetrachloride, etc.

 

(v) It liquefies to a deep blue liquid at 161.2 K which can be solidified to a black violet solid at about 80.6 K.

 

(vi) In contrast to dioxygen which is paramagnetic, ozone is diamagnetic.

 

(vii) The small concentrations of ozone are harmless. However, if the concentration rises above about 100 ppm, breathing becomes uncomfortable resulting in headache and nausea.

 

Chemical Properties of Ozone 

The important chemical properties of ozone are discussed below :

 

(1) Action with litmus

Ozone is neutral towards litmus because it does not give any colour change with blue or red litmus.

(2) Decomposition

Ozone is thermodynamically unstable with respect to oxygen because it results in liberation of heat (ΔH is negative) and increase in entropy (ΔS is positive). These two factors reinforce each other resulting in large negative Gibb’s free energy change (ΔG is negative) for its conversion into oxygen. Therefore, the high concentrations of ozone can be dangerously explosive.

 

On heating above 475 K, it decomposes readily to form oxygen gas :

 

2O₃ → 3O₂

ΔH = – 285.4 kJ

 

The reaction is catalysed by substances like platinum black, manganese dioxide, copper oxide, etc.

 

(3) Oxidising agent

Ozone is a stronger oxidising agent than molecular oxygen because ozone has higher energy content and decomposes to give atomic oxygen as :

O₃ → O₂ + O

Therefore, ozone oxidises a number of non-metals and other reducing agents. 

 

For example

(a) Oxidation of compounds

 

(i) Ozone oxidises black lead sulphide to white lead sulphate :

O₃ → O₂ + O ]×4

PbS + 4O →  PbSO₄ 

————————————

PbS + 4O₃ → PbSO₄ +4O₂

————————————–

Similarly, ozone oxidises sulphides of copper, zinc, cadmium, etc. to their corresponding sulphates.

ZnS + 4O₃ → ZnSO₄ + 4O₂

CuS + 4O₃ → CuSO₄ + 4O₂

CdS + 4O₃ → CdSO₄ + 4O₂

 

(ii) Ozone oxidises halogen acids to corresponding halogens

O₃ → O₂ + O

2HCl + O → H₂O + Cl₂

—————————————

2HCl +O₃  → H₂O + Cl₂ + O₂

_{————————————————}

 

(iii) Ozone oxidises nitrites to nitrates

O₃  → O₂ + O 

KNO₂ + O₃ —-> KNO₃ + O₂

————————————–

KNO₂ + O₃ → KNO₃ + O₂

—————————————

 

(iv) Ozone oxidises acidified ferrous salts to ferric salts

O₃  → O₂ + O 

2 FeSO₄ + H₂SO₄ + O —–> Fe₂(SO₄)₃ + H₂O

————————————————————

2 FeSO₄ + H₂SO₄ + O₃ —–>Fe₂(SO₄)₃ + H₂O + O₂

_{————————————————————————–}

 

(v) Ozone oxidises Potassium iodide to iodine

_{O3 → O2 + O}

_{2kI +} H₂O _{+ O → 2KOH +}  I₂

_{———————————————-}

2KI+ H₂O + O_{3 →} 2KOH + I₂ + O₂

_{———————————————–}

_{(vi) Ozone oxidises potassium ferrocyanide to potassium ferricyanide}

O₃ → O₂ + O 

2K₄[Fe(CN)₆] + H₂O + O → 2K₃[Fe(CN)₆] + 2KOH

———————————————————————–

2K₄[Fe(CN)₆] + H₂O + O₃ → 2K₃[Fe(CN)₆] + 2KOH + O₂

————————————————————————-

 

(vii) Ozone oxidises potassium manganate to potassium permanganate

O₃ → O₂ + O

2K₂MnO₄ + O + H₂O _→ 2KMnO₄ + 2KOH 

———————————————————-

2K₂MnO₄ + O₃ + H₂O _→ 2KMnO₄ + 2KOH + O₂

_{———————————————————————-}

 

 

(b) Oxidation of non-metals

 

(viii) Ozone oxidises moist iodine to iodic acid

O₃ → O₂ + O ]×5

I₂ + 5 (O) → I₂O₅

I₂O₅ + H₂O _→ 2 HIO₃

———————————————

I₂ + 5O₃ + + H₂O _→ 2 HIO₃ + 5 O₂

_{——————————————————}

(ix) Ozone oxidises Sulphur to sulphuric acid

O₃  → O₂ + O ]×3

S + 3O → SO₃

SO₃ + H₂O _→ H₂SO₄

——————————————

S + 3O₃ + H₂O _→ H₂SO₄ + 3O₂

——————————————

 

(x) Ozone oxidises Phosphorus to Phosphoric acid

O₃  → O₂ + O ]×10

P₄ + 10 O → P₄O₁₀

P₄O₁₀ + 6H₂O _→ 4 H₃PO₄ 

—————————————————-

P₄ + 10 O₃ + 6 H₂O _→ 4 H₃PO₄ + 10O₂

—————————————————-

 

(c) Oxidation Of metalloids

 

(i) Oxidation Of moist arsenic

O_{3 →} O₂ + O ]×5

2As + 5O → As₂O₅

As₂O₅ + 3H₂O _→ 2H₃AsO₄

————————————————–

2As + 5O₃ + 3H₂O _→ 2H₃AsO₄ + 5O₂

————————————————–

 

(ii) Oxidation of moist antimony

O_{3 →} O₂ + O

2Sb + 5O _→ Sb₂O₅

Sb₂O₅ + 3H₂O _→ 2H₃SbO₄

————————————————–

2Sb + 5O₃ + 3 H₂O _→ 2H₃SbO₄ + 5O₂

—————————————————

 

(d) Oxidation of metals

 

(i) Silver is oxidised to silver oxide

O_{3 →} O₂ + O

2Ag + O → Ag₂O

——————————-

2Ag + O_{3 →} Ag₂O + O

——————————-

 

(ii) Mercury is oxidised to mercurous oxide 

O_{3 →} O₂ + O

2Hg + O _→ Hg₂O 

———————————-

2Hg + O₃ → Hg₂O  + O₂

———————————–

 

(4) Bleaching agent

Due to the oxidising action of ozone, it acts as a mild bleaching agent as well as a sterilising agent. It acts as a bleaching agent for vegetable colouring matter.

 

Vegetable colouring matter + O_{3 ^→} Oxidised coloured matter + O₂

(colourless)

 

(5) Formation of ozonides
Ozone reacts with alkenes in the presence carbon tetrachloride (CCl₄) to form an ozonide.

For example:

 

(6) Action with peroxides

Ozone reacts with peroxides such as hydrogen peroxides and barium peroxide and liberates oxygen.

H₂O₂ + O_{3 →} H₂O + 2O₂

Ba₂O₂ + O₃ → BaO + 2O₂

 

Structure of Ozone

 

(1) The structure of ozone molecule is angular.

 

(2) The O-O-O bond angle is 116.8° and 0-0 bond length is 127.8 pm.

 

(3) The bond length in ozone molecule is intermediate between single and double bonds in oxygen atoms (single 0-0 bond = 148 pm, double 0=0 bond = 122 pm). Ozone molecule thus, is considered to be a resonance hybrid of the following two resonating structures (a) and (b).

 

 

Uses of Ozone

 

(1) Ozone is used for disinfecting and sterilising water because ozone has germicidal properties.

 

(2) It is used for bleaching, flour delicate fabrics, oils, flour, starch, ivory, etc.

 

(3) It is used for purifying air of crowded places such as cinemas, underground railways, auditoriums, tunnels, mines, etc. and for destroying objectionable odours in slaughter houses.

 

(4) It is used in industry for the manufacture of potassium permanganate, artificial silk, synthetic camphor etc.

 

(5) It is used in the laboratory for the ozonolysis of organic compounds.