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Ozone is an extremely reactive gas which has been composed of three oxygen atoms. Ozone is both a natural as well as a man-made product. Ozone occurs in the Earth's upper atmosphere i.e., the stratosphere and lower atmosphere the troposphere. Being in a condition where ozone is found in the atmosphere, ozone affects life on Earth in good or bad ways.
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Key Terms: Oxygen, Group 15 Elements, Ozone, atoms, chemical reactions, Boiling Point
Structure of of ozone
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Structure of of Ozone
In the above diagram, the + and - show that the ozone molecules are short of electrons on the locations where these signs occur which means that ozone is a dipolar molecule and thereby causes ozone to have characteristic properties. It is seen that ozone reacts very selectively and is electrophilic.
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Physical Characteristics
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| Symbol | O3 |
| Atomic weight | 48g/mol |
| Melting point | -192,5 °C |
| Boiling point | -119,5 °C |
| Relative density (in air) | 1,7 kg/m3 |
| Solubility | Soluble in water |
| Odour | Pungent |
| Appearance | Pale blue gas |
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Chemical Properties
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Chemical Properties of Ozone
Ozone dissolves in water resulting in the formation of hydrogen peroxide. The chemical equation is given below.
O3 + 3H2O → 3H2O2
Ozone reacts with lead sulfide resulting in the formation of lead sulfate. The chemical equation is given below.
3PbS + 4O3 → 3PbSO4
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Chemical Reactions
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With metals
It is seen that ozone oxidizes with most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state. For example:
Cu + O3 → CuO + O2
Ag + O3 → AgO + O2
With nitrogen and carbon compounds
It also oxidizes nitric oxide to nitrogen dioxide:
NO + O3 → NO2 + O2
The above reaction is accompanied by chemiluminescence and therefore NO2 can be further oxidized to nitrate radical:
NO2 + O3 → NO3 + O2
In the above reaction, NO3 formed can react with NO2 to form N2O5.
Note that solid nitronium perchlorate can be made from NO2, ClO2, and O3 gases:
NO2 + ClO2 + 2O3 → NO2ClO4 + 2O2
It does not react with ammonium salts, but ozone oxidizes ammonia to ammonium nitrate:
2 NH3 + 4O3 → NH4NO3 + 4O2 + H2O
It should be noted that ozone reacts with carbon to form carbon dioxide at room temperature also:
C + 2O3 → CO2 + 2O2
With sulfur compounds
We will see that ozone oxidizes sulfides to sulfates.
Instance : lead(II) sulfide is oxidized to lead(II) sulfate:
PbS + 4O3 → PbSO4 + 4O2
In the reaction, the sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide:
S + H2O + O3 → H2SO4
3SO2 + 3 H2O + O3 → 3H2SO4
When at the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:
H2S + O3 → SO2 + H2O
At the aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid:
H2S + O3 → S + O2 + H2O
3H2S + 4O3 → 3H2SO4
Read More: Chemical Properties of Sulphur Dioxide
Preparations
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Process the preparation of ozone
Ozone can be prepared by passing a silent electric discharge through dry, unadulterated, and cold oxygen in an extraordinary device. The device that discharges ozone is known as the ozoniser. When we process the preparation of ozone, we obtain the gas of up to 10% concentration and this is an endothermic process and we must take care to complete it at high temperatures and that is the reason we use a silent electric discharge for the preparation of ozone.
When we want to produce higher concentrations of ozone, what we can do is that we can use the process of fractional liquefaction of an Oxygen and ozone mixture. By the fractional liquefaction of both the products, we can produce ozone at higher concentrations.
Read More: Manufacturing process of sulphuric acid
How does atmospheric ozone affect human health?
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Some Important Effects of Ozone on Humans
We have witnessed that ozone has two properties of interest to human health. First and foremost, ozone absorbs Ultraviolet light thereby reducing human exposure to harmful UV radiation that causes skin cancer and cataracts. Secondly, when we inhale the ozone gas, it reacts chemically with many biological molecules in the respiratory tract which gradually leads to a number of adverse health effects and health problems. Therefore, inhaling ozone gas can be dangerous.
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Uses of ozone
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Industrially, ozone is used to:
- Purify laundry in hospitals, food factories, care homes, etc
- Ozone helps in deodorized air and objects, such as after a fire. This process is extensively used in fabric restoration
- Ozone kills the bacteria on food or on contact surfaces
- It is seen that water-intensive industries such as breweries and dairy plants can make effective use of dissolved ozone as a replacement to chemical sanitizers such as peracetic acid, hypochlorite or heat.
- Purify cooling towers and control legionella with reduced chemical consumption, water bleed-off and increased performance.
- Helps in sanitizing swimming pools and spas
- Ozone kills the insects in stored grain.
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Tests of Ozone
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- Ozone turns alcoholic solution of benzidine brown
- The ozone gas turns moist starch-iodide paper blue
- Ozone does not reduce the acidified solution of KMnO4 ,K2Cr2O7.
Tests of Ozon
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Sample Questions
Ques. Give reasons why are Cl or Br atoms so effective in removing ozone molecules in the stratosphere? (2 marks)
Ans. They catalyze the conversion of O3 to O2. 105 O3 are converted for each Cl or Br atom:
O3 + Cl à O2 + ClO
ClO + O3 à O2 + O2 + Cl
ClO + O à O2 + Cl
2O3 à 3O2
Ques. What do you think is an ozone hole observed over Antarctica at certain times of the year? (1 mark)
Ans. It is because sunlight releases Cl on ice crystals in the form of ClONO2 (ClONO2 + hn à ClO + NO2), and we are off to the catalytic conversion of O3 to O2. The phenomenon occurs once per year in the Antarctic spring.
Ques. Is Ozone paramagnetic? (1 mark)
Ans. No, ozone is not paramagnetic.
Ques. Is ozone polar? (1 mark)
Ans. Yes, ozone is a polar gas.
Ques. Is ozone chiral? (1 mark)
Ans. No, ozone is not chiral.
Ques. Is ozone denser than air? (1 mark)
Ans. No, ozone is not denser than air
Ques. Does Ozone attract organic molecules containing double bonds? (1 mark)
Ans. Yes, Ozone attracts organic molecules containing double bonds.
Ques. Is Ozone the most thermodynamically stable form of oxygen? (1 mark)
Ans. No, ozone is not the most thermodynamically stable form of oxygen.
Ques. What is the structure of an O3 Molecule? (Delhi 2009)
Ans. Structure of ozone:
Ques. Account for the following:
(a) (i) Ozone is thermodynamically unstable.
(ii) Solid PCl5 is ionic in nature.
(iii) Fluorine forms only one oxoacid HOF.
(b) Draw the structure of (i) BrF5 (ii) XeF4 (Delhi 2016)
Ans. (a) (i) Ozone is thermodynamically very unstable because:
- The decomposition of ozone into oxygen is exothermic in nature. (ΔH = -ve)
- There is also increase in entropy which in turn makes ΔG -ve and reaction spontaneous.
(ii) In solid state, PCl5 exists as [PCl3]+ [PCl6]– in which the cation is tetrahedral and anion is octahedral. Because of the presence of strong attractive forces, it is a solid.
(iii) Due to absence of d-orbitals in fluorine, it can only form one oxoacid i.e., HOF.
(b) (i) Structure of BrF5:
Ques. Account for the following :
(i) PCl5 is more covalent than PCl3.
(ii) Iron on reaction with HCl forms FeCl2 and not FeCl3.
(iii) The two 0-0 bond lengths in the ozone molecule are equal. (Delhi 2014)
Ans. (i) In PCl5, phosphorus has +5 oxidation state and has less tendency to loose electrons than in +3 of PCl3. Therefore, PCl5 has more tendency to share e-1s than PCl3.
(ii) Because HCl on reaction with iron liberates H2 gas which prevents the formation of ferric chloride.
(iii) Due to resonance the two oxygen atoms have partial double bond character and thus have same bond length i.e. 128 pm
Ques. Which aerosol depletes ozone layer? (All India 2013)
Ans. Aerosols like foams, sprays etc. contain freons which are responsible for depletion of ozone layer.
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