CHAPTER-7

CLASS-XII

THE p- BLOCK ELEMENTS

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Prepared by

Mr. B.SENTHAMIZHSELVAN

PGT CHEMISTRY

JNV MYSORE

KARNATAKA

HYDERABAD REGION

PART -3

Contents

• Group 16 elements : Oxygen family
• Introduction
• Occurrence, General group Characters
• Periodic trends
• Preparation, Physical and Chemical Properties and uses of :
• Dioxygen and Ozone
• Classification of oxides

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PART - 3

GROUP 16 ELEMENTS [ CHALCOGENS]

Oxygen

GROUP 16

Sulphur

Selenium

Tellurium

Polonium

Group 16 includes:

1. Oxygen (O)
2. Sulphur (S)
3. Selenium (Se)
4. Tellurium (Te)
5. Polonium (Po)

GROUP 16 ELEMENTS- ELECTRONIC CONFIGURATION: ns²np⁴

OCCURRENCE OF OXYGEN:

• Oxygen is the most abundant of all the elements on earth.
• Oxygen forms about 46.6% by mass of earth’s crust.
• Dry air contains 20.946%

oxygen by volume.

OCCURRENCE OF SULPHUR:

• The abundance of sulphur in the earth’s crust is only 0.03-0.1%. Combined sulphur exists primarily as sulphates such as

gypsum CaSO4.2H2O, epsom salt MgSO4.7H2O, baryte BaSO4 and

sulphides such as galena PbS, zinc blende ZnS, copper pyrites CuFeS2.

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• Traces of sulphur occur as hydrogen sulphide in volcanoes. Organic

materials such as eggs, proteins, garlic, onion, mustard, hair and wool

contain sulphur.

� Po:

Selenium and tellurium are also found as metal selenides and tellurides in sulphide ores.

Polonium occurs in nature as a decay product of thorium and uranium minerals.

Atomic and Ionic Radii

• Due to increase in the number of shells, atomic and ionic radii increase from top to bottom in the group.
• The size of oxygen atom is, however, exceptionally small.

o

Se

Po

Te

S

IONIZATION ENTHALPY

• Ionisation enthalpy decreases down the group. It is due to increase in size. However, the elements of this group have lower ionisation enthalpy values compared to those of Group15 in the corresponding periods.
• This is due to the fact that Group 15 elements have extra stable half-filled p orbitals electronic configurations.

Electron Gain Enthalpy

• Because of the compact nature of oxygen atom, it has less negative electron gain enthalpy than sulphur.
• However, from sulphur onwards the value again becomes less negative up to polonium.

Electronegativity

• Next to fluorine, oxygen has the highest electronegativity value amongst

the elements.

• Within the group, electronegativity decreases with an increase in atomic number.
• This implies that the metallic character

increases from oxygen to polonium

Q. Why water is a liquid and H2S is a gas though oxygen and sulphur belong to the same group?

Ans. Oxygen atom has a small size and high electronegativity, therefore, it is capable of forming hydrogen bonds which is not possible in H₂S. Due to the presence of hydrogen bonding it has a compact structure and is present in liquid state.

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Q. Why does the stability of +4 oxidation state increase down the group while that of +6 decrease ?

Ans. The general outer electronic configuration of 16 group elements is ns²np⁴ . Therefore, the common oxidation state would be +4 and +6 . Stability of +4 will increase due to the inert pair effect.

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Q. Why does oxygen limit its covalency to 4? OR Why cannot oxygen expand its covalency beyond 4 while the other elements can do so?

Ans. This is due to the absence of the d-orbitals in the oxygen atom.

Q :Elements of Group 16 generally show lower value of first ionisation

enthalpy compared to the corresponding periods of group 15. Why?

Ans : Due to extra stable half-filled p orbitals electronic configurations of Group 15 elements, larger amount of energy is required to remove electrons compared to Group 16 elements.

PHYSICAL PROPERTIES OF OXYGEN FAMILY ELEMENTS

CHEMICAL PROPERTIES OF OXYGEN FAMILY ELEMENTS

• The stability of -2 oxidation state decreases down the group.

Polonium hardly shows –2 oxidation state.

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• Since electronegativity of oxygen is very high, it shows only negative oxidation state as –2 except in the case of OF2 where its oxidation state is + 2.

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• Other elements of the group exhibit + 2, + 4, + 6 oxidation states but + 4 and + 6 are more common.

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• Sulphur, selenium and tellurium usually show + 4 oxidation state in their compounds with oxygen and + 6 with fluorine.

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• The stability of + 6 oxidation state decreases down the group and stability of + 4 oxidation state increase (inert pair effect). Bonding in +4 and +6 oxidation states are primarily covalent.

Anomalous behavior of Oxygen:

Reactivity towards Hydrogen

Reactivity towards Hydrogen

• All the elements combine with hydrogen to form hydrides of the type H₂E
• Acidic character increases from top to bottom because of decrease in bond dissociation enthalpy down the group
• Thermal stability decreases down the group because of the increase in the bond length
• Except water all hydrides have reducing property and this increases down the group.

Reasoning Questions:

Q: H₂S is less acidic than H₂Te. Why?

A: Due to the decrease in bond (E–H) dissociation enthalpy down the group, acidic character increases.

Q: H₂Te is the strongest reducing agent amongst the group 16 hydrides. Why?

A: Down the group bond enthalpy decreases making the loss of H easier, hence reducing property increases. Te is the last member, hence the strongest reducing agent.

Physical properties of hydrides:

Reactivity towards Oxygen:

• All the elements form oxides of the formula EO₂ and EO_3.
• Reducing properties of dioxides decreases down the group.
• All oxides are acidic in nature .

Q: Reducing property of dioxide decreases from SO₂ to TeO₂ . Why? Or SO₂ is reducing while TeO₂ is an oxidising agent. Why?

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Sulphur is reducing agent because SO2 has d orbital so it can easily expand. Its +4 os to +6 os and can be behave as reducing agent.

But in TeO2,Te is heaver element.electrons are reluctant to take part in bond formation due to poor shielding of d- and f-orbitals thus lower oxidation states (+2, -2) becomes more stable( inert pair effect) and hence TeO2 is oxidising reagent.

Reactivity towards Halogens

• They form a large number of halides of the formula EX₆, EX₄, EX₂
• The stability of the halides decreases down the group, in the order F > Cl > Br > I ( Stability decreasing )
• Among Hexahalides, Hexa fluorides are the only stable halides. They are gaseous in nature and have octahedral structure.
• Sulphur hexafluoride, SF₆ is exceptionally stable for steric reasons.

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• Amongst tetrafluorides, SF₄ is a gas, SeF₄ a liquid and TeF₄ a solid.
• These fluorides have sp³d hybridisation and thus, have trigonal bipyramidal structures in which one of the equatorial positions is occupied by a lone pair of electrons. This geometry is also regarded as see-saw geometry.
• All elements except selenium form dichlorides and dibromides. These dihalides are formed by sp³ hybridisation and thus, have tetrahedral structure.
• The well known monohalides are dimeric in nature. Examples are S₂F₂, S₂Cl₂, S₂Br₂, Se₂Cl₂ and Se₂Br₂.
• These dimeric halides undergo disproportionation as given below:

2Se₂Cl₂ → SeCl₄ + 3Se

DIOXYGEN ( O₂ )

Preparation of Dioxygen:

1. From KClO₃ :

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2. From Oxides :

(Thermal decomposition of oxides of metals)

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3. From decomposition of hydrogen peroxide:

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1. On large scale it can be prepared from water or air. Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode.��2. Industrially, dioxygen is obtained from air by first removing carbon dioxide and water vapour and then, the remaining gases are liquefied and fractionally distilled to give dinitrogen and dioxygen.

Properties of O₂

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• Dioxygen is odourless and colourless
• It is paramagnetic inspite of having ‘even’ number of electrons
• It reacts with nearly all metals and

non-metals except gold, platinum and some noble gases.

Some of the common reactions of Dioxygen are:

USES OF DIOXYGEN GAS

• In addition to its importance in normal respiration and combustion processes, oxygen is used in oxyacetylene welding, in the manufacture of many metals, particularly steel. Oxygen cylinders are widely used in hospitals, high altitude flying and in mountaineering.
• The combustion of fuels, e.g., hydrazines in liquid oxygen, provides tremendous thrust in rockets.

OXIDES

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Oxides are a binary compound of oxygen with another element is called oxide.

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Oxides can be simple (MgO, Al₂O₃) or mixed (Pb₃O₄, Fe₃O₄).

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Another way of classification of simpe oxides is-

a) Acidic

b) Basic

c) Amphoteric

d) Neutral

Ex: CO₂, SO₂

Ex : Na₂O, CaO

Ex : Al₂0₃, ZnO

Ex: CO, NO, N₂O

Reactions showing Al₂O₃ as amphoteric oxide

• Ozone is an allotropic forms of oxygen
• It is formed from atmospheric oxygen in the presence of sunlight
• Ozone protects the earth’s surface from excessive concentration of ultraviolet (UV) radiations
• It is a pale blue gas, dark blue liquid or violet-black solid

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OZONE

PREPARATION OF OZONE GAS

• When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs. The product is known as ozonised oxygen.

3O₂ → 2O₃ ΔH^O (298 K) = +142 kJ mol^–1

• Since the formation of ozone from oxygen is an endothermic process, it is necessary to use a silent electrical discharge in its preparation to prevent its decomposition.
• If concentrations of ozone greater than 10 per cent are required, a battery of ozonisers can be used, and pure ozone (b.p. 385 K) can be condensed in a vessel surrounded by liquid oxygen.

• Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat (ΔH is negative) and an increase in entropy (ΔS is positive). These two effects reinforce each other, resulting in large negative Gibbs energy change (ΔG) for its conversion into oxygen.

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• Due to the ease with which it liberates atoms of nascent oxygen (O₃ → O₂ + O), it acts as a powerful oxidising agent. For e.g., it oxidises lead sulphide to lead sulphate and iodide ions to iodine.

PbS(s) + 4O₃(g) → PbSO₄(s) + 4O₂(g)

2I–(aq) + H₂O(l) + O₃(g) → 2OH–(aq) + I₂(s) + O₂(g)

Q: How is O₃ estimated

quantitatively?

A: When ozone reacts with an excess of potassium iodide solution, iodine is liberated which can be titrated against a standard solution of sodium thiosulphate. This is a quantitative method for estimating O₃ gas.

• The structure of ozone is as follows

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Q: All the O-O bonds in ozone are equivalent. Explain why?

Ans: The two oxygen-oxygen bond lengths in the ozone

molecule are identical (128 pm) and the molecule is angular

as expected with a bond angle of about 117o. It is a

resonance hybrid of two main forms.

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• Ozone saves us from the UV radiations but there is a depletion in the ozone concentration in the upper layer of the atmosphere due to supersonic jet airoplanes and use of freons which are used in the aerosol sprays and as refrigerants .

USES OF OZONE

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• It is used as a germicide, disinfectant and for sterilising water.

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• It is also used for bleaching oils, ivory, flour, starch, etc. It acts as an oxidising agent in the manufacture of potassium permanganate.