This article is on the P Block Elements Notes Class 12 of Chemistry. The notes on P Block Elements of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the P Block Elements chapter of Class 12 for the students so that they can give a quick glance of the chapter.
This chapter of Class 12 has been divided into four articles. This article (Part 2) includes the Group 16 elements. The second article (Part 1) is on Group 15 elements. The third article (Part 3) is on Group 17 elements. At last, the fourth article (Part 4) is on Group 18 elements.
P block Elements Part 2
Group 16 Elements
O, S, Se, Te and Po constitute group 16 of the periodic table. These are collectively known as chalcogens (ore-forming). Their general valence shell electronic configuration is ns2 np4.
Oxygen is the most abundant of all the elements on earth. Like water, it comprises 89% by weight of the oceans. It occurs as ozone (O3) in the upper atmosphere. Oxygen occurs in the free form as O2 and makes 20.946% by volume of the atmosphere.
Sulphur is the sixteenth most abundant element and constitutes 0.03-0.1% by mass of the earth’s crust. It occurs in the combined form as sulphide ores as galena PbS, zinc blende ZnS, copper pyritesCuFeS2 and sulphate ores. Other elements such as selenium and tellurium occur as metal selenides and tellurides in sulphide ores. Polonium occurs in nature as a decay product of thorium and uranium minerals.
Different physical properties of group 16 elements are given below
They are smaller than corresponding elements of group 15 due to increased nuclear charge. Down the group, they increase mainly due to the addition of a new shell.
The elements of group 16 show a number of oxidation states. Oxygen shows —2 oxidation state except in the case of OF2 (+2), O2F2 (+1) and H2O2 (-1). Other elements of this group exhibit +2, +4 and +6 oxidation states but +4 and +6 are common. The stability of the +6 oxidation state decreases and +4 oxidation state increases down the group due to inert pair effect.
Ionisation enthalpy decreases down the group. It is due to the increase in size. However, the ionisation enthalpy of the elements of this group has lower values than those of group 15 due to the fact that group 15 elements have extra stable half-filled p-orbitals electronic configurations.
Electron Gain Enthalpy:
The oxygen atom, because of the compact nature, has less negative electron gain enthalpy than sulphur. However, the value again becomes less negative from sulphur onwards up to polonium.
Oxygen has the highest electronegativity value amongst the elements of this group, next to fluorine of group 17. Within the group; electronegativity decreases with an increase in atomic number.
These elements are little metallic because of IE. Down the group, metallic character increases due to the decrease in IE. Thus, O and S are non-metals but Se and Te have little metallic character while Po is metallic.
Melting and Boiling Points:
Melting and boiling points increase regularly from O to Te due to an increase in size and hence van der Waals’ forces. The last element Po has low melting and boiling point than Te because of the maximum inert pair effect.
Chemical properties of group 16 elements are
They form hydrides of the formula H2X.
(i) Thermal stability decreases down the group because the size of the atom increases and hence, bond length (E-H) decreases.
(ii) Acidic character increases down the group due to the decrease in bond enthalpy of H – E bond down the group.
(iii) Reducing nature: Except H2O, all are reducing agent. The reducing character increases down the group.
(iv) Boiling points: H2O has the highest boiling point due to extensive H-bonding. After a sudden drop from H2O to H2S, boiling point gradually increases from H2S to H2Te due to the increase in van der Waals’ forces.
These elements form MX2, MX4 and MX6 type halides (X = F, Cl, Br and I). All hexafluorides are gaseous and octahedral in shape. SF6 is exceptionally stable for steric reasons. MX4 (SF4 SeF4, etc) type halides have sp3d-hybridisation and have a trigonal bipyramidal structure in which one of the equatorial positions is occupied by a lone pair of electrons (see-saw geometry).
Dihalides are sp3 -hybridised and have a tetrahedral shape.
These elements form MO2 and MO3 (where M = S, Se, Te, Po) type oxides. SO2 is reducing while TeO2 is an oxidising agent. Both types of oxides are acidic in nature.
A binary compound of oxygen with other elements is called oxide. In many cases, one element forms two or more oxides. Oxides can be simple (MgO and Al2O3) or mixed (Pb3O4 and Fe3O4 ).
Dioxygen (O2) and sulphur in vapour state (S2) are paramagnetic in nature. It can be prepared from water or air on large scale. Electrolysis of water leads to the release of hydrogen at the cathode and oxygen at the anode. Dioxygen is obtained by the liquefaction of air followed by the distillation in industries to give dinitrogen and dioxygen.
Uses of Dioxygen:
O2 cylinders are widely used in hospitals, high altitude flying and in mountaineering. It is also used in oxyacetylene welding, in the manufacture of many metals mainly steel.
It is an allotropic form of oxygen. It is formed from oxygen in the presence of sunlight at a height of about 20 km. It is a pale blue gas, dark blue liquid and violet-black solid having a strong characteristic smell. It is a diamagnetic. It acts as a powerful oxidising agent due to the ease with which it liberates nascent oxygen atoms.
Uses of Ozone:
It is used as a strong oxidising agent in the manufacture of potassium permanganate (KMnO4) bleaching agent germicide and as a disinfectant and for sterilising water. It is used for bleaching oils, ivory, flour, starch, etc. The two oxygen-oxygen bond lengths in O3 molecule is identical and the molecule is angular.
Sulphur is the second member of the oxygen family. Both rhombic and monoclinic sulphur haveS8 molecules. The S8 ring in both forms has a crown shape. The ring adopts the chair form in cyclo S6.
Compounds of Sulphur:
- Sulphur Dioxide (SO2):
It is produced in the roasting of sulphide ores as a by-product. It behaves as a reducing agent (when moist). The two S— O bonds in SO2 molecule are equal due to two resonating structures. SO2 molecule is angular.
Uses of Sulphur Dioxide:
Used in refining petroleum and sugar, and in bleaching wool and silk. Liquid SO2 is used as a solvent and preparation of industrial chemicals like H2SO4, NaHSO3, _a_I-1503, are manufactured from sulphur dioxide.
- Oxoacids of Sulphur:
Sulphur forms a number of oxoacids such as
H2SO3 = Sulphurous acid H2SO4 = Sulphuric acid (king of chemicals)
H2S2O3 = Thiosulphuric acid H2S2O6 = Dithionic acid
H2SO5 = Peroxymonosulphuric acid
They are known in aqueous solution or in the form of their salts.
Structures of some important oxoacids of sulphur are
- Sulphuric Acid:
It is manufactured by the contact process. It forms two series of salts; i.e., normal sulphates and acid sulphates. The larger value of Ka indicates that H2SO4 is largely dissociated into H+ and HSO4– ions. Greater the value of Ka, the stronger is the acid.
2 SO2 (g) + O2 (g) –V2O5—> 2 SO3 (g)
SO3 + H2SO4 —–> H2S2O7
Concentrated H2SO4 acts as a strong dehydrating agent. It removes water even from organic compounds.
C12H12O11 –H2SO4—> 12C + 11H2O
Uses of Sulphuric Acid:
It is used in a heavy amount (e.g., ammonium sulphate, superphosphate, etc) in the manufacture of fertilisers, in petroleum refining and manufacture of paints, pigments and dyestuff intermediates. It is also used in storage batteries, in the manufacturing of nitrocellulose membrane and in detergent industry and metallurgical application like electroplating, galvanising, etc.
Some Important Reactions:
(i) For Dioxygen (O2)
2 KClO3 –∆/MnO4—>2KCl + 3O2 (g)
2PbO2 (s) —-> 2PbO(s) + O2 (g)
2H2O2(aq) —-> 2H2O (l) + O2 (g)
2 SO2 + O2— V2O5—-> 2 SO3 [Catalytic oxidation]
(ii) For Ozone (03) [Oxidising agent]
3O2 —> 2O3
PbS (s) + 4O3 (g) > PbSO4 (s) + 4O2 (g)
(iii) For Sulphur Dioxide:
2 SO2 + O2— V2O5—-> 2 SO3
SO2( g) + H2O (I) ——> H2SO3 (aq)
SO2 (g) + Cl2 (g) —–> SO2Cl2 (l)
(iv) For Sulphuric Acid (H2SO4 ):
Cu + 2 H2SO4 (conc.) –> CuSO4 + SO2 + 2H2O
3 S+ 2 H2SO4 (conc.) —–> 3SO2 + 2H2O
This article has tried to highlight all the important points of P Block Elements: Group 16 Elements in the form of notes for class 12 students in order to understand the basic concepts of the chapter.
Check Part 1 of the Chapter Here: Group 15 Elements – p block Elements
Check Part 3 of the Chapter Here: Group 17 Elements – p block Elements
Check Part 4 of the Chapter Here: Group 18 Elements – p block Elements