The p-Block Elements Class 12th Notes - Free NCERT Class 12 Chemistry Chapter 7 Notes - Download PDF

The ncert Class 12 Chemistry chapter 7 Notes talks about the p-block elements. The main topics covered in Class 12 Chemistry chapter 7 notes are general trends in the Chemical properties of elements of groups 15,16,17 and 18; preparation, properties and applications of N2 and phosphorus and some of their important compounds; preparation, properties and uses of oxygen and ozone and chemical properties of some simple oxides; allotropic forms of sulphur, chemical properties of its important compounds; preparation, properties and uses of chlorine and hydrochloric acid; chemistry of interhalogens; uses of noble gases; and importance of these elements in our day to day life. These the p-block elements Class 12 notes also include some FAQs to clarify some of the most frequent and important questions.

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NCERT Class 12 Chemistry Chapter 7 Notes

Group 15 Elements

Group 15 elements are Nitrogen, phosphorus, arsenic, antimony, and bismuth. As we move down the group there is a transition from non-metallic to metallic character through metalloidic property, here phosphorus and nitrogen are non-metals, arsenic and antimony metalloids and bismuth is a metal.

Electronic Configuration:

The electronic configuration of group 15 elements can be written as . They have an extra stable electronic configuration, since the s orbital of these elements is completely filled and p orbitals are half-filled.

Atomic and Ionic radii:

Generally, covalent and ionic radii increase as we go down the group. There is a considerable

increase in covalent radius from N to P due to the addition of new orbitals. However, as we move from As to Bi we only observe a small increase in covalent radius. This is because of the poor shielding effect provided by fully filled d or f orbitals, for which valence electrons are more tightly pulled towards the nucleus.

Ionisation enthalpy:

With the increase in atomic size, the ionisation enthalpy of this group decreases down the group.

Electronegativity:

The electronegativity value also decreases down the group with increasing atomic size.

Physical properties:

Due to a decrease in ionisation enthalpy and increase in atomic size, the metallic character increases down the group, therefore nitrogen and phosphorus are non-metals, arsenic and antimony metalloids and bismuth is a metal. The boiling points, increase down the group but the melting point increases till arsenic and then decreases to bismuth.

Chemical properties:

Oxidation states: +3, +5, and –3 are the most prevalent oxidation states. Because of the decrease in electronegativity caused by the rise in atomic size, the probability to show a –3 oxidation state diminishes down the group. Due to the inert pair effect, the stability of the +5 oxidation state falls while the stability of the +3 oxidation state improves. The oxidation states of nitrogen are from –3 to +5. In an acidic media, nitrogen and phosphorus with oxidation states ranging from +1 to +4 undergo oxidation and reduction. This is referred to as disproportionation.

Reactivity towards Hydrogen: Group 15 elements form trihydrides, ; with the decrease in bond dissociation energy down the group, the stability of hydrides decreases down the group .

Reactivity towards Oxygen: Group 15 elements form trioxides () and pentoxides (). Atomic size increases down the group and thus its ability to accept electrons decreases. As the distance between valence shell and nucleus decreases the ability to accept electrons decreases. By Lewis, definition acid accepts a pair of valence electrons. Hence the acidic properties of oxides of group 15 decrease while basic properties increase down the group.

Reactivity towards Halogens: Group 15 elements react with halogens to form trihalides and pentahalides. Nitrogen can't form pentahalides because, as it doesn't have d-orbitals. Trihalides have a pyramidal shape and covalent characteristic decreases down the group. Pentahalides behave as lewis acids because of the presence of vacant d – orbitals and have trigonal bipyramidal shapes.

Reactivity towards Metal: All elements of group 15 react with metals to form binary compounds in the –3 oxidation state.

Dinitrogen

Preparation: Following are some reactions to prepare dinitrogen—

Properties: Dinitrogen is a colourless, tasteless, odourless and non-toxic gas. Dinitrogen is inert at room temperature due to the high bond enthalpy of bond.

Uses: Dinitrogen is used in the manufacturing of ammonia and other industrial chemicals containing nitrogen, (e.g., calcium cyanamide). Its application also includes where an inert atmosphere is necessary, like that in the iron and steel industry, inert diluent for reactive chemicals. Liquid dinitrogen finds application as a refrigerant to preserve biological materials, in cryosurgery and food items.

Ammonia

Preparation: Following are some reactions to prepare dinitrogen—

Commercially, ammonia is manufactured by Haber’s process as given below:

At a pressure of , a temperature of along with catalyst iron oxide with small amounts of and s used to to aid in attaining the equilibrium, faster.

Properties: Ammonia is a colorless gas. The molecule has a trigonal pyramidal shape with nitrogen atom at the top position and three hydrogen atoms at the bottom. It has a pungent smell. The presence of a lone pair on the nitrogen makes it a lewis base.

Uses: Ammonia is used to prepare various nitrogenous fertilisers and in the manufacture of some inorganic nitrogen compounds, such as nitric acid.

Nitric Acid

Preparation: On a large scale, nitric acid is prepared by Oswald’s process:

Properties: Nitric acid is a colourless liquid, in the gaseous phase, it exists as a planar molecule.

Brown Ring Test: This test is used to detect the presence of nitrates ions in the solution. In this test Fe2+ ions reduce nitrates to nitric oxides. Nitric oxides further react with Fe2+ to form a brown-colored complex. The procedure for the brown ring test is - in the aqueous solution containing nitrate ion, dilute ferrous sulfate is added. Now in that mixture concentrated sulphuric acid is added along the sides of the test tube. A brown ring is formed which confirms the presence of nitrate in the solution.

(Brown)

Uses: Ammonium nitrate is produced from nitric acid. Ammonium nitrate is used as fertilizers and other nitrates are used in manufacturing explosives and pyrotechnics. Nitroglycerin, trinitrotoluene and other organic nitro compounds are also prepared from nitric acid.

Phosphorous

White Phosphorous: It is a very reactive discrete tetrahedral molecules, which glows in dark. It is a translucent waxy solid which is but insoluble in water and has low ignition temperature, therefore, kept underwater.

Red Phosphorous: It is a polymeric structure consisting of chains of P4 units linked together. It is less reactive than white phosphorus, has an iron-grey lustre and does not glow in dark, however, it is not soluble in water and .

Black Phosphorous: It exists in two forms – black phosphorus and black phosphorus, it is very less reactive and has opaque monoclinic or rhombohedral crystals.

Phosphine

Preparation:

Properties: It is a highly poisonous colourless gas with a rotten fish smell. It explodes in contact with oxidising agents like , and vapours of and . It is slightly soluble in water

Uses: The spontaneous combustion of phosphine is used in Holme’s signals, wherein containers containing calcium carbide and calcium phosphide are pierced and then thrown in the sea, the gases evolved to burn and serve as a signal.

Phosphorous Halides

The halides of phosphorous are mainly of two types: and .

Phosphorus Trichloride
It is a colourless oily liquid and has a pyramidal shape with hybridisation.

Preparation:

Phosphorus pentachloride

It is a yellow-white powder. In gaseous and liquid phases, it has a trigonal bipyramidal structure and in the solid phase, it exists as an ionic solid .

Preparation:

Group 16 Elements

This group is called a group of chalcogens. It contains Oxygen, sulphur, selenium, tellurium and polonium.

Electronic Configuration:

The valence shell electronic configuration of this group element is .

Atomic and Ionic radii:

Both the radii increase down the group with an increase in the number of shells.

Ionisation enthalpy:

Ionisation enthalpy decreases down the group with an increase in atomic size. However, this group has lower ionisation enthalpy in comparison to group 15 due to extra stable half-filled p orbitals electronic configuration.

Electronegativity:

The electronegativity value decreases down the group with an increase in atomic size.

Physical properties:

Here oxygen and sulphur are non-metals, selenium and tellurium metalloids, and polonium is a metal. All these elements exhibit allotropy.

Chemical properties:

Oxidation states: The stability of the -2 oxidation state reduces as we progress through the group.

Polonium is rarely found in the –2 oxidation state. Because oxygen's electronegativity is so strong, it only has a negative oxidation state of –2, except in the instance of OF2, which has a positive oxidation state of + 2. Other elements in the group have oxidation states of + 2, + 4, and + 6, however + 4 and + 6 are more prevalent. In their compounds with oxygen, sulphur, selenium, and tellurium normally have a + 4 oxidation state, while fluorine has a + 6 oxidation state. The stability of the + 6 oxidation state reduces as the group progresses, while the stability of the + 4 oxidation state rises (inert pair effect).

Reactivity towards Hydrogen: Bond dissociation enthalpy H-E decreases (where E = O, S, Se, Te, or Po) down the group. We know that the more the bond enthalpy the more thermal stability will be. So the stability of hydrides also decreases from to .

Reactivity towards Oxygen: Group 16 elements react with oxygen to form oxides of EO2 and EO3 types, where E = S, Se, Te or Po.

Reactivity towards Halogens: Group 16 elements form a large number of halides of the type EX6 , EX4 , EX 2, where E = O, S, Se, Te, or Po and X= F, Cl, Br, or I. The stability of the halides is in the order of. So among hexhalides, hexafluoride will be the most stable.

Dioxygen

Preparation: Following are some reactions for the preparation of dioxygen in the laboratory:

Properties: Dioxygen is a colourless and odourless gas, which is soluble in water at 293 K. It is paramagnetic in nature even though it has an even number of electrons.

Uses: It is very essential in normal respiration and combustion processes. It is used in oxyacetylene welding, in the manufacturing of steel and as oxygen cylinders in hospitals.

Ozone

Preparation: On passage of a slow dry stream of oxygen through a silent electrical discharge, conversion of oxygen to ozone occurs. The product is known as ozonized oxygen.

Properties: Pure ozone exists as pale blue gas, dark blue liquid and violet-black solid. In the ozone molecule, the two oxygen-oxygen bond lengths are identical at 128 pm and the molecule is with a bond angle of .

Uses: It can be used as a germicide, disinfectant, and water sterilizer. It's also used to bleach oils, ivory, flour, and starch, among other things.

Sulphur

Rhombic Sulphur/(α-sulphur): This allotrope is yellow in colour. When roll sulphur solution in CS₂ is evaporated, rhombic sulphur crystals emerge. It is insoluble in water but soluble in benzene, alcohol, and ether to some extent. It dissolves easily in CS₂.

Monoclinic sulphur (β-sulphur): It has a melting point of 393 K and a specific gravity of 1.98. It dissolves in CS₂. This Sulphur is made by melting rhombic sulphur and then cooling it until a crust is formed.

Both rhombic and monoclinic Sulphur is found as . Both varieties of the ring have a crown shape and are puckered. The ring takes on the chair shape in cyclo-. is the dominating species at high temperatures (1000 K) and, like , is paramagnetic.

Sulphur Dioxide

Preparation: Following are some reactions for the preparation of Sulphur dioxide:

Properties: Sulphur dioxide is a pungent-smelling colourless gas and is soluble in water.

Uses: It is used to refine petroleum and sugar, bleach wool and silk and as antichlor, disinfectant and preservative. Liquid is used to dissolve a number of organic and inorganic chemicals.

Sulphuric acid

Preparation: Sulphuric acid is manufactured in three steps by Contact Process:

Burning of Sulphur/sulphide ores in the air in order to generate.

Convert to with oxygen in presence of a catalyst ()

Absorption of in to produce Oleum ().

Properties: Sulphuric acid is a colourless, thick, and oily liquid. Sulphuric acid's chemical reactions are caused by the following characteristics: Low volatility, strong acidic character, high affinity for water, and capacity to operate as an oxidizer.

Uses: It is utilised in fertiliser manufacturing (e.g., ammonium sulphate, superphosphate), petroleum refining, pigment, paint, and dye manufacturing, detergent industry, and metallurgical uses.

Group 17 elements

This group is called a group of Halogens and constitutes Fluorine, chlorine, bromine, iodine and astatine as group elements.

Electronic Configuration:

The valence shell electronic configuration of this group element is .

Atomic and Ionic radii:

Both the radii increase down the group with an increase in the number of shells. Due to the maximum effective nuclear charge, this group has the smallest atomic radii in their respective periods.

Ionisation enthalpy:

Decreases down the group with an increase in atomic size.

Electronegativity:

The electronegativity value decreases down the group with increased atomic size. Fluorine is the most electronegative element among all the other elements of the periodic table.

Physical properties:

Fluorine and chlorine are the gases, bromine is a liquid whereas iodine exists as a solid. With the increase in atomic number, the melting and boiling points also increase. Halogens are all coloured. This is due to the absorption of visible light, which causes the outer electrons to be excited to a higher energy level. The order of bond dissociation enthalpy is given as , due to relatively large electron-electron repulsion among the lone pairs in molecule where they are much closer to each other than in the case of .

Chemical properties:

Oxidation states: Halogens exhibit –1 oxidation state. However, chlorine, bromine and iodine also exhibit + 1, + 3, + 5 and + 7 O.S. The halogens are all extremely reactive. In reaction with metals and non-metals, halogens form halides.

Reactivity towards Hydrogen: Group 17 elements form hydrogen halides with hydrogen but the affinity for reaction with hydrogen decreases from fluorine to iodine due to poor orbital overlap.

Reactivity towards Oxygen: Group 17 elements form many oxides with oxygen, but they are usually unstable.

Reactivity towards Halogens: Group 17 elements react with metals to form metal halides. Halides in a higher oxidation state are more covalent than halides in a lower oxidation state.

Reactivity of halogens towards other halogens: Group 17 elements combine amongst themselves to form a number of compounds known as interhalogens.

Chlorine

Preparation: Following are a few reactions for the preparation of Chlorine:

Chlorine is manufactured by Deacon’s process:

It is also prepared by electrolysis of brine solution, wherein chlorine is liberated at the anode.

Properties: It has a green-yellow colour with a pungent and suffocating odour. It
weighs 2-5 times heavier than air. It is soluble in water. However, Chlorine water on standing loses its yellow colour as a result of the formation of HCl and HOCl. Chlorine forms chlorides when it reacts with a variety of metals and nonmetals. It produces HCl when it combines with hydrogen-containing substances.

Uses: Compounds of chlorine have many applications. ClO2 is used as a bleaching agent in water treatment, paper, pulp and textiles. It is used in the purification of gold and platinum, where sodium chloride and hydrochloric acid are used for electrolysis. It is also used in the manufacture of dyes, drugs, and organic compounds like CCl₄ , DDT etc. It can be used to prepare poisonous gas like phosgene, tear gas, and mustard gas.

Hydrogen chloride

Preparation: In the laboratory, it is obtained by heating sodium chloride with concentrated sulphuric acid.

Properties: HCl is a colourless and pungent-smelling gas that is easily liquefied to a colourless liquid at b.p. of 189 K and freezes to a white crystalline solid at f.p. of 159 K.

Uses: It is used to produce chlorine gas by electrolysis or by reacting with potassium permanganate. is manufactured by reacting with ammonia. It is also used to manufacture glucose. It can be used to extract glue from bones and purify black bones.Its application includes medicine and laboratory reagents.

Interhalogen Compounds

Preparation: The interhalogen compounds can be prepared by the direct combination of halogen on lower interhalogen compounds

Properties: These compounds are all covalent molecules and are diamagnetic in nature

Uses: Fluorinating agents made out of interhalogen chemicals are quite beneficial. In the enrichment of 235-U, ClF₃ and BrF₃ are used to produce UF₆.

Group 18 elements

The elements of this group are called noble elements. It comprises helium, neon, argon, krypton, xenon and radon.

Electronic Configuration:

The valence shell electronic configuration of this group element is .

Atomic and Ionic radii:

Both the radii increase down the group with an increase in the number of shells.

Ionisation enthalpy:

Decreases down the group with an increase in atomic size.

Physical properties:

Noble gases are monoatomic in nature. They have no colour, no odour, and no taste. In water, they are just slightly soluble. Because weak dispersion forces are the only type of interatomic interaction in these elements, they have very low melting and boiling temperatures. It has the remarkable property of diffusing through the majority of popular laboratory materials such as rubber, glass, and plastics.

Chemical properties:

Noble gases are the least reactive in general. The following factors contribute to the inertness of noble elements:

Firstly, noble gases except helium () have completely filled the electronic configuration of .

Secondly, they have high ionisation enthalpy and positive electron gain enthalpy.

Uses: Helium is used in filling balloons for meteorological observation, a cryogenic agent. Neon is particularly used in discharge tubes and fluorescent bulbs. Argon provides an inert atmosphere in high-temperature metallurgical processes and is used as filling in electric bulbs.

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