NEET Chemistry Notes p-Block Elements – Group-17 Elements Halogens

NEET Chemistry Notes p-Block Elements – Group-17 Elements Halogens

Group-17 Elements Halogens

Group-17 Elements Halogens

Group 17 members are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Astatine is a radioactive element. The halogens are highly reactive non-metallic elements. Fluorine is most electronegative atom and strong oxidising agent.

Important Properties

General Configuration and Oxidation State

Ground state and excited state in fluorine will be same due to the absence of d-orbitals and oxidation state of fluorine is always -1. All the halogens are highly reactive. Halogens are strong oxidising agent and their oxidising power decreases down the group

The reaction of  I₂with water is non-spontaneous. However, I can be oxidised by oxygen in acidic medium.

• Bond Dissociation Enthalpy

The Order of bond dissociation enthalpy is

• Fluorine shows anomalous behaviour because of its very small size and high electronegativity. F₂ has less bond enthalpy than Cl₂ and Br₂ and the absence of d-orbitals in valence shell.

Chlorine (Cl 2)

Preparation

Chlorine is obtained by the electrolysis of brine (concentrated NaCl solution). Chlorine is liberated at anode (electrolytic process). It is also obtained as a byproduct in many chemical industries.

Properties

It is a greenish yellow gas with pungent and suffocating odour. It is soluble in water.

Some Important Compounds of Halogen

• Hydrogen chloride HCl
• Interhalogen compounds
• Oxoacids of halogen
•  Oxides

Hydrogen Chloride (HCl)

• In laboratory, it is prepared by heating sodium chloride with concentrated sulphuric acid.
  
• HCl gas can be dried by passing through concentrated sulphuric acid.
•  It is a colourless and pungent smelling gas. Its aqueous solution is called hydrochloric acid.
• When three parts of concentrated HCl and one part of concentrated HN0₃ are mixed, aqua-regia is formed which is used for dissolving noble metals, e.g. gold, platinum.

Interhalogen Compounds

• When two different halogens react with each other, interhalogen compounds are formed.
  These compounds are covalent and diamagnetic in nature. They are volatile solids or liquids except ClF which is a gas at 298 K. Interhalogen compounds are more reactive than halogens (except fluorine).
• The XY₃ type compounds have bent T shape, XY₅ type compounds have square pyramidal shape and IF₇ has pentagonal bipyramidal structure.

 Oxoacids of Halogens

Higher oxoacids of fluorine such as HF0₂,HF0₃ do not exist because fluorine is most electronegative and has the absence of d-orbitals. +3 oxidation state of bromine and iodine are unstable due to inert pair effect, therefore HBrO₂ and HI0₂ do not exist.
Decreasing order of acidic nature

Structures of Oxoacids of Chlorine

Pseudo halide ions are stronger ligands than halide ions and these can function as ambidentate ligands as they are made up of two hetero atoms.

Oxides

• Fluorine forms two oxides, OF₂ and 0₂F₂, but only OF₂ is thermally stable at 298 K. 0₂F₂ oxidises plutonium to PuF₆ and the reaction is used for removing plutonium as PuF₆ from spent nuclear fuel.
• Chlorine forms a number of oxides such as, Cl₂0, Cl₂0₃, Cl₂O_g, Cl₂0₇, Cl0₂. Cl0₂ is used as a bleaching agent for paper pulp, textiles and in water treatment.
• Br₂0, Br0₂, Br0₃ are the least stable bromine oxides and exist only at low temperatures. They are very powerful oxidising agents.
•  The iodine oxides, i.e.I₂0₄, I₂0₅,1₂0₇ are insoluble solids and decompose on heating. I₂0₅ is a very good oxidising agent and is used in the estimation of carbon monoxide.

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