Summary notes for AQA A-Level Chemistry Unit 3.2.3 - Group 7(17), the halogens by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Trends in Properties Down Group 7, Identification of Halide Ions, Uses of Chlorine and Chlorate(I)
Group 7(17), the Halogens
• Fluorine = poisonous pale yellow gas.
• Chlorine = poisonous green gas.
• Bromine = toxic red-brown volatile liquid - it forms a red-brown vapour.
• Iodine = shiny grey solid - it sublimes to form a violet vapour w/ gentle heating.
Trends in Properties Down Group 7
Electronegativity
Electronegativity decreases down the group because…
• no. of shells ↑ down the group ∴ size of atoms ↑ so…
• bonding electrons in a covalent bond are further from the nucleus + there is more shielding.
• ∴ weaker attraction between +vely charged nucleus + bonding pair of electrons.
• ∴ element has lower electronegativity.
Boiling Points
B.p.s increase down the group because…
• Size of diatomic molecules ↑ down the group.
• Larger molecules have more electrons, leading to greater induced dipole-dipole forces.
• ∴ greater van der Waals’s forces between molecules.
• ∴ more energy required to overcome the greater van der Waals’s forces as you go down the group.
Oxidising Ability
An oxidising agent accepts electrons.
Oxidising power decreases down the group because…
• Size of ions ↑ down the group.
• ∴ outer electrons are more shielded + further away from the nucleus.
• ∴ electrostatic force of attraction by nucleus on the additional electron becomes weaker down the
group.
• ∴ harder to gain an electron.
Displacement reactions
∴ from our knowledge of oxidising powers of halogens ↓ down the group we would expect…
… uorine to displace all other halogens from solutions of halide compounds.
… chlorine to displace bromide + iodide from solutions of bromide + iodide compounds.
… bromine to displace iodide from a solution of an iodide compound.
Displacement conclusion table:
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