2.3 Group 7 - The Halogens

trend in atomic radius down group 7

increases due to additional electron shells

trend in reactivity down group 7

decreases as atomic radius increases and there is more electron shielding so it is harder to attract an electron

trend in first ionisation energy down group 7

decreases as greater atomic radius and more shielding

trend in boiling point down group 7

increases as VdW strength increases as there are more electrons in the atom

trend in electronegativity down group 7

decreases as bigger atoms have more shielding so weaker attraction to the positive nucleus

trend in oxidising power down group 7

decreases are more shielding and a greater atomic radius

• a halogen displaces any halide below it, e.g. Cl displaced Br

halide ions

good reducing agents as they donate electrons to species being reduced when they are oxidised. reducing power increases down the group

reaction of concentrated sulfuric acid and sodium chloride

steamy fumes, blue litmus turns red

• NaCl + H₂SO₄ —> NaHSO₄ + HCl

reaction of concentrated sulfuric acid and sodium bromide

• NaBr + H₂SO₄ —> NaHSO₄ + HBr - steamy fume of HBr and Br

• 2H⁺ + 2Br^- + H₂SO₄ —> SO₂ + 2H₂O + Br₂ - steamy fumes of sulfur dioxide, orange gas of bromine

reaction of concentrated sulfuric acid and sodium iodide

• NaI + H₂SO₄ —> NaHSO₄ + HI - steamy fumes

  • 8H⁺ + 8I^- + H₂SO₄ —> H₂S + 4H₂O + 4I₂ - egg smell (H₂S) and yellow sulfur

how to test for halide ions with silver nitrate

acidify samples with HNO₃ to remove carbonate or hydroxide impurities before adding silver nitrate

adding silver nitrate to chloride ions

white precipitate formed that dissolves in dilute ammonia

adding silver nitrate to bromide ions

cream precipitate formed that dissolves in concentrated ammonia

adding silver nitrate to iodide ions

yellow precipitate formed that doesn’t dissolve in ammonia

chlorine reaction with cold water

disproportionation as Cl is oxidised and reduced

• Cl₂ + H₂O —> ClO^- + Cl^-- + 2H⁺

• universal indicator turns red (acid HCl) but then colorless as HClO is a bleach

what happens to chlorine in UV light?

2Cl₂ + 2H₂O —UV—> 4HCl + O₂

this mean chlorine is rapidly lost from swimming pool, especially shallow ones, so must be replenished

alternative to direct chlorination of swimming pools

NaClO + H₂O ⇌ Na⁺ + OH^—+ HClO

This must be kept acidic or the equilibrium shifts left, removing HClO

reaction of chlorine with cold, dilute NaOH

Cl₂ + 2NaOH —> NaCl + NaClO + H₂O

NaClO is the active ingredient in bleach

test for sulfate ions (SO₄^2—)

use barium chloride (BaCl₂), which reacts to form a white precipitate

test for carbonate ions (CO₃^2—)

add an acid, e.g. HCl, which fizzes as CO₂ is produced. To test for CO₂, bubble gas produced through limewater, which turns cloudy in it

test for hydroxide ions (OH^—)

red litmus turns blue-purple

flame test for Ca²⁺

brick red

flame test for Sr²⁺

red

flame test for Ba²⁺

pale green

test for ammonium ions (NH⁺)

ammonia gas is given off, which is a bas so red litmus would turn blue

• to produce ammonia faster, add NaOH