Group 7

ELECTRONEGATIVITY FALLS DOWN GROUP 7

• Because as halogens get bigger
• Bonding pair gets further away from the nucleus
• So is less attracted to it --> BECOMES LESS ELECTRONEGATIVE

MELTING AND BOILING POINTS INCREASE DOWN GROUP 7

• As molecules get bigger, more intermolecular forces between molecules
• Stronger van der waals forces
• More energy required to turn into liquid or gas
• MELTING AND BOILING POINTS RISE

OXIDISING ABILITY FALLS DOWN GROUP 7

• Oxidation is LOSS of electrons
• Fluorine is too powerful an oxidising agent to do solution reactions
• Chlorine can take electrons from bromide and iodide ions so is more powerful than bromine and iodine
• Bromine is a more powerful oxidising agent than iodine

REDUCING ABILITY OF HALIDE IONS INCREASES DOWN GROUP 7

• When a halide ion acts as a reducing agent it GIVES electrons to something else
• The bigger the halide ion the further the electrons are from the nucleus
• The more they are screened by inner electron shells
• Weaker attraction between outer electrons and nucleus
• Easier to lose electrons as you go down the group

Reactions of halide salts and conc. sulfuric acid

• NaCl(s) + H₂SO₄(l) ==> NaHSO₄(s) + HCl(g)
• 2NaBr(s) + 2H₂SO₄(l) ==> NaHSO₄(s) + Br₂(g) + SO₂(g) + 2H₂O(l)
• 8NaI(s) + 5H₂SO₄(l) ==> 4Na₂SO₄(s) + 4I₂(g/s) + H₂S(g) + 4H₂O(l)
• NaF(s) + H₂SO₄(l) ==> NaHSO₄(s) + HF(g)

• The test has to be done in solution
• Solution is acidified by adding dilute nitric acid to the silver nitrate
• The nitric acid reacts with and removes other ions that might give a precipitate with silver nitrate
• F- = NO PRECIPITATE
• Cl- = WHITE PRECIPITATE
• Br- = PALE CREAM PRECIPITATE
• I- = PALE YELLOW PRECIPITATE
• The precipitates are the insoluble silver halides: AgCl, AgBr and AgI

Ammonia is used to confirm the halides:

• AgCl + Ammonia --> precipitate dissolves to give a colourless solution
• AgBr + Ammonia --> precipitate remains in dilute ammonia but dissolves to colourless in conc. ammonia
• AgI + Ammonia --> precipitate is insoluble in ammonia solution of any concentration

• Chloride is very reactive and undergoes reaction with water molecules forming a mixture of hydrochloric acid and chlorate (I)
• Cl₂ + H₂O HCl + HOCl
• Humans are tolerant of small doses of chlorine, whilst low conc. of chlorine kill small microorganisms, therefore water treatment plants add chlorine to kill micro-organisms and make water fit for human consumption.
• When chlorine is bubbled through cold sodium hydroxide, a disproportionation reaction occurs and chlorine is simultaneously oxidised and reduced:
• 3Cl₂ + 6NaOH --> NaClO₃ + 5NaCl + 3H₂O
• The sodium chlorate (V) produced (NaClO3) is a useful weedkiller and oxidising agent.