2.3 Halogens Flashcards
What are the physical appearances of fluorine, chlorine, bromine, and iodine at room temperature?
• Fluorine: very pale yellow gas, highly reactive
• Chlorine: greenish, reactive gas, poisonous in high concentrations
• Bromine: red liquid, gives off dense brown/orange poisonous fumes
• Iodine: shiny grey solid, sublimes to purple gas
What is the trend in melting and boiling points down Group 7 (halogens)? Why?
They increase down the group due to more electrons in larger molecules, causing stronger van der Waals forces that require more energy to break.
What is the trend in electronegativity down Group 7 and why?
Electronegativity decreases down the group because atomic radius increases (more shells), so the nucleus is less able to attract bonding electrons.
What is the rule for displacement reactions between halogens and halide ions?
A halogen with stronger oxidising power will displace one with weaker power from its compound. Oxidising strength decreases down the group.
What are the observations when halogens are added to potassium halide solutions?
• Cl₂ + KBr → yellow solution (Br₂)
• Cl₂ + KI → brown solution (I₂)
• Br₂ + KI → brown solution (I₂)
• No reaction with same or more reactive halide
Colour Key:
• Cl₂: very pale green (often colourless)
• Br₂: yellow solution
• I₂: brown solution, sometimes black solid
Give the ionic equations for halogen displacement reactions.
• Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
• Cl₂ + 2I⁻ → 2Cl⁻ + I₂
• Br₂ + 2I⁻ → 2Br⁻ + I₂
What is the purpose of nitric acid in halide testing with silver nitrate?
To remove carbonate ions (which would form Ag₂CO₃ precipitate) and prevent false positives.
What are the results of silver nitrate test with halide ions? Include equations.
F⁻: no precipitate
Cl⁻: white ppt → Ag⁺ + Cl⁻ → AgCl
Br⁻: cream ppt → Ag⁺ + Br⁻ → AgBr
I⁻: pale yellow ppt → Ag⁺ + I⁻ → AgI
How do the Ag halide precipitates react with ammonia?
AgCl: dissolves in dilute NH₃ → colourless solution
AgBr: dissolves in concentrated NH₃
AgI: does not dissolve
Why does reducing power of halide ions increase down the group?
Larger ions lose electrons more easily due to weaker nuclear attraction on the outer shell.
What happens when NaF and NaCl react with conc. H₂SO₄? Include reactions and observations.
NaF + H₂SO₄ → NaHSO₄ + HF (white steamy fumes)
NaCl + H₂SO₄ → NaHSO₄ + HCl (white steamy fumes)
Only acid-base reactions, no redox
What are the reactions and observations for NaBr with conc. H₂SO₄?
Acid-base: NaBr + H₂SO₄ → NaHSO₄ + HBr (white fumes)
Redox:
2H⁺ + 2Br⁻ + H₂SO₄ → Br₂ (orange fumes) + SO₂ (colourless gas) + 2H₂O
Half equations for Br⁻ with H₂SO₄.
Oxidation: 2Br⁻ → Br₂ + 2e⁻
Reduction: H₂SO₄ + 2H⁺ + 2e⁻ → SO₂ + 2H₂O
What are the reactions and observations for NaI with conc. H₂SO₄?
NaI + H₂SO₄ → NaHSO₄ + HI (white fumes)
Redox reactions:
I₂ (black solid/purple fumes)
SO₂ (colourless gas)
S (yellow solid)
H₂S (bad egg smell gas)
Half equations for I⁻ with H₂SO₄.
I⁻ → I₂ + 2e⁻
H₂SO₄ + 2H⁺ + 2e⁻ → SO₂
H₂SO₄ + 6H⁺ + 6e⁻ → S
H₂SO₄ + 8H⁺ + 8e⁻ → H₂S
What is disproportionation and give an example with chlorine and water.
Disproportionation is when an element is both oxidised and reduced.
Cl₂ + H₂O ⇌ HCl + HClO
• Universal indicator turns red (acid), then colourless (bleaching by HClO)
What happens when chlorine water is exposed to sunlight?
Cl₂ + H₂O + sunlight → 4H⁺ + 4Cl⁻ + O₂
• Chlorine fades, colourless gas (O₂) evolves
What is chlorine’s use in water treatment?
Chlorine kills bacteria in drinking water and swimming pools. Health benefits outweigh toxicity.
What happens when chlorine reacts with cold, dilute NaOH?
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
• Forms bleach; solution fades from greenish to colourless
What are IUPAC names for NaClO and NaClO₃?
NaClO: sodium chlorate(I)
NaClO₃: sodium chlorate(V)
What are IUPAC names for K₂SO₄ and K₂SO₃?
K₂SO₄: potassium sulfate(VI)
K₂SO₃: potassium sulfate(IV)
