2.3 Group 7, halogens
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What is the trend in electronegativity down group 7?
• Decreases • The outer electron gets further away from the nucleus, due to shielding. • Reduces the attraction so harder to attract bonding electrons.
What is the trend in boiling point down group 7?
• Increases • Larger atoms have more electrons • More VDWS • So need more energy to overcome them
What is the trend in oxidising ability down group 7?
• Decreases • Higher up halogens displace those lower down • They have a higher electronegativity so have a higher affinity to taking electrons
How to prove the oxidising ability trend?
Use displacement reactions between halide ions in an aqueous solution.
What is the trend in reducing ability down group 7?
• Increases • Larger ions going down, so can lose their outer electrons more easily • Due to weaker attraction and bigger AR
How to prove the reducing ability trend?
Sodium Halide + Sulfuric Acid
NaCl + H₂SO₄ =
NaHSO₄ + HCl
NaBr + H₂SO₄ =
• NaHSO₄ + HBr • 2Br¯ = Br₂ • 2Br¯ + 2H⁺ + H₂SO₄ = SO₂ + 2H₂O + Br₂
NaI + H₂SO₄ =
• NaHSO₄ + HI • 2I¯ = I₂ • 2I¯ + 2H⁺ + H₂SO₄ = SO₂ + 2H₂O + I₂ • 6I¯ + 6H⁺ + H₂SO₄ = S + 4H₂O + 3I₂ • 8I¯ + 8H⁺ + H₂SO₄ = H₂S + 4H₂O + I₂
What does chlorine as a reducing agent produce?
Nothing, no reduction
What does bromine as a reducing agent produce?
SO₂, gas with choking smell
What does iodine as a reducing agent produce?
S, yellow solid SO₂, gas with choking smell H₂S, gas with rotten egg smell
What are the tests for halides? (2 types)
• Silver nitrate - Ammonia
Outline the silver nitrate test
• Add nitric acid (to acidify carbonate ions) • Add silver nitrate • AgX precipitate form
AgCl colour
white
AgBr colour
cream
AgI colour
yellow
In what type of ammonia does Cl¯ dissolve in?
dilute
In what type of ammonia does Br¯ dissolve in?
concentrated
In what type of ammonia does I¯ dissolve in?
none, doesn’t dissolve
Chlorine + water (no UV) =
Cl₂(g) + H₂O(l) ⇌ HClO (aq) + HCl (aq) forms chlorate ions (+1 ox state) [Cl₂ oxidised] forms chloride ions (-1 ox state) [Cl₂ reduced]
What is a disproportionation reaction?
A reaction where a substance is simultaneously oxidised and reduced, giving two different products.
Chlorine + water (in sunlight) =
2Cl₂(g) + 2H₂O(l) → 4HCl (l) + O₂(l)
Chlorine + Sodium Hydroxide =
Cl₂(g) + 2NaOH(aq) → NaClO (aq) + NaCl (aq) + H₂O(l) forms chlorate ions (+1 ox state) [Cl₂ oxidised] forms chloride ions (-1 ox state) [Cl₂ reduced]