Chemistry - 3.2.3 Group 7, the halogens

Describe and explain the general trend in atomic radius across Period 3?

• Atomic radius decreases across the period (1)
• Nuclear charge increases (1)
• Same shielding → stronger nuclear attraction (1)

Compare and explain the differences between the reactions of H₂SO₄ with NaCl, NaBr and NaI

• NaCl + H₂SO₄ → HCl (acid-base, not redox) (1)
• NaBr + H₂SO₄ → HBr, then Br₂ + SO₂ (1)
• NaI + H₂SO₄ → HI, then I₂ + H₂S/S/SO₂ (1)
• Trend: Halide reducing power increases down group (1)

Halogens are used in disproportionation reactions with both water and cold, dilute sodium hydroxide. Compare the reactions, giving balanced equations, and state the uses of the products formed.

• With water: Cl₂ + H₂O → HCl + HClO (1)
• With cold NaOH: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (1)
• Both are disproportionation (1)
• NaClO/HClO used as bleach/disinfectant (1)
• NaCl formed as a by-product (1)

Explain the variation in first ionisation energy across Period 3 from sodium to argon.

• General increase across period (1)
• Increased nuclear charge, same shielding (1)
• Dip at Al: 3p electron higher energy than 3s (1)
• Dip at S: paired electrons in 3p repel (1)

Explain why the first ionisation energy of aluminium is lower than that of magnesium.

• Al electron removed from 3p orbital (1)
• 3p higher in energy than 3s (1)
• Easier to remove → lower ionisation energy (1)

Explain why the reaction between sodium iodide and concentrated sulfuric acid produces a bad smell and a yellow solid. Write equations to support your answer.

• NaI + H₂SO₄ → HI + NaHSO₄ (1)
• HI reduces H₂SO₄ → H₂S + S + I₂ (1)
• Yellow solid = sulfur (1)
• Bad smell = H₂S (1)

Describe and explain the trend in reducing power of the halide ions down the group.

• Reducing power increases down group (1)
• Ions larger with more shielding (1)
• Electron lost more easily (1)
• Attraction between nucleus and outer electrons weaker (1)