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ChemistryMediumClass 12

Halogen Compounds — HF, HCl, HBr, HI comparisons

p-block Elements (Class 12 — Groups 15, 16, 17, 18)

10

JEE Qs

8%

Hard

60

min

Focus on understanding the underlying reasons (bond enthalpy, hydrogen bonding, atomic size, electronegativity) for each trend, rather than just memorizing the orders, as JEE often tests conceptual understanding with reasoning.

🧮 Key Formulas

Acid dissociation: HX(aq) + H₂O(l) ⇌ H₃O⁺(aq) + X⁻(aq)
Bond dissociation enthalpy (Δ_diss_H): Energy required to break the H-X bond. Directly proportional to thermal stability.
Reducing strength: Ease of oxidation of X⁻ to X₂.
Boiling point: Determined by intermolecular forces (hydrogen bonding in HF, van der Waals forces in HCl, HBr, HI).

✅ Key Points for JEE

  • 1Acidic strength order: HI > HBr > HCl > HF. This is primarily due to the decrease in H-X bond dissociation enthalpy down the group, making H⁺ release easier. (Bond length increases, bond strength decreases).
  • 2Thermal stability order: HF > HCl > HBr > HI. This is directly proportional to the H-X bond dissociation enthalpy; stronger bonds lead to higher thermal stability.
  • 3Reducing strength order of halide ions: I⁻ > Br⁻ > Cl⁻ > F⁻ (HI > HBr > HCl > HF as reducing agents). This trend is due to the increasing ease of oxidation of the halide ion (larger size, lower electronegativity, weaker attraction for valence electrons) down the group.
  • 4Boiling point order: HF > HI > HBr > HCl. HF has an abnormally high boiling point due to strong intermolecular hydrogen bonding. For HCl, HBr, and HI, boiling point increases with molecular mass due to increasing van der Waals forces.

⚠️ Common Mistakes

  • ✕Confusing acidic strength with electronegativity; students often incorrectly assume HF is the strongest acid due to F's high electronegativity. The primary factor for hydracids in the same group is bond dissociation enthalpy, not electronegativity.
  • ✕Incorrectly ordering boiling points, especially misplacing HF by overlooking the role of hydrogen bonding, or not considering the increase in van der Waals forces with molecular size for the others.
  • ✕Mixing up the trends for thermal stability and acidic strength or reducing strength. Remember that high thermal stability (strong bond) often means low acidic strength for hydracids, and vice versa.

NCERT Chapters

  • Class 12 Chemistry Part 1 Ch 7: p-Block Elements