UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper I
Q1(f). Melting points of bromine and iodine chloride
Question
Br₂ melts at 7.2 ° C whereas ICl melts at 27.2 ° C. Explain.
Answer in brief
ICl is polar and has dipole-related intermolecular attractions in addition to dispersion; Br₂ is nonpolar and relies mainly on dispersion.
Key points
- The two molecules have similar molar masses, so their mass difference alone does not explain the stated melting-point contrast.
- The heteronuclear I–Cl bond is polar, allowing oriented dipole interactions in its solid; symmetric Br₂ has no permanent dipole.
- Stronger cohesive interactions and crystal packing can raise the energy needed for melting.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversMelting points of bromineiodine chlorideIntermolecular forces
All parts of Question 1
- 1(a) Permanganate and iron(II)5 marks
- 1(b) Carbon isotopes and stability5 marks
- 1(c) Diagonal relationship5 marks
- 1(d) Conductivity of phosphoric acid5 marks
- 1(e) Oxidation of Fe(II) and Mn(II)5 marks
- 1(f) Melting points of bromine and iodine chloride5 marks
- 1(g) Small dipole moment of CO5 marks
- 1(h) Acidity of concentrated HF5 marks
- 1(i) Temperature and conductivity5 marks
- 1(j) Diborane with excess ammonia5 marks