Skip to content

Why is boiling point of… (Class 12 Chemistry 2023, Q22)

questionbuddy.in/q/cbse-12-chemistry-2023-56-5-1-q22-boiling-point-dichlorobenzene-higher-than

ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2023: question 22

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section B, question 22, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 2 marks · Very short answerHaloalkanes and Haloarenes

Why is boiling point of o-dichlorobenzene higher than p-dichlorobenzene but melting point of para isomer is higher than ortho isomer ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Boiling depends on intermolecular forces in the liquid; compare dipole moments of the two isomers.

Hint 2

Melting depends on how well molecules pack in the solid; which isomer is more symmetrical?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Boiling point: o-dichlorobenzene is polar (the two C–Cl dipoles do not cancel), while in p-dichlorobenzene they cancel (μ = 0). The stronger dipole–dipole forces in the ortho isomer give it the higher boiling point.

    1 mark

  2. Melting point: p-dichlorobenzene is more symmetrical, so its molecules pack more closely and fit better in the crystal lattice; more energy is needed to break the lattice, so it has the higher melting point.

    1 mark

Answer: o-isomer: higher dipole moment → higher b.p.; p-isomer: symmetrical, better packing → higher m.p.

Where the marks are

  • B.p.: higher polarity / dipole–dipole interactions in o-isomer (1 mark)
  • M.p.: symmetry and better crystal packing of p-isomer (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Haloalkanes and Haloarenes, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 56/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.