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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2016 · Paper I

Q1(a)(ii). Dipole moments of BF₃ and PF₃

  • 5 marks
  • Answered
Question

Comment on the dipole moments of BF₃ and PF₃ molecules.

Answer in brief

BF₃ is trigonal planar: its three equal B–F bond dipoles cancel, μ=0. PF₃ is pyramidal: the P–F bond dipoles add to a resultant, so μ≠0; the lone pair on P acts against it, so the moment is small.

Key points

  • The molecular dipole is the vector sum of the bond dipoles (and of any lone-pair moment). Both molecules have strongly polar bonds; the shape decides the sum.
  • B has three valence electrons and no lone pair. The three B–F bonds lie in a plane at 120°. Three equal vectors at 120° add to zero: μ(BF₃)=0 (non-polar, although each B–F bond is very polar).
  • P has three bond pairs and one lone pair, so the molecule is a trigonal pyramid. The three P–F dipoles point down, away from the lone pair, and their vertical components add.
  • The lone pair on P has its own moment pointing up, opposite to the bond resultant. The two partly cancel, so PF₃ has only a small dipole moment.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. The molecular dipole is the vector sum of the bond dipoles (and of any lone-pair moment). Both molecules have strongly polar bonds; the shape decides the sum.
  2. BF₃: B has three valence electrons and no lone pair. The three B–F bonds lie in a plane at 120°. Three equal vectors at 120° add to zero: μ(BF₃)=0 (non-polar, although each B–F bond is very polar).
  3. PF₃: P has three bond pairs and one lone pair, so the molecule is a trigonal pyramid. The three P–F dipoles point down, away from the lone pair, and their vertical components add.
  4. The lone pair on P has its own moment pointing up, opposite to the bond resultant. The two partly cancel, so PF₃ has only a small dipole moment.
  5. The same contrast as NH₃/NF₃: when bond dipoles and lone pair point the same way the moment is large; when they oppose it is small.

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Model answer to Q1(a)(ii), 2016 Paper I: solution page 3
Solution page 3 of 46
Read the whole 2016 solution2016 question paper

All parts of Question 1