Chemistry · Chemical bonding and molecular structure · chapter 4 of 9

Class 11 Chemistry Chapter 4: Chemical Bonding and Molecular Structure case study question with answers

One case-based question of the kind CBSE-pattern school exams ask: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

A student collects dipole moments, in debye (D), for some molecules from a data table based on NCERT. A dipole moment is the product of charge and distance of separation, and for a polyatomic molecule it is the vector sum of the bond dipoles, so it depends on the shape as well as on the polarity of the bonds. 1 D = 3.33564 × 10⁻³⁰ C m.

Molecule	μ (D)	Shape
HF	1.78	linear
HCl	1.07	linear
HBr	0.79	linear
HI	0.38	linear
NH₃	1.47	trigonal pyramidal
CHCl₃	1.04	tetrahedral
CCl₄	0	tetrahedral

Part 1 · 1 mark

Among the hydrogen halides in the table, the bond with the least polarity is

(a) H–F(b) H–Cl(c) H–I(d) H–Br

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Answer: (c) H–I

HI has the smallest dipole moment, 0.38 D: iodine is the least electronegative halogen, so the shared pair is displaced least.

Part 2 · 1 mark

The dipole moment of HBr expressed in C m is

(a) 0.237 × 10⁻³⁰ C m(b) 4.22 × 10⁻³⁰ C m(c) 7.90 × 10⁻³⁰ C m(d) 2.64 × 10⁻³⁰ C m

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Answer: (d) 2.64 × 10⁻³⁰ C m

0.79 × 3.33564 × 10⁻³⁰ = 2.64 × 10⁻³⁰ C m.

Part 3 · 2 marks

Both CHCl₃ and CCl₄ are tetrahedral, yet only one has a dipole moment. Explain.

Answer and marking scheme

In CCl₄ four identical C–Cl bond dipoles point to the corners of a regular tetrahedron and their vector sum is zero. In CHCl₃ one C–Cl bond is replaced by C–H, whose bond dipole is different, so the bond dipoles no longer cancel and a resultant of 1.04 D remains.

CCl₄: equal bond dipoles cancel by symmetry (1)
CHCl₃: C–H differs from C–Cl, dipoles do not cancel (1)

Show, with a crossed arrow on the Lewis structure of HCl, the direction of shift of electron density, and state how this direction compares with that of the dipole moment vector by convention.

Answer and marking scheme

H–Cl is drawn with the cross at H (positive end) and the arrow head at Cl (negative end), showing electron density shifted towards the more electronegative Cl. By convention the dipole moment vector points from the negative centre to the positive centre, opposite to the crossed arrow.

crossed arrow with cross at H, head at Cl (1)
opposite to the conventional dipole moment vector (1)

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