Question

In which of the following option/options, the order of arrangement does not agree with the variation of property indicated against it?
(a) BF\(_3\)>NF\(_3\)>NH\(_3\) (Dipole moment)
(b) HgCl\(_2\)>NH\(_4\)\(^+\)>SF\(_4\) (Bond angle)
(c) NH\(_3\)<H\(_2\)O<HF (Strength of intermolecular hydrogen bonding)
(d) H - I>H - Br>H – Cl (Bond length)

• A. a, b and c
• B. a only
• C. c and d only
• D. d only

Hint:

For dipole moment questions, always consider both bond polarity and molecular geometry. Symmetrical molecules (like BF\(_3\), CCl\(_4\), CO\(_2\)) have a net dipole moment of zero even if their bonds are polar.

Answer 1

The Correct Option is B

To solve this question, we need to evaluate each option's validity based on the given property trend.

1.  Option (a): BF\(_3\)>NF\(_3\)>NH\(_3\) - Dipole moment:
   • BF\(_3\) is a symmetrical molecule with zero dipole moment.
   • NF\(_3\), on the other hand, has a lower dipole moment than NH\(_3\), because the nitrogen-fluorine bond is less polar than the nitrogen-hydrogen bond, and the lone pair's effect is less dominant in NF\(_3\).
   • The actual correct order should be NH\(_3\) > NF\(_3\) > BF\(_3\). Therefore, this statement does not agree with the variation in dipole moments, making it incorrect.
2. Option (b): HgCl\(_2\)>NH\(_4\)\(^+\)>SF\(_4\) - Bond angle:
   • HgCl\(_2\) is linear with a bond angle of 180°. NH\(_4\)\(^+\) is tetrahedral with a bond angle of approximately 109.5°, and SF\(_4\) is seesaw-shaped, with bond angles less than 109.5°.
   • The order given in the option is correct.
3. Option (c): NH\(_3\)<H\(_2\)O<HF - Strength of intermolecular hydrogen bonding:
   • HF forms the strongest hydrogen bonds among the given molecules, followed by H\(_2\)O, then NH\(_3\).
   • The order given is correct.
4. Option (d): H - I>H - Br>H – Cl - Bond length:
   • The bond length decreases with increasing bond strength, and the bond strength increases as we move up the halogen group.
   • Therefore, H–I > H–Br > H–Cl is the correct bond length order.

Based on the above analysis, the correct answer is that only option (a) fails to follow the trend indicated. Hence, the correct answer is: a only.