Question

which is the correct order for the following properties?
A. Acidity order: $Si{F_4} < SiC{l_4} < SiB{r_4} < Si{I_4}$
B. Melting point: $N{H_3} > Sb{H_3} > As{H_3} > P{H_3}$
C. Boiling point: $N{H_3} > Sb{H_3} > As{H_3} > P{H_3}$
D. Dipole moment: $N{H_3} > Sb{H_3} > As{H_3} > P{H_3}$

Answer 1

Hint: Acidity of a molecule is the ability of a molecule to accept an electron pair. Melting and boiling point of molecules depends on intermolecular forces or van der waals forces, strong is the forces higher is the melting and boiling point. Dipole moment is a vector quantity that describes two opposite charges separated by a distance.

Complete Step by step answer: in option A, order of acidity of halides of silicon is not correct. As Lewis acidity can be defined as the ability of compounds to accept an electron pair. As F is more electronegative than Cl, therefore there is more positive charge over Si. Silicon can expand its oxidation state up to +6. Silicon can accept an electron pair in case of $Si{F_4}$ and forms ${[Si{F_6}]^{2 - }}$ . therefore order should be $Si{F_4} > SiC{l_4} > SiB{r_4} > Si{I_4}$
In option B and C, melting and boiling point increases with increase in intermolecular force. Strong is the intermolecular force between the molecules higher will be the melting and boiling point. As we go down in a group, size of the molecule increases, mass increases and hence van der waals forces also increases and therefore on going down the group the melting and boiling point increases. But $N{H_3}$has more boiling and melting point than phosphine and arsenic hydride.
So the correct order for melting point will be $N{H_3} > Sb{H_3} > As{H_3} > P{H_3}$
Correct order of boiling point will be $Sb{H_3} > N{H_3} > As{H_3} > P{H_3}$.
In option D, dipole moment is a vector quantity which depends on magnitude as well as direction. Ammonia has trigonal pyramidal geometry. It has the highest dipole moment, as nitrogen is more electronegative than hydrogen , so the direction of dipole moment will go towards nitrogen. Lone pairs are more electronegative as they are closer to the nucleus. hence the total dipole moment direction will be too upward and adding the magnitude gives it the highest dipole moment among all.

   Structure of ammonia
On the other hand, the electronegativity of P = 2.19, As= 2.18, Sb= 2.05 while that of H= 2.20. when we will compare the electronegativity of them, hydrogen is more electronegative. Hence the direction of dipole moment of the bond will be towards hydrogen and dipole moment of between lone pairs will be upwards and since dipole moment is vector quantity therefore dipole moments get cancelled out some.

Since the electronegativity difference between Sb and hydrogen is more, hence the order will be $NH3 > SbH3 > AsH3 > PH3$.

So the correct option is D.

Note: the dipole moment is a measure of the separation of positive and negative electrical charges within a system, that means it is a measure of the system’ s overall polarity. Basically tells about the charge separation. Since the dipole moment is a vector quantity, its value depends on magnitude as well as direction. So except for ammonia, the hydrogen becomes the negative end. And the dipole vector cancels out.