今日知识点：Intermolecular forces and boiling points

例题

Q: 

Nitrogen, N2, and carbon monoxide, CO, both have Mr = 28.

The boiling point of N2 is 77 K.

The boiling point of CO is 82 K.

What could be responsible for this difference in boiling points?

A   CO molecules have a permanent dipole, the N2 molecules are not polar.

B   N2 has σand π bonding, CO has σ bonding only.

C   N2 has a strong N≡N bond, CO has C=O bond.

D   The CO molecule has more electrons than the N2 molecule.

解析

Boling points are determined by the intermolecular forces.

Stronger the intermolecular forces, higher energy required to separate the particles, therefore higher boiling points.

N2 is a covalent molecule with a symmetrical structure, so it is a non-polar molecule. The intermolecular forces between N2 is dispersion forces ( or van der Waal forces )

C=O is a covalent molecule with a nonsymmetrical structure, so it is a polar molecules. O has a higher electronegativity, so the O end is partially negatively charged. The intermolecular forces between CO is dispersion forces and dipole-dipole attraction.

CO has a stronger intermolecular forces than N2，therefore higher boiling point.

Choose  A

下面我们为大家准备了一道同类型的题目，请大家一起来试试解答。

At room temperature and pressure, H2O is a liquid and H2S is a gas.

What is the reason for this difference?

A   O has higher first and second ionization energies than S。

B   The covalent bond between O and H is a stronger than the covalent bond between S and H.

C   There is significant hydrogen bonding between H2O molecules but not between H2S molecules.

D    The instantaneous dipole-induced dipole forces between H2O molecules are stronger than the instantaneous dipole-induced dipole forces between H2S molecules.

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正确答案： C