氢键 - 定义、效果、类型、性质

氢键是分子之间的静电偶极-偶极接触，具有某些共价键合特性。氢键强且定向，导致原子间距离更短，相互作用伙伴数量少。其每摩尔氢键的强度范围为 4 kJ 至 50 kJ。在氢键中，一对孤对富电子供体原子，特别是第二排元素如氮、氧和氟之间的相互作用，以及氢与一个更具电负性的原子或基团之间的键的反键分子轨道之间的相互作用是称为部分分子间键合。

虽然氢键在静电和共价上都有贡献，但它们的作用程度仍然存在争议。目前的研究表明，主要贡献是共价的。

氢键

Hydrogen bonding is the development of hydrogen bonds, which are a type of attractive intermolecular force caused by the dipole-dipole interaction between a hydrogen atom bound to a strongly electronegative atom and another very electronegative atom nearby. Hydrogen is covalently bound to the more electronegative oxygen atom in water molecules (H₂O). 

由于一个水分子的氢原子与另一个 H2O 分子的氧原子之间的偶极-偶极相互作用，在水分子中发生氢键。

在这种情况下，OH键中的电子键对非常接近氧核（由于氧和氢的电负性差异很大）。结果，氢原子产生部分正电荷（+），而氧原子产生部分负电荷（-）。

一个水分子（带+电荷）的氢原子和另一个水分子（带-电荷）的氧原子之间的静电相互作用现在可以产生氢键。因此，氢键是一种独特的分子间吸引力，仅当氢原子与高电负性原子结合时才会发生。与典型的偶极-偶极力和色散力相比，氢键主要是强键。然而，它们比实际的共价或离子连接弱。

Conditions required for Hydrogen Bonding

• A highly electronegative atom must be coupled to the hydrogen atom in the molecule. The polarisation of a molecule is proportional to its electronegativity.
• The electronegative atom should be modest in size. The greater the electrostatic attraction, the smaller the size.

氢键对元素的影响

1. 离解： HF 在水中离解并释放出二氟离子而不是氟离子。在 HF 中，这是由于氢键。 HCl、HBr 和 HI 分子不形成氢键。这就解释了为什么不存在 KHCl ₂ 、KHBr ₂和 KHI ₂等化学物质。
2. 缔合：由于氢键，羧酸分子以二聚体形式存在。此类化合物的分子量是根据其简单公式计算得出的分子量的两倍。

氢键的例子

• 水——一个高度负电的氧原子与水分子中的一个氢原子相连。共享的电子对更多地被氧原子吸引，分子的这一端变为负，而氢原子变为正。

• 氟化氢——最强的氢键是由具有最高电负性的氟形成的。

• 氨——它包含具有强电负性并与氢原子偶联的氮原子。

• 酒精和羧酸

Strength of Hydrogen Bond: The hydrogen bond is a relatively weak one. Hydrogen bonds have a strength that is halfway between weak van der Waals forces and strong covalent bonds. The attraction of the shared pair of electrons, and hence the atom’s electronegativity, determines the hydrogen bond’s dissociation energy.

氢键的性质

1. 挥发性——在不同分子之间结合氢键的化合物的沸点更高，因此它们的挥发性更低。
2. 溶解性——由于水和醇分子之间可能发生氢键，低级醇可溶于水。
3. 冰的密度比水低——在固体冰的情况下，氢键使水分子形成笼状结构。事实上，每个水分子都四面体连接到其他四个水分子。在固态下，分子不像液态那样紧密排列。当冰融化时，这种类似外壳的结构会坍塌，使分子靠得更近。结果，对于相同数量的水，水的体积减少，而密度增加。因此，在 273 K 时，冰的密度低于水。冰因此而漂浮。
4. 粘度和表面张力——氢键存在于具有相关分子的化合物中。结果，它们的流程变得更加复杂。它们具有高表面张力并具有较高的粘度。

Why do hydrogen-bonded compounds have high melting and boiling points?

The melting and boiling temperatures of hydrogen-bonded compounds are unusually high. The elevated melting and boiling points of hydrogen-bonded compounds are attributable to the additional energy required to break these bonds.

• At room temperature, H₂O is a liquid, whereas H₂S, H₂Se, and H₂Te are all gases. Hydrogen bonding produces links in the water molecules, resulting in water having a higher boiling point than the other chemicals.
• Because there is hydrogen bonding in NH₃, but not in PH₃, ammonia has a higher boiling point than PH₃.
• The presence of hydrogen bonding accounts for hydrogen fluoride’s particularly high boiling point among the halogen acids.
• Because ethanol contains hydrogen bonds, it has a higher boiling point than diethyl ether.

氢键的类型

H键有两种类型，标记如下：

1. 分子间氢键 -当相同或不同物质的分子之间形成氢键时，就会发生分子间氢键。例如，水、酒精和氨中的氢键。
2. 分子内氢键 -分子内氢键是指在单个分子内发生的氢键。它出现在具有两个基团的化合物中，其中一个基团具有与电负性原子相连的氢原子，而另一个具有与另一个基团的电负性较小的原子相连的高电负性原子。这种联系是在一组的电负性更高的原子和另一组的氢原子之间建立的。

示例问题

问题 1：状态八位字节规则。

回答：

Atoms are most stable when their valence shells are filled with eight electrons, according to the octet rule. It is based on the observation that the atoms of the major group elements have a proclivity for chemical bonding in such a way that each atom in the resulting molecule has eight electrons in the valence shell. Only the core group elements are subject to the octet rule.

问题2：影响Ionic键形成的因素有哪些？

回答：

Factors affecting the formation of Ionic Bond are:- 

• Ionization Enthalpy
• Electron Gain Enthalpy
• Lattice Energy

问题 3：列出Ionic化合物的任意三个特征。

回答：

Characteristics of Ionic Compounds are as follows:-

1. The melting and boiling points of ionic compounds are usually quite high. This is due to the strong electrostatic forces that hold ions together in ionic compounds.
2. Ionic compounds are frequently found in solid form.
3. Ionic compound solutions are excellent electrical conductors. In their molten condition, they are also good conductors of electricity.

问题4：什么是氢键？

回答：

When a hydrogen atom is coupled to a highly electronegative atom, the shared pair of electrons are attracted more by this atom, and the molecules’ negative end becomes slightly negative while the positive end becomes slightly positive.The negative end of one molecule attracts the positive end of the other, resulting in the formation of a weak bond. This connection is referred to as Hydrogen Bonding.

问题5：氢键需要什么条件？

回答：

Conditions required for Hydrogen Bonding are:

• A highly electronegative atom must be coupled to the hydrogen atom in the molecule. The polarisation of a molecule is proportional to its electronegativity.
• The electronegative atom should be modest in size. The greater the electrostatic attraction, the smaller the size.