📚 Hydrogen Bonding: Formation and Its Impact on Properties | 氢键的形成及其影响
Hydrogen bonding is one of the most important intermolecular forces in A-Level Chemistry. It explains why water has a surprisingly high boiling point, why ice floats, and why DNA can store genetic information. In this revision article, we will examine the conditions for hydrogen bond formation, compare its strength with other intermolecular forces, and explore its broad effects on physical and biological properties.
氢键是 A-Level 化学中最重要的一类分子间作用力。它解释了为什么水的沸点异常高、为什么冰能浮在水面上，也解释了 DNA 为什么能储存遗传信息。在这篇复习文章中，我们将系统梳理氢键的形成条件、与其它分子间作用力的强弱比较，以及它对物理性质和生命现象的广泛影响。
1. What Is Hydrogen Bonding? | 什么是氢键？
A hydrogen bond is a strong type of permanent dipole–dipole interaction between a hydrogen atom covalently bonded to an electronegative atom, and a lone pair of electrons on another electronegative atom. The most common elements involved are nitrogen, oxygen and fluorine.
氢键是永久偶极－偶极作用力中一种较强类型，它存在于一个与电负性原子以共价键结合的氢原子，与另一个电负性原子上的孤对电子之间。最常见的参与元素是氮、氧和氟。
For a hydrogen bond to form, three conditions must be satisfied:
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The hydrogen atom must be covalently bonded to a very electronegative atom such as N, O or F. This polarises the H atom strongly, giving it a significant partial positive charge (δ+).
氢原子必须与 N、O 或 F 等电负性很强的原子成键，这会使氢原子被强烈极化，带上显著的部分正电荷（δ+）。
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The other molecule must have a lone pair on a highly electronegative atom such as N, O or F, which acts as the electron-pair acceptor region.
另一个分子必须具有位于高电负性原子（如 N、O 或 F）上的孤对电子，孤对电子所在的区域充当接受体。
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The hydrogen bond is strongest when the donor arrangement is nearly linear, for example A−H···B, where the dots represent the hydrogen bond.
当给体排列接近直线时氢键最强，例如 A−H···B，其中虚线点代表氢键。
2. Why Does Hydrogen Bonding Occur? | 氢键为什么会形成？
In a covalent bond between hydrogen and a highly electronegative atom, the shared pair of electrons is pulled away from hydrogen. The hydrogen atom therefore acquires a positive partial charge, while the electronegative atom acquires a negative partial charge. Because hydrogen has no inner core electrons, its positive charge is very concentrated at the nucleus, producing an intense local electric field.
在氢原子与高电负性原子形成的共价键中，共用电子对被拉向电负性更强的一侧。氢原子因此带上部分正电荷，而电负性较大的原子带上部分负电荷。由于氢原子没有内层电子，它的正电荷高度集中在原子核附近，形成强烈的局部电场。
This concentrated δ+ hydrogen atom is then attracted to the lone pair of an adjacent molecule. The attraction is mainly electrostatic, but it is unusually strong because the small size of the hydrogen atom allows the two molecules to approach each other very closely.
这个高电荷密度的 δ+ 氢原子随后会被邻近分子的孤对电子吸引。这种作用本质上主要是静电吸引，但由于氢原子半径极小，两个分子可以靠得非常近，因此氢键强度比普通偶极－偶极作用大得多。
We can represent the interaction between two water molecules as:
O−H···O
Here, the left O−H bond is the donor, while the right oxygen atom with its lone pair is the acceptor.
在上式中，左侧 O−H 键是给体，右侧带有孤对电子的氧原子是接受体。
3. How Strong Is a Hydrogen Bond? | 氢键有多强？
Hydrogen bonds are much stronger than ordinary dipole–dipole forces and London dispersion forces, but much weaker than covalent or ionic bonds. Their typical strength ranges from about 10 to 40 kJ mol⁻¹. A covalent bond usually requires 150 to 400 kJ mol⁻¹ to break.
氢键比普通偶极－偶极作用力和伦敦色散力强得多，但又远弱于共价键或离子键。氢键的典型强度约为 10 到 40 kJ mol⁻¹，而断裂一个共价键通常需要 150 到 400 kJ mol⁻¹。
| Interaction 作用类型 | Typical strength 典型强度 (kJ mol⁻¹) | Relative strength 相对强弱 |
|---|---|---|
| London dispersion forces 伦敦色散力 | 1–10 | Weakest 最弱 |
| Permanent dipole–dipole interactions 永久偶极－偶极作用 | 5–20 | Intermediate 中等 |
| Hydrogen bond 氢键 | 10–40 | Strong intermolecular force 很强的分子间作用力 |
| Covalent bond 共价键 | 150–400 | Strongest chemical bond 最强化学键 |
Hydrogen bonds are especially important in molecules of low relative molecular mass. Without hydrogen bonds, very light molecules such as H₂O and NH₃ would exist as gases at room temperature; in fact, hydrogen bonds allow them to remain liquid over a wide range of temperatures.
氢键对低相对分子质量的分子尤其重要。如果没有氢键，H₂O、NH₃ 这类很轻的分子在室温下都会是气体；事实上，正是氢键使它们在很大温度范围内保持液态。
4. Hydrogen Bonding in Water and Ice | 水与冰中的氢键
Water is the classic example of hydrogen bonding. Each water molecule contains two O−H bonds and two lone pairs on oxygen. Therefore, each water molecule can form up to four hydrogen bonds with surrounding water molecules.
水是氢键的经典例子。每个水分子含有两个 O−H 键和两对位于氧原子上的孤对电子。因此，每个水分子最多能与周围水分子形成四个氢键。
These hydrogen bonds give water several anomalous physical properties:
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High boiling point and large enthalpy of vaporisation, because substantial energy is needed to break the hydrogen bonds between molecules.
沸点高且汽化焓大，因为大量分子间氢键需要能量来断裂。
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High specific heat capacity, because absorbed heat is used to break hydrogen bonds rather than rapidly increasing the kinetic energy of the molecules.
比热容大，因为吸收的热量首先被用于破坏氢键，而不是迅速增大分子的平均动能。
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High surface tension, because surface molecules are pulled strongly inward by hydrogen-bonded neighbours.
表面张力大，因为表面分子受到邻近氢键作用向内强烈牵拉。
In ice, the water molecules arrange into an open tetrahedral lattice. Each oxygen atom is connected to four neighbouring oxygen atoms by hydrogen bonds. This network creates large empty spaces, so ice has a lower density than liquid water and floats.
在冰中，水分子排列成开阔的四面体晶格。每个氧原子通过氢键与相邻的四个氧原子相连。这种网状结构留下了大量空隙，因此冰的密度小于液态水，能够浮在水面上。
When ice melts, some hydrogen bonds are broken, allowing molecules to pack more closely together. This explains why water is densest at about 4 °C, rather than at its freezing point.
当冰融化时，部分氢键被破坏，分子可以排列得更紧密。这也解释了为什么水在约 4 °C 时密度最大，而不是在冰点处最大。
5. Effect on Boiling Points of Hydrides | 氢键对氢化物沸点的影响
Hydrogen bonding explains the anomalous boiling points of the hydrides of nitrogen, oxygen and fluorine. For groups 15, 16 and 17 of the Periodic Table, the hydride with the lightest relative molecular mass often has an unexpectedly high boiling point, because it can form effective hydrogen bonds.
氢键可以解释氮、氧、氟的氢化物沸点异常的原因。在第 15、16、17 族中，相对分子质量最小的氢化物往往具有异常高的沸点，因为它们能形成有效氢键。
The table below shows the boiling points of some hydrides. Notice that H₂O has a boiling point of 100 °C, much higher than H₂Te, despite having a far lower relative molecular mass.
下表中列出了一些氢化物的沸点。注意 H₂O 的沸点为 100 °C，尽管其相对分子质量远小于 H₂Te，其沸点却高得多。
| Hydride 氢化物 | Relative molecular mass 相对分子质量 | Boiling point 沸点 (°C) |
|---|---|---|
| H₂O | 18 | 100 |
| H₂S | 34 | −60 |
| H₂Se | 81 | −41 |
| H₂Te | 130 | −2 |
| HF | 20 | 20 |
| HCl | 36.5 | −85 |
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