Intermolecular Forces and Hydrogen Bonding | 分子间作用力与氢键

📚 Intermolecular Forces and Hydrogen Bonding | 分子间作用力与氢键

In Edexcel A Level Chemistry, understanding intermolecular forces is essential for explaining physical properties such as boiling point, solubility, viscosity and surface tension. These forces act between molecules, not within molecules, and are much weaker than covalent, ionic or metallic bonds.

在 Edexcel A Level 化学中，理解分子间作用力对于解释沸点、溶解性、粘度和表面张力等物理性质至关重要。这些力作用于分子之间，而不是分子内部，并且比共价键、离子键或金属键弱得多。

1. What Are Intermolecular Forces? | 什么是分子间作用力？

Intermolecular forces are the attractive or repulsive forces that exist between neighbouring molecules. They are often called weak forces because they are typically 1-10% of the strength of an average covalent bond, yet they determine whether a substance is a gas, liquid or solid at room temperature.

分子间作用力是相邻分子之间存在的吸引或排斥力。它们常被称为弱作用力，因为其强度通常只有平均共价键强度的 1-10%，但它们决定了物质在室温下是气体、液体还是固体。

It is important to distinguish between intermolecular forces and intramolecular bonds. Intramolecular bonds, such as covalent bonds, hold atoms together inside a molecule, while intermolecular forces hold separate molecules together in a liquid or solid lattice.

区分分子间作用力和分子内化学键非常重要。共价键等分子内化学键将原子连接在分子内部，而分子间作用力则将独立的分子维系在液体或固体中。

There are three main types of intermolecular forces studied at A Level: London dispersion forces, permanent dipole-dipole forces and hydrogen bonds. They all originate from the distribution of electrons in molecules and their relative strengths can be ranked as follows:

A Level 阶段主要研究三种分子间作用力：伦敦色散力、永久偶极-偶极作用力和氢键。它们都来源于分子中电子的分布，相对强度可以排序如下：

London forces < permanent dipole-dipole forces < hydrogen bonds

This ordering assumes molecules of similar size or molar mass; a large non-polar molecule may have stronger London forces than a small polar molecule.

这个顺序通常假定分子的尺寸或摩尔质量相近；一个较大的非极性分子可能比一个较小的极性分子具有更强的伦敦色散力。

2. London Dispersion Forces | 伦敦色散力

London dispersion forces, also called instantaneous dipole-induced dipole forces, act between all atoms and molecules. They arise because electrons are constantly moving, and at any instant the electron density around a molecule may become unevenly distributed.

伦敦色散力，也称为瞬时偶极-诱导偶极力，存在于所有原子和分子之间。它的产生是因为电子在不断运动，在任意瞬间，分子周围的电子密度可能不均匀分布。

This uneven distribution creates a temporary or instantaneous dipole. This temporary dipole can repel or attract electrons in a neighbouring molecule, inducing another dipole. The attraction between the instantaneous dipole and the induced dipole is the London force.

这种不均匀分布产生一个瞬时偶极。这个瞬时偶极可以排斥或吸引邻近分子中的电子，从而诱导出另一个偶极。瞬时偶极与诱导偶极之间的吸引力就是伦敦色散力。

The strength of London forces increases with the number of electrons in a molecule because larger electron clouds are more polarisable. This explains why boiling points generally increase down a homologous series or group as molar mass increases.

伦敦色散力的强度随着分子中电子数量的增加而增大，因为更大的电子云更容易被极化。这解释了为什么随着摩尔质量增大，同系物或同一族的沸点通常会升高。

For example, in the alkanes, CH₄ has a boiling point of -162 °C while C₈H₁₈ has a much higher boiling point of 126 °C. The larger octane molecule has more electrons and a larger surface area, leading to stronger London forces.

例如，在烷烃中，CH₄ 的沸点为 -162 °C，而 C₈H₁₈ 的沸点高得多，为 126 °C。较大的辛烷分子拥有更多电子和更大的表面积，因此伦敦色散力更强。

3. Permanent Dipole-Dipole Forces | 永久偶极-偶极作用力

Permanent dipole-dipole forces occur between polar molecules. A polar molecule has a permanent separation of positive and negative charge because it contains polar bonds arranged so that their dipoles do not cancel.

永久偶极-偶极作用力存在于极性分子之间。极性分子由于含有极性键且偶极矩不相互抵消，因此具有正电荷和负电荷的永久分离。

The positive end of one polar molecule attracts the negative end of another polar molecule. This force is stronger than London forces for molecules of comparable size, but weaker than hydrogen bonding.

一个极性分子的正电端会吸引另一个极性分子的负电端。对于大小相近的分子，这种力比伦敦色散力强，但比氢键弱。

A common example is hydrogen chloride, HCl. The chlorine atom is more electronegative than hydrogen, so the H-Cl bond is polar, with Hδ⁺ and Clδ⁻. Adjacent HCl molecules align with opposite charges facing each other.

一个常见的例子是氯化氢 HCl。氯原子的电负性比氢原子大，因此 H-Cl 键是极性键，分别带有 Hδ⁺ 和 Clδ⁻。相邻的 HCl 分子会以相反电荷相对的方式排列。

When comparing substances with similar molar masses, polar molecules typically have higher boiling points than non-polar molecules because dipole-dipole forces add to the London forces that are always present.

在比较摩尔质量相近的物质时，极性分子的沸点通常高于非极性分子，因为永久偶极-偶极作用力会叠加在始终存在的伦敦色散力之上。

4. Hydrogen Bonding: Definition and Requirements | 氢键：定义与形成条件

Hydrogen bonding is the strongest type of intermolecular force. It is a special type of permanent dipole-dipole attraction that occurs when a hydrogen atom is covalently bonded to a very electronegative atom: nitrogen, oxygen or fluorine.

氢键是最强的分子间作用力。它是一种特殊的永久偶极-偶极吸引力，发生在氢原子与电负性很强的氮、氧或氟原子形成共价键时。

The hydrogen atom carries a significant positive charge because the bonding electrons are pulled towards the electronegative N, O or F atom. This highly polar bond allows a lone pair on a neighbouring N, O or F atom to form a strong directional attraction.

由于成键电子被拉向电负性很强的 N、O 或 F 原子，氢原子带有明显的正电荷。这种高度极化的键使相邻的 N、O 或 F 原子上的孤对电子能够形成强烈的方向性吸引力。

A hydrogen bond can be represented as X-H···Y, where X and Y are both N, O or F. The dotted line shows the hydrogen bond, while the solid line shows the covalent bond inside the molecule.

氢键可以表示为 X-H···Y，其中 X 和 Y 都是 N、O 或 F。虚线表示氢键，实线表示分子内部的共价键。

Water is the most important example. Each H₂O molecule has two O-H bonds and two lone pairs on oxygen, so it can form hydrogen bonds with up to four neighbouring water molecules in a tetrahedral arrangement.

水是最重要的例子。每个 H₂O 分子有两个 O-H 键和氧原子上的两个孤对电子，因此它可以与最多四个相邻水分子形成四面体排列的氢键。

Hydrogen bonding is responsible for the unusual properties of water, including its relatively high boiling point, high surface tension, and the fact that ice is less dense than liquid water.

氢键是水具有异常性质的原因，包括相对较高的沸点、高表面张力，以及冰的密度小于液态水的现象。

5. Explaining Anomalous Boiling Points of Hydrides | 解释氢化物沸点的异常

A classic Edexcel exam question asks why NH₃, H₂O and HF have much higher boiling points than the heavier hydrides in their groups. The answer is hydrogen bonding.

Edexcel 考试中一个经典问题是：为什么 NH₃、H₂O 和 HF 的沸点远高于同族中较重的氢化物。答案是氢键。

For most hydrides, boiling point increases down a group because London forces increase with increasing molar mass. However, NH₃, H₂O and HF break this trend because their small, highly electronegative N, O and F atoms allow strong hydrogen bonding between molecules.

大多数氢化物的沸点沿族向下升高，因为伦敦色散力随摩尔质量增加而增强。然而，NH₃、H₂O 和 HF 打破了这一趋势，因为它们较小且电负性很强的 N、O 和 F 原子使分子之间能够形成强氢键。

Hydride	Boiling point / °C
CH₄	-162
NH₃	-33
H₂O	100
HF	20

The table shows that water has an exceptionally high boiling point for such a small molecule. Without hydrogen bonding, H₂O would be expected to boil well below 0 °C on the basis of London forces alone.

表中显示，对于如此小的分子，水的沸点异常高。如果没有氢键，仅依据伦敦色散力预测，H₂O 的沸点应远低于 0 °C。

Water has a higher boiling point than HF even though F is more electronegative than O because each H₂O molecule can form two hydrogen bonds through its two O-H hydrogens, while each HF molecule has only one hydrogen atom available for hydrogen bonding.

尽管 F 的电负性比 O 大，但水的沸点仍高于 HF，因为每个 H₂O 分子可以通过两个 O-H 氢原子形成两个氢键，而每个 HF 分子只有一个可用于形成氢键的氢原子。

6. Comparing IMF Strength and Boiling Points | 比较分子间作用力强度与沸点

When substances have similar molar masses, the type and strength of intermolecular forces determine their boiling points. The substance with the weakest intermolecular forces usually has the lowest boiling point.

当物质的摩尔质量相近时，分子间作用力的类型和强度决定了它们的沸点。分子间作用力最弱的物质通常沸点最低。

For example, butane (CH₃CH₂CH₂CH₃), propanal (CH₃CH₂CHO) and propan-1-ol (CH₃CH₂CH₂OH) have similar molar masses. Butane is non-polar, propanal is polar, and propan-1-ol can form hydrogen bonds.

例如，丁烷 (CH₃CH₂CH₂CH₃)、丙醛 (CH₃CH₂CHO) 和正丙醇 (CH₃CH₂CH₂OH) 的摩尔质量相近。丁烷是非极性分子，丙醛是极性分子，而正丙醇可以形成氢键。

Butane has only London forces, so it has the lowest boiling point.

丁烷只有伦敦色散力，因此沸点最低。
Propanal has London forces and permanent dipole-dipole forces, so its boiling point is higher.

丙醛既有伦敦色散力又有永久偶极-偶极作用力，因此沸点较高。
Propan-1-ol has London forces, permanent dipole-dipole forces and hydrogen bonds, giving it the highest boiling point of the three.

正丙醇具有伦敦色散力、永久偶极-偶极作用力和氢键，因此三者中沸点最高。

Always compare molar mass first when explaining boiling point trends. If molar masses are similar, then compare the types of intermolecular force present.

解释沸点趋势时，应先比较摩尔质量。如果摩尔质量相近，再比较分子间作用力的类型。

7. Solubility and ‘Like Dissolves Like’ | 溶解性与“相似相溶”

Solubility depends on the balance between solute-solute, solvent-solvent and solute-solvent interactions. A solute dissolves well when the solute-solvent forces are strong enough to overcome the forces between solute particles and between solvent particles.

溶解性取决于溶质-溶质、溶剂-溶剂和溶质-溶剂相互作用之间的平衡。当溶质-溶剂作用力足够强，能够克服溶质粒子之间和溶剂粒子之间的作用力时，溶质就能很好地溶解。

The simple rule ‘like dissolves like’ means that polar or hydrogen-bonding solutes tend to dissolve in polar solvents such as water, while non-polar solutes tend to dissolve in non-polar solvents such as hexane.

“相似相溶”这一简单规则意味着极性或能形成氢键的溶质倾向于溶解在水等极性溶剂中，而非极性溶质倾向于溶解在己烷等非极性溶剂中。

Ethanol is completely miscible with water because both molecules can form hydrogen bonds. The -OH group in ethanol can hydrogen bond with water molecules, while the non-polar ethyl group also allows ethanol to dissolve many non-polar substances.

乙醇能与水完全互溶，因为两种分子都能形成氢键。乙醇中的 -OH 基团可以与水分子形成氢键，而非极性的乙基也使乙醇能够溶解许多非极性物质。

In contrast, octane does not dissolve in water. Water molecules strongly hydrogen bond to each other, and octane cannot form strong attractions with water, so dissolving would require breaking many hydrogen bonds for little energy gain.

相比之下，辛烷不溶于水。水分子之间会形成强氢键，而辛烷无法与水形成强吸引力，因此溶解辛烷需要破坏大量氢键却几乎没有能量补偿。

8. Viscosity, Surface Tension and Density Anomalies | 粘度、表面张力与密度异常

Intermolecular forces also affect viscosity and surface tension. Stronger intermolecular forces lead to higher viscosity because molecules resist flowing past one another.

分子间作用力还影响粘度和表面张力。分子间作用力越强，粘度越高，因为分子抵抗相对流动的能力更强。

Surface tension arises because molecules at the surface of a liquid experience an unbalanced pull from molecules below. Liquids with strong intermolecular forces, especially hydrogen-bonded liquids like water, have high surface tension.

表面张力的产生是因为液体表面的分子受到下方分子的不平衡拉力。分子间作用力强的液体，尤其是水等能形成氢键的液体，具有很高的表面张力。

Water has an unusually high surface tension, which allows small insects to walk on its surface. This is directly caused by the network of hydrogen bonds between water molecules.

水的表面张力异常高，使小型昆虫能在水面上行走。这直接由水分子之间的氢键网络造成。

Ice is less dense than liquid water because hydrogen bonding holds water molecules in a relatively open tetrahedral lattice in the solid state. When ice melts, some hydrogen bonds break and molecules can pack more closely together.

冰的密度小于液态水，因为在固态下，氢键将水分子固定在相对开放的四面体晶格中。当冰融化时，部分氢键断裂，分子可以更紧密地堆积在一起。

This density anomaly is rare among liquids and is important for aquatic life, as ice floats and insulates the liquid water below.

这种密度异常在液体中很少见，对水生生物很重要，因为冰能漂浮并隔绝下方的液态水。

9. Intermolecular Forces in Organic Chemistry | 有机化学中的分子间作用力

Intermolecular forces explain many trends in organic chemistry. Alcohols have much higher boiling points than alkanes of similar molar mass because alcohols can form hydrogen bonds, while alkanes cannot.

分子间作用力可以解释有机化学中的许多趋势。醇的沸点远高于摩尔质量相近的烷烃，因为醇能形成氢键，而烷烃不能。

Carboxylic acids have even higher boiling points than alcohols of similar molar mass because two carboxylic acid molecules can form two hydrogen bonds with each other, creating a dimer held together by a pair of O-H···O interactions.

羧酸的沸点甚至高于摩尔质量相近的醇，因为两个羧酸分子可以彼此形成两个氢键，通过一对 O-H···O 相互作用形成二聚体。

Amines and alcohols both form hydrogen bonds because they contain N-H and O-H bonds respectively. However, O-H hydrogen bonds are generally stronger than N-H hydrogen bonds because oxygen is more electronegative than nitrogen.

胺和醇都能形成氢键，因为它们分别含有 N-H 和 O-H 键。但 O-H 氢键通常比 N-H 氢键强，因为氧的电负性大于氮。

Haloalkanes are polar molecules and have permanent dipole-dipole forces,

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Intermolecular Forces and Hydrogen Bonding | 分子间作用力与氢键