A-Level Chemistry: Intermolecular Forces — Types, Strength Comparison, and Effects on Physical Properties

A-Level 化学：分子间作用力 — 类型、强度比较及其对物理性质的影响

Intermolecular forces are one of the most fundamental and frequently examined topics in A-Level Chemistry. Understanding these forces is essential for explaining physical properties such as boiling points, melting points, solubility, and volatility. This article provides a comprehensive bilingual guide covering all types of intermolecular forces, their relative strengths, and how to apply this knowledge in exam-style questions.

分子间作用力是 A-Level 化学中最基础、最常考的主题之一。理解这些力对于解释物理性质（如沸点、熔点、溶解度和挥发性）至关重要。本文提供全面的中英双语指南，涵盖所有类型的分子间作用力、它们的相对强度，以及如何在考试题型中应用这些知识。

1. Intramolecular vs Intermolecular Forces

分子内力 vs 分子间力

Before diving into intermolecular forces, it is crucial to distinguish them from intramolecular forces — the forces that hold atoms together within a molecule:

在深入分子间作用力之前，必须将其与分子内力区分开来——即把原子保持在分子内部的力：

Intramolecular forces (分子内力): Covalent bonds, ionic bonds, and metallic bonds — these hold atoms together within a molecule or compound. Covalent bond energy: ~150–800 kJ mol⁻¹.
Intermolecular forces (分子间力): Forces between separate molecules — much weaker, typically 1–40 kJ mol⁻¹.

The key point for exams: When a molecular substance melts or boils, intermolecular forces are overcome, NOT covalent bonds. The molecules themselves remain intact; only the forces between them are disrupted. This is why simple molecular substances have relatively low melting and boiling points compared to giant covalent or ionic structures.

考试的关键点：当分子物质熔化或沸腾时，被克服的是分子间作用力，而非共价键。分子本身保持完整，只有它们之间的力被破坏。这就是为什么简单分子物质与巨型共价或离子结构相比，熔点和沸点相对较低。

2. Types of Intermolecular Forces

分子间作用力的类型

There are three main types of intermolecular forces, listed here from weakest to strongest:

分子间作用力主要有三种类型，从最弱到最强排列如下：

2.1 London Dispersion Forces (Instantaneous Dipole–Induced Dipole)

伦敦色散力（瞬时偶极–诱导偶极）

London dispersion forces exist between all molecules — polar and non-polar alike. They arise from the constant motion of electrons within molecules:

伦敦色散力存在于所有分子之间——无论是极性分子还是非极性分子。它们源于分子内电子的不断运动：

At any instant, the electron distribution in a molecule may become asymmetrical, creating a temporary (instantaneous) dipole.
This instantaneous dipole can induce a dipole in a neighbouring molecule by repelling its electrons.
The resulting electrostatic attraction between the two temporary dipoles is the London dispersion force.

在任何瞬间，分子中的电子分布可能变得不对称，产生一个临时（瞬时）偶极。
这个瞬时偶极可以通过排斥邻近分子的电子，诱导该分子产生偶极。
两个临时偶极之间的静电吸引力就是伦敦色散力。

Factors affecting London dispersion forces:

影响伦敦色散力的因素：

Number of electrons (电子数量): More electrons → larger electron cloud → more easily polarised → stronger London forces. This explains why boiling points increase down Group 18 (noble gases): He (−269°C) < Ne (−246°C) < Ar (−186°C) < Kr (−152°C) < Xe (−108°C).
Surface area / molecular shape (分子表面积/形状): Molecules with greater surface contact (e.g., straight-chain alkanes) have stronger London forces than branched isomers. For example, n-pentane (bp 36°C) vs neopentane (bp 9.5°C) — both C₅H₁₂ but different shapes.

2.2 Permanent Dipole–Dipole Interactions

永久偶极–偶极相互作用

Permanent dipole–dipole interactions occur between polar molecules — molecules that have a permanent separation of charge due to differences in electronegativity between bonded atoms.

永久偶极–偶极相互作用发生在极性分子之间——由于键合原子之间电负性差异而具有永久电荷分离的分子。

The δ+ end of one polar molecule is attracted to the δ− end of another. This is an additional force on top of London dispersion forces.

一个极性分子的 δ+ 端被另一个极性分子的 δ− 端吸引。这是在伦敦色散力之上的额外力。

Key exam examples:

常考例子：

Molecule	Polar?	Reason
HCl	Yes ✓	Cl is more electronegative than H (δ+H–Clδ−)
CO₂	No ✗	Linear shape — dipoles cancel
H₂O	Yes ✓	Bent shape — dipoles do not cancel
CCl₄	No ✗	Tetrahedral — individual bond dipoles cancel
CHCl₃	Yes ✓	Tetrahedral but asymmetrical — dipoles do not cancel
CH₃COCH₃ (propanone)	Yes ✓	C=O bond is polar

Exam tip: Always draw the molecular shape first. If the molecule is symmetrical, the individual bond dipoles may cancel, making the molecule non-polar overall — despite having polar bonds.

考试技巧：始终先画出分子形状。如果分子是对称的，单个键偶极可能会抵消，使分子整体为非极性——尽管有极性键。

2.3 Hydrogen Bonding

氢键

Hydrogen bonding is the strongest type of intermolecular force (typically 10–40 kJ mol⁻¹). It is a special, stronger case of permanent dipole–dipole interaction that occurs only when hydrogen is covalently bonded to a very electronegative atom with a lone pair of electrons.

氢键是最强的分子间作用力类型（通常为 10–40 kJ mol⁻¹）。它是一种特殊的、更强的永久偶极–偶极相互作用，仅当氢与具有孤对电子的高电负性原子共价键合时才会发生。

Requirements for hydrogen bonding:

氢键的形成条件：

A hydrogen atom covalently bonded to a highly electronegative atom: N, O, or F (nitrogen, oxygen, fluorine).
The electronegative atom must have at least one lone pair of electrons.

一个氢原子与高电负性原子共价键合：N、O 或 F（氮、氧、氟）。
该电负性原子必须至少有一对孤对电子。

Classic exam examples:

典型考试例子：

Water (H₂O): The δ+H on one water molecule is attracted to the lone pair on the δ−O of another. This explains water’s anomalously high boiling point (100°C) compared to H₂S (−60°C), H₂Se (−41°C), and H₂Te (−2°C).
Ammonia (NH₃): N–H···N hydrogen bonds. Explains why ammonia has a higher boiling point (−33°C) than PH₃ (−88°C).
Hydrogen fluoride (HF): F–H···F hydrogen bonds — the strongest due to fluorine’s extreme electronegativity. Explains HF’s boiling point (19.5°C) compared to HCl (−85°C).
Alcohols (ROH): O–H···O hydrogen bonds. Explains why ethanol (bp 78°C) has a much higher boiling point than ethane (bp −89°C), despite similar molecular mass.
Carboxylic acids (RCOOH): Form dimers through two hydrogen bonds. Explains why ethanoic acid (bp 118°C) boils higher than propan-1-ol (bp 97°C), even though the alcohol can also hydrogen-bond — the dimer is more stable.

水 (H₂O)：一个水分子上的 δ+H 被另一个水分子上 δ−O 的孤对电子吸引。这解释了水异常高的沸点 (100°C)，相比之下 H₂S (−60°C)、H₂Se (−41°C) 和 H₂Te (−2°C) 要低得多。
氨 (NH₃)：N–H···N 氢键。解释了为什么氨的沸点 (−33°C) 高于 PH₃ (−88°C)。
氟化氢 (HF)：F–H···F 氢键——由于氟的极端电负性而最强。解释了 HF 的沸点 (19.5°C) 与 HCl (−85°C) 的比较。
醇类 (ROH)：O–H···O 氢键。解释了为什么乙醇 (bp 78°C) 的沸点远高于乙烷 (bp −89°C)，尽管分子质量相似。
羧酸 (RCOOH)：通过两个氢键形成二聚体。解释了为什么乙酸 (bp 118°C) 的沸点高于丙-1-醇 (bp 97°C)，尽管醇也能形成氢键——二聚体更稳定。

Common misconception: Students often mistakenly think HCl can form hydrogen bonds. It cannot! Chlorine, despite being electronegative, is too large — its lone pairs are in the 3p orbital, which is too diffuse to form strong hydrogen bonds. Hydrogen bonding is ONLY between H and N, O, or F.

常见误区：学生常常错误地认为 HCl 可以形成氢键。它不能！氯尽管电负性较强，但原子太大——其孤对电子在 3p 轨道中，过于弥散而无法形成强氢键。氢键仅存在于 H 与 N、O 或 F 之间。

3. Strength Comparison

强度比较

The hierarchy of intermolecular forces from weakest to strongest:

分子间作用力从最弱到最强的层级：

London dispersion forces < Permanent dipole–dipole < Hydrogen bonding

伦敦色散力 < 永久偶极–偶极 < 氢键

However, this is an oversimplification. A molecule with very strong London forces (due to many electrons) can have a higher boiling point than a smaller molecule with hydrogen bonding. For example:

然而，这是一个过度简化。具有非常强伦敦色散力的分子（由于电子数量多）可能比具有氢键的较小分子有更高的沸点。例如：

Octane (C₈H₁₈): only London forces, bp = 126°C
Water (H₂O): hydrogen bonding, bp = 100°C
Methanol (CH₃OH): hydrogen bonding, bp = 65°C

Octane boils higher than both water and methanol despite having “only” London forces, because its large electron cloud (66 electrons vs water’s 10) generates substantial London dispersion forces.

辛烷的沸点比水和甲醇都高，尽管它”只有”伦敦色散力，因为其大的电子云（66 个电子 vs 水的 10 个）产生了可观的伦敦色散力。

Exam strategy for comparing boiling points:

比较沸点的考试策略：

First, check for hydrogen bonding (N–H, O–H, or F–H bonds present?).
If H-bonding is present in both, compare the number/extent of hydrogen bonds (e.g., water forms 2 per molecule on average; alcohols form fewer due to steric hindrance).
Then check for permanent dipoles (polar bonds in an asymmetrical molecule).
Finally, compare number of electrons (more electrons → stronger London forces).

首先检查是否存在氢键（有 N–H、O–H 或 F–H 键吗？）。
如果两者都有氢键，比较氢键的数量/程度（例如，水平均每个分子形成 2 个氢键；由于空间位阻，醇形成的氢键较少）。
然后检查永久偶极（不对称分子中的极性键）。
最后，比较电子数量（电子越多 → 伦敦色散力越强）。

4. Effects on Physical Properties

对物理性质的影响

4.1 Boiling Point and Melting Point

沸点和熔点

Stronger intermolecular forces → more energy required to separate molecules → higher boiling/melting point.

分子间作用力越强 → 分离分子所需的能量越多 → 沸点/熔点越高。

Trend down Group 17 (halogens): F₂ (bp −188°C) → Cl₂ (bp −34°C) → Br₂ (bp 59°C) → I₂ (bp 184°C). All are non-polar, so only London forces operate. The increasing number of electrons down the group explains the trend.

第 17 族（卤素）的趋势：F₂ (bp −188°C) → Cl₂ (bp −34°C) → Br₂ (bp 59°C) → I₂ (bp 184°C)。全部是非极性的，所以只有伦敦色散力在起作用。族内向下电子数增加解释了这一趋势。

Anomalous boiling points of H₂O, HF, and NH₃: These three compounds have significantly higher boiling points than expected from the trend of their group hydrides, due to hydrogen bonding.

H₂O、HF 和 NH₃ 的异常沸点：这三种化合物的沸点明显高于其族氢化物的趋势预期，这是由于氢键的存在。

Group 16 Hydrides	bp (°C)	Group 17 Hydrides	bp (°C)	Group 15 Hydrides	bp (°C)
H₂O	100	HF	19.5	NH₃	−33
H₂S	−60	HCl	−85	PH₃	−88
H₂Se	−41	HBr	−67	AsH₃	−62
H₂Te	−2	HI	−35	SbH₃	−17

4.2 Solubility

溶解度

The rule of thumb: “Like dissolves like” — substances with similar intermolecular forces tend to be mutually soluble.

经验法则：“相似相溶”——具有相似分子间作用力的物质往往相互可溶。

Polar solvents dissolve polar/ionic solutes: Water (polar, H-bonding) dissolves ethanol, NaCl, sugars.
Non-polar solvents dissolve non-polar solutes: Hexane dissolves iodine (I₂), oils, and fats.

极性溶剂溶解极性/离子溶质：水（极性，氢键）溶解乙醇、NaCl、糖类。
非极性溶剂溶解非极性溶质：己烷溶解碘 (I₂)、油脂。

Why alcohols are miscible with water — up to a point: Methanol, ethanol, and propan-1-ol are fully miscible with water because the –OH group can hydrogen-bond with water. However, as the hydrocarbon chain lengthens (butan-1-ol → pentan-1-ol → hexan-1-ol), the non-polar alkyl chain increasingly disrupts the hydrogen-bonding network, and solubility decreases. Hexan-1-ol is only sparingly soluble in water.

为什么醇与水可混溶——但有上限：甲醇、乙醇和丙-1-醇与水完全混溶，因为–OH 基团可以与水形成氢键。然而，随着烃链增长（丁-1-醇 → 戊-1-醇 → 己-1-醇），非极性烷基链越来越破坏氢键网络，溶解度降低。己-1-醇仅微溶于水。

4.3 Viscosity

粘度

Liquids with stronger intermolecular forces tend to be more viscous (resistant to flow). For example, glycerol (propane-1,2,3-triol) is highly viscous because each molecule has three –OH groups, enabling extensive hydrogen bonding between molecules.

分子间作用力更强的液体往往更粘稠（抵抗流动）。例如，甘油（丙-1,2,3-三醇）非常粘稠，因为每个分子有三个 –OH 基团，使得分子间能够形成广泛的氢键。

5. Common Exam Question Patterns

常见考试题型

Q1: Explain the trend in boiling points of the hydrogen halides (HF, HCl, HBr, HI).

Answer approach: HF has the highest boiling point (19.5°C) due to hydrogen bonding. HCl (−85°C) → HBr (−67°C) → HI (−35°C) show a gradual increase due to increasing number of electrons (stronger London forces). The large drop from HF to HCl is because HCl cannot hydrogen-bond.

答题方法：HF 因氢键而具有最高的沸点 (19.5°C)。HCl (−85°C) → HBr (−67°C) → HI (−35°C) 呈逐渐上升趋势，因为电子数增加（伦敦色散力更强）。从 HF 到 HCl 的大幅下降是因为 HCl 不能形成氢键。

Q2: Why does ice float on water?

Answer: In ice, each water molecule forms four hydrogen bonds in a tetrahedral arrangement, creating an open lattice structure. This structure is less dense than liquid water, where hydrogen bonds are continuously breaking and reforming, allowing molecules to pack more closely. Maximum density of water occurs at 4°C.

答案：在冰中，每个水分子以四面体排列形成四个氢键，形成一个开放晶格结构。这种结构的密度低于液态水，在液态水中氢键不断断裂和重新形成，使分子能够更紧密地排列。水的最大密度出现在 4°C。

Q3: Compare the boiling points of butane (C₄H₁₀) and propanone (CH₃COCH₃).

Answer: Propanone (bp 56°C) boils higher than butane (bp −0.5°C), despite similar molecular size. Both have London forces, but propanone is polar (C=O bond) and therefore has permanent dipole–dipole interactions in addition to London forces. More energy is required to overcome both types of forces.

答案：丙酮 (bp 56°C) 的沸点高于丁烷 (bp −0.5°C)，尽管分子大小相似。两者都有伦敦色散力，但丙酮是极性的（C=O 键），因此除了伦敦色散力外还有永久偶极–偶极相互作用。克服这两种力需要更多能量。

Q4: Explain the solubility trend of the alcohols in water.

Answer: Short-chain alcohols are miscible with water because the –OH group hydrogen-bonds with water. As chain length increases, the non-polar alkyl portion dominates, disrupting water’s hydrogen-bonding network. The energy cost of breaking water’s H-bonds eventually outweighs the energy gain from alcohol–water H-bonding.

答案：短链醇与水混溶，因为–OH 基团与水形成氢键。随着链长增加，非极性烷基部分占主导地位，破坏水的氢键网络。打破水的氢键所消耗的能量最终超过了醇-水氢键所获得的能量。

6. Summary Table

总结表

Force Type	Present In	Typical Energy	Key Factor
London Dispersion	All molecules	1–10 kJ mol⁻¹	Number of electrons
Permanent Dipole–Dipole	Polar molecules only	3–15 kJ mol⁻¹	Electronegativity difference + asymmetry
Hydrogen Bonding	Molecules with H–N, H–O, or H–F	10–40 kJ mol⁻¹	Presence of N/O/F with lone pair

力类型	存在于	典型能量	关键因素
伦敦色散力	所有分子	1–10 kJ mol⁻¹	电子数量
永久偶极–偶极	仅极性分子	3–15 kJ mol⁻¹	电负性差异 + 不对称性
氢键	含 H–N、H–O 或 H–F 的分子	10–40 kJ mol⁻¹	存在有孤对电子的 N/O/F

7. Key Takeaways / 核心要点

Intermolecular forces are between molecules, not within them. Covalent bonds remain intact during phase changes of simple molecular substances.
London dispersion forces exist between ALL molecules and increase with electron count.
Permanent dipole–dipole interactions occur only in polar molecules and are additional to London forces.
Hydrogen bonding is the strongest intermolecular force and requires H bonded to N, O, or F.
Boiling point comparison strategy: Check H-bonding first → then polarity → then electron count.
Like dissolves like: Matching intermolecular force types between solute and solvent favours solubility.

分子间作用力存在于分子之间，而非分子内部。简单分子物质相变时共价键保持完整。
伦敦色散力存在于所有分子之间，并随电子数量增加而增强。
永久偶极–偶极相互作用仅发生在极性分子中，是伦敦色散力之外的额外力。
氢键是最强的分子间作用力，需要 H 与 N、O 或 F 键合。
沸点比较策略：首先检查氢键 → 然后检查极性 → 最后检查电子数量。
相似相溶：溶质和溶剂之间分子间作用力类型的匹配有利于溶解。

This article was written for A-Level Chemistry students studying intermolecular forces. Practice applying these concepts to past paper questions for the best exam preparation. For more A-Level Chemistry revision resources, explore our full collection of exam-board-specific study guides.

本文为学习分子间作用力的 A-Level 化学学生而编写。将所学应用到真题练习中，是备考的最佳方式。更多 A-Level 化学复习资源，请浏览我们针对各考试局的完整学习指南。

A-Level Chemistry: Intermolecular Forces — Types, Strength Comparison, and Effects on Physical Properties | A-Level 化学：分子间作用力详解