📚 Intermolecular Forces: Edexcel A-Level Chemistry 2.18 | 分子间作用力：Edexcel A-Level 化学 2.18
In Edexcel A-Level Chemistry, specification point 2.18 focuses on the nature of three types of intermolecular forces: London dispersion forces, permanent dipole-dipole interactions, and hydrogen bonds. These forces hold molecules together in liquids and solids, and they control physical properties such as boiling point and solubility.
Edexcel A-Level 化学考纲 2.18 重点研究三类分子间作用力的本质：伦敦色散力、永久偶极-偶极相互作用和氢键。这些作用力使分子在液态和固态中聚集，并决定了沸点、溶解度等物理性质。
1. Topic Overview and Specification Map | 主题概览与考纲定位
Specification point 2.18 asks you to understand how London dispersion forces, permanent dipole-dipole interactions, and hydrogen bonds arise and to identify them in given molecules. This topic is often assessed alongside trends in boiling points, solubility, and the comparison of intermolecular forces with covalent, ionic, and metallic bonding.
考纲 2.18 要求理解伦敦色散力、永久偶极-偶极相互作用和氢键如何产生，并能在给定分子中识别它们。该主题常与沸点变化趋势、溶解度规律，以及分子间作用力与共价键、离子键、金属键的对比一起考查。
The learning objectives can be broken down into four examinable skills that you should be able to demonstrate without hesitation.
这一考点的学习目标可以拆解为四项可考查技能，你应当能够毫不犹豫地展示出来。
- Describe the origin of each of the three types of intermolecular force. | 描述三类分子间作用力各自的成因。
- Identify which forces are present in a named molecule or between two named molecules. | 判断某一给定分子中或两个给定分子之间存在哪些作用力。
- Explain trends in melting point, boiling point and solubility using intermolecular forces. | 用分子间作用力解释熔点、沸点和溶解度的变化趋势。
- Compare the strength of intermolecular forces with the strength of covalent, ionic and metallic bonds. | 把分子间作用力的强度与共价键、离子键、金属键进行比较。
In addition, the same specification area links directly to the shapes of molecules, because molecular geometry determines whether a molecule has a permanent dipole at all.
此外，这一考纲内容与分子形状直接相关，因为分子几何构型决定了分子是否具有永久偶极。
2. What Intermolecular Forces Actually Are | 分子间作用力究竟是什么
Intermolecular forces are weak attractive forces that act between separate molecules, or between separate groups of atoms, rather than within a single molecule. They are electrostatic in origin: they all arise from the attraction between regions of partial positive charge and regions of partial negative charge.
分子间作用力是作用在独立分子之间（或独立原子团之间）的弱吸引作用，而不是作用在单个分子内部。它们本质上都是静电作用：都源于部分正电荷区域与部分负电荷区域之间的吸引。
Because these forces are weak compared with covalent bonds, they are broken when a simple molecular substance melts or boils, while the covalent bonds inside each molecule stay intact.
由于这些作用力与共价键相比非常弱，简单分子物质熔化或沸腾时被破坏的是分子间作用力，而每个分子内部的共价键保持完整。
Boiling a simple molecular substance: intermolecular forces break, covalent bonds do not
The three types you must know, in order of typical strength, are:
你必须掌握的三类作用力，按典型强度由弱到强排列为：
| Type | 类型 | Typical strength | 典型强度 | Present in | 存在条件 |
| London dispersion forces 伦敦色散力 | ≈ 0.05–5 kJ mol⁻¹ | All molecules and atoms 所有分子与原子 |
| Permanent dipole-dipole 永久偶极-偶极 | ≈ 5–20 kJ mol⁻¹ | Polar molecules only 仅极性分子 |
| Hydrogen bonding 氢键 | ≈ 10–40 kJ mol⁻¹ | H bonded to N, O or F 与 N、O、F 相连的 H |
Note that the ranges overlap. This is why exam answers should always talk about relative strength in context rather than quoting a single number.
注意这些范围存在重叠，因此在答题时应结合具体情境讨论相对强弱，而不是机械地给出某个具体数值。
3. London Dispersion Forces | 伦敦色散力
London dispersion forces exist between all molecules and all noble gas atoms, whether polar or non-polar. They arise from the constant motion of electrons. At any instant, the electron cloud of a molecule may be unevenly distributed, producing a temporary dipole with a partial negative charge at one end and a partial positive charge at the other.
伦敦色散力存在于所有分子和所有稀有气体原子之间，无论极性还是非极性。它们源于电子的持续运动。在任意瞬间，分子的电子云可能分布不均，产生一个瞬时偶极：一端带部分负电荷，另一端带部分正电荷。
This temporary dipole then induces a dipole in a neighbouring molecule, so the two molecules attract each other very briefly. The effect is transmitted along a chain of molecules, which is why dispersion forces act between large numbers of molecules at once.
这个瞬时偶极又会使相邻分子产生诱导偶极，于是两个分子在极短时间内相互吸引。这种效应会沿分子链传递，因此色散力可同时作用于大量分子之间。
The strength of London dispersion forces depends on two main factors:
伦敦色散力的强度主要取决于两个因素：
- The number of electrons, which increases with relative molecular mass and with the size of the electron cloud. | 电子数：随相对分子质量和电子云体积增大而增多。
- The surface area of contact between molecules, which is greatest for long unbranched chains. | 分子间的接触表面积：长直链分子的接触面积最大。
The classic evidence is the halogen series, where boiling point increases steadily down the group.
经典证据是卤素系列，其沸点随族序数增加而稳定升高。
F₂ < Cl₂ < Br₂ < I₂ (boiling point rises as electrons increase)
The noble gases show the same pattern: He < Ne < Ar < Kr < Xe. Alkanes also follow it, so hexane has a much higher boiling point than methane despite both being non-polar.
稀有气体呈现相同规律：He < Ne < Ar < Kr < Xe。烷烃也是如此，因此己烷的沸点远高于甲烷，尽管两者都是非极性分子。
4. Permanent Dipole-Dipole Interactions | 永久偶极-偶极相互作用
A permanent dipole-dipole interaction occurs between polar molecules that already possess a permanent dipole. Such a dipole exists when two atoms of different electronegativity are joined by a covalent bond and the molecule is not symmetrical overall, so the bond dipoles do not cancel out.
永久偶极-偶极相互作用发生在已具有永久偶极的极性分子之间。当电负性不同的两个原子形成共价键，且分子整体不对称、各键偶极无法抵消时，分子就会具有永久偶极。
The more electronegative atom carries the partial negative charge, written δ⁻, and the less electronegative atom carries the partial positive charge, written δ⁺.
电负性较大的原子带部分负电荷，记作 δ⁻；电负性较小的原子带部分正电荷，记作 δ⁺。
In a liquid or solid, the δ⁺ end of one molecule is attracted to the δ⁻ end of a neighbouring molecule, so the molecules line up in an ordered arrangement. This attraction is stronger than London dispersion forces because the charges are permanent rather than instantaneous.
在液体或固体中，一个分子的 δ⁺ 端吸引相邻分子的 δ⁻ 端，使分子按一定方向排列。这种吸引比伦敦色散力强，因为电荷是永久的而非瞬时的。
Two points that examiners frequently test are worth stating explicitly:
有两点考官经常考查，值得明确强调：
- Polar molecules also have London dispersion forces. Permanent dipole-dipole interactions are additional to them, not a replacement. | 极性分子同样存在伦敦色散力，永久偶极-偶极相互作用是叠加在其上的，而非取而代之。
- A molecule can contain polar bonds and still have no permanent dipole, provided the geometry makes the bond dipoles cancel. CO₂ and CCl₄ are the standard examples. | 分子中含有极性键也可能没有永久偶极，只要几何构型使各键偶极相互抵消。CO₂ 与 CCl₄ 是标准例子。
5. Hydrogen Bonding | 氢键
A hydrogen bond is a particularly strong form of permanent dipole-dipole interaction that occurs only when hydrogen is covalently bonded to one of the three most electronegative elements: nitrogen, oxygen or fluorine. Because these elements pull electrons strongly, the H atom carries a large partial positive charge, δ⁺.
氢键是一种特别强的永久偶极-偶极相互作用，仅当氢原子与电负性最大的三种元素之一——氮、氧或氟——以共价键相连时才存在。由于这些元素强烈吸引电子，H 原子带有较大的部分正电荷 δ⁺。
This δ⁺ hydrogen is then attracted to a lone pair of electrons on an N, O or F atom of a neighbouring molecule. The interaction is written in exam answers as X–H···Y, where the dashed line represents the hydrogen bond.
这个 δ⁺ 氢原子随后吸引相邻分子中 N、O 或 F 上的孤对电子。答题时写成 X–H···Y，虚线即代表氢键。
X–H···Y where X and Y are N, O or F
The two essential conditions are therefore:
因此两个必要条件为：
- An H atom attached directly to N, O or F, giving a strongly polar X–H bond. | 氢原子直接与 N、O 或 F 相连，形成强极性 X–H 键。
- A lone pair on a small, highly electronegative N, O or F atom in a second molecule. | 另一个分子中 N、O 或 F 上存在孤对电子。
Hydrogen fluoride, water and ammonia all hydrogen bond. Hydrogen chloride does not, because chlorine is not electronegative enough, even though it also has lone pairs. Methanol and ethanol hydrogen bond because of the –OH group, while ethanoic acid can form dimers through two hydrogen bonds.
氟化氢、水和氨都能形成氢键。氯化氢则不能，因为氯的电负性不够大，尽管它也有孤对电子。甲醇和乙醇因含 –OH 基团而能形成氢键，乙酸则可通过两个氢键形成二聚体。
Hydrogen bonds are strong enough to produce dramatic physical effects, such as the anomalously high boiling point of water compared with hydrogen sulfide.
氢键强度足以产生显著的物理效应，例如水的沸点明显高于硫化氢。
6. Comparing the Strength of the Three Forces | 三种作用力的强度比较
For a fair comparison, you must compare molecules of similar size, because London dispersion forces depend strongly on the number of electrons. A useful exam-level ordering, for molecules of comparable mass, is:
若要公平比较，必须选择大小相近的分子，因为伦敦色散力强烈依赖于电子数。在质量相近的分子之间，考试层面的常用排序为：
London dispersion < permanent dipole-dipole < hydrogen bonding < covalent bond
A quantitative illustration makes the scale clear: breaking a hydrogen bond requires roughly 10 to 40 kJ mol⁻¹, whereas breaking an O–H covalent bond in water requires about 460 kJ mol⁻¹.
一个定量示例可以说明这个数量级：断裂一个氢键约需 10 至 40 kJ mol⁻¹，而断裂水中的 O–H 共价键约需 460 kJ mol⁻¹。
Molecules of similar molar mass can therefore have very different boiling points depending on which forces operate.
因此，摩尔质量相近的分子，其沸点可能相差很大，取决于存在哪些作用力。
| Substance | 物质 | Mᵣ | Forces present | 存在的作用力 | Boiling point | 沸点 |
| CH₄ 甲烷 | 16 | Dispersion only | 仅色散力 | −162 °C |
| H₂O 水 | 18 | Dispersion + H-bonding | 色散力 + 氢键 | 100 °C |
| H₂S 硫化氢 | 34 | Dispersion + dipole-dipole | 色散力 + 偶极 | −60 °C |
Water boils far above methane and hydrogen sulfide even though it has fewer electrons than H₂S, and the sole reason is hydrogen bonding.
水的沸点远高于甲烷和硫化氢，尽管它的电子数比 H₂S 少，唯一原因就是氢键。
7. Explaining Boiling Point Trends | 解释沸点变化趋势
Almost every exam question on this topic asks you to explain a trend in boiling point. A reliable three-step structure is: state which forces are present, compare their strength and the energy needed to overcome them, then link this to the physical property.
这一考点几乎所有题目都要求解释沸点趋势。可靠的三步结构是：指出存在哪些作用力，比较其强弱以及克服它们所需的能量，最后联系到相应的物理性质。
The main trends you should be able to explain are summarised below.
你应当能够解释的主要趋势总结如下。
- Down a group in the halogens or noble gases: more electrons and a larger electron cloud, so stronger dispersion forces and higher boiling point. | 卤素或稀有气体同族向下：电子数和电子云体积增大，色散力增强，沸点升高。
- Along the alkane series: more electrons and greater surface contact, so stronger dispersion forces. | 烷烃同系物递增：电子数增多、接触面积增大，色散力增强。
- Between branched and unbranched isomers: branching reduces the contact area, so dispersion forces are weaker and the boiling point is lower. | 支链与直链异构体之间：支链使接触面积减小，色散力减弱，沸点降低。
- Between a polar and a non-polar molecule of similar mass: the polar one has additional dipole-dipole interactions, so its boiling point is higher. | 质量相近的极性与非极性分子之间：极性分子还有偶极-偶极作用，沸点更高。
- Between molecules that hydrogen bond and those that do not: the hydrogen-bonded substance boils much higher. | 能形成氢键与不能形成氢键的物质之间：前者沸点高得多。
Always use the plural phrase intermolecular forces, and always state that these forces must be overcome, rather than saying that bonds are broken.
务必使用复数表述"分子间作用力"，并说明需要克服这些作用力，而不要说"化学键被断裂"。
8. Solubility and the Like-Dissolves-Like Rule | 溶解度与相似相溶规则
Solubility is also governed by intermolecular forces. The general principle is that substances dissolve best in solvents whose intermolecular forces are similar in type and strength to their own, often summarised as like dissolves like.
溶解度同样由分子间作用力决定。一般原则是：物质最容易溶解在分子间作用力类型和强度与之相似的溶剂中，这常被概括为相似相溶。
The process can be analysed in three energy steps that must balance favourably overall.
这一过程可以分解为三个能量步骤，总体上必须有利于溶解。
- Energy is needed to overcome forces between solvent molecules. | 需要能量克服溶剂分子之间的作用力。
- Energy is needed to overcome forces between solute particles. | 需要能量克服溶质粒子之间的作用力。
- Energy is released when new solute-solvent interactions form. | 形成新的溶质-溶剂相互作用时释放能量。
A polar solute such as ethanol dissolves readily in water because both can form hydrogen bonds; the new hydrogen bonds release a similar amount of energy to that needed to break the old ones.
乙醇这类极性溶质易溶于水，因为两者都能形成氢键；新形成的氢键释放的能量与打断原有氢键所需的能量相近。
A non-polar solute such as iodine does not dissolve appreciably in water, because replacing strong hydrogen bonds between water molecules with weak dispersion forces to iodine is energetically unfavourable. Iodine does dissolve in hexane, where dispersion forces dominate in both solute and solvent.
碘这类非极性溶质在水中几乎不溶，因为用与碘之间的弱色散力取代水分子之间的强氢键在能量上是不利的。碘可溶于己烷，因为溶质和溶剂中都以色散力为主。
9. Intermolecular Forces versus Chemical Bonds | 分子间作用力与化学键的区别
A common exam trap is confusing intermolecular forces with the covalent bonds inside a molecule. The distinction is fundamental and appears in multiple-choice questions as well as written ones.
常见失分点是把分子间作用力与分子内部的共价键混淆。这一区别是根本性的，既出现在简答题中，也出现在选择题中。
| Feature | 特征 | Intermolecular forces | 分子间作用力 | Covalent bonds | 共价键 |
| Location 位置 | Between molecules 分子之间 | Within a molecule 分子内部 |
| Strength 强度 | Weak, kJ mol⁻¹ scale | 弱，约 0.05–40 kJ mol⁻¹ | Strong, hundreds of kJ mol⁻¹ | 强，数百 kJ mol⁻¹ |
| Broken when 断裂时机 | Melting and boiling 熔化、沸腾 | Chemical reaction 化学反应 |
This difference explains why simple molecular substances have low melting and boiling points despite containing strong covalent bonds.
这一差异解释了为何简单分子物质尽管含有强共价键，熔点和沸点却很低。
The same idea also explains the physical properties of giant covalent structures such as diamond and silicon dioxide, where every atom is held by covalent bonds in a continuous lattice, so melting requires breaking covalent bonds and temperatures are extremely high.
同样道理也解释了金刚石、二氧化硅等巨型共价结构的物理性质：每个原子都通过共价键连接成连续晶格，因此熔化必须断裂共价键，温度极高。
10. Special Cases: Ice, Water and Anomalous Behaviour | 特例：冰、水与反常现象
Hydrogen bonding produces several anomalous properties of water that examiners like to use as context.
氢键导致水出现若干反常性质，考官常以此作为命题情境。
In ice, each water molecule forms four hydrogen bonds in a regular tetrahedral arrangement. This open lattice contains large empty spaces, so ice is less dense than liquid water and floats.
在冰中，每个水分子以规则四面体方式形成四个氢键。这种开放晶格包含大量空隙，因此冰的密度小于液态水，会浮在水面上。
When ice melts, the lattice collapses only partially, so the density increases on melting. This is unusual, because most substances are denser as solids.
冰熔化时晶格只是部分塌缩，因此熔化后密度增大。这一点很不寻常，因为大多数物质的固态密度更大。
Hydrogen bonding also gives water a high specific heat capacity and a high surface tension, and it explains why water has an unexpectedly high enthalpy of vaporisation.
氢键还使水具有较高的比热容和表面张力，也解释了水为何具有异常高的汽化焓。
Other hydrogen-bonded systems worth remembering include hydrogen fluoride, which hydrogen bonds despite having a low molar mass, and ammonia, whose hydrogen bonding is weaker than water because nitrogen is less electronegative than oxygen.
其他值得记住的氢键体系包括氟化氢——尽管摩尔质量低却能形成氢键，以及氨——其氢键弱于水，因为氮的电负性小于氧。
11. Practical Evidence and Experimental Links | 实验证据与实操联系
Edexcel papers often link this theory to practical work. Knowing where the evidence comes from makes explanations easier to construct under time pressure.
Edexcel 试题常把这一理论与实验操作联系起来。了解证据来源有助于在限时条件下快速组织解释。
Simple comparative experiments can demonstrate the effect of intermolecular forces.
简单的对比实验可以直观展示分子间作用力的影响。
- Comparing the time taken for equal volumes of different liquids to evaporate shows the relative strength of their intermolecular forces. | 比较等体积不同液体的蒸发时间，可反映其分子间作用力的相对强弱。
- Measuring the temperature change on dissolving a solid in different solvents links to solute-solvent interactions. | 测定固体在不同溶剂中溶解时的温度变化，可联系溶质-溶剂相互作用。
- Comparing the viscosity of long-chain alkanes or alcohols shows how chain length and hydrogen bonding affect flow. | 比较长链烷烃或醇的黏度，可说明链长与氢键对流动性的影响。
Chromatography also depends on intermolecular forces, since separation relies on the different strengths of interaction between the mobile phase, the stationary phase and the components of a mixture.
色谱法同样依赖分子间作用力，因为分离取决于流动相、固定相与混合物组分之间相互作用的强弱差异。
12. Common Errors and Exam Technique | 常见错误与答题技巧
Most lost marks in this topic come from a small number of repeated mistakes. Checking your answers against the list below is a fast way to protect your grade.
这一考点的失分大多来自少数几个反复出现的错误。用下面的清单检查答案，是快速保分的有效方法。
- Writing intermolecular force in the singular. Use the plural, since many forces act together. | 把"分子间作用力"写成单数。应使用复数，因为大量作用力共同起作用。
- Saying that covalent bonds break when a simple molecular liquid boils. They do not. | 声称简单分子液体沸腾时共价键断裂，实际上并不会。
- Claiming that hydrogen bonding requires only a polar X–H bond, without mentioning the lone pair on N, O or F. | 只提到极性 X–H 键就断言存在氢键，未提及 N、O 或 F 上的孤对电子。
- Forgetting to state δ⁺ and δ⁻ when describing dipole-dipole interactions. | 描述偶极-偶极作用时忘记标出 δ⁺ 与 δ⁻。
- Ignoring dispersion forces in polar molecules. | 忽略极性分子中同样存在的色散力。
- Quoting a numerical strength without comparing it with the energy needed to break the forces in that specific molecule. | 只给出强度数值，却未与断裂该具体分子中作用力所需的能量进行比较。
A reliable answering template for a three-mark explanation question is: name the forces present, compare them with the forces in the substance you are comparing against, then state the consequence for the boiling point.
对于 3 分的解释题，可靠的答题模板是：指出存在的作用力，与对照物质中的作用力进行比较，最后说明对沸点的影响。
Revising this topic effectively means practising identification of forces from displayed and skeletal formulae, because that is the skill most frequently tested in the first part of a question.
有效复习这一考点意味着反复练习从结构式和骨架式中识别作用力，因为这是题目第一部分最常考查的技能。
13. Quick Revision Summary | 快速复习总结
Keep this condensed summary in your revision notes and read it immediately before the exam.
把这份精简总结放进复习笔记，并在考前立即通读一遍。
- All molecules and noble gas atoms have London dispersion forces; strength rises with electron number and contact surface area. | 所有分子和稀有气体原子都有伦敦色散力；强度随电子数和接触表面积增大而升高。
- Polar molecules have permanent dipole-dipole interactions in addition to dispersion forces. | 极性分子在色散力之外还有永久偶极-偶极相互作用。
- Hydrogen bonding needs H bonded directly to N, O or F, plus a lone pair on N, O or F in another molecule. | 氢键需要 H 直接连在 N、O 或 F 上，且另一分子中 N、O 或 F 上有孤对电子。
- Typical strengths: dispersion < dipole-dipole < hydrogen bond < covalent bond. | 典型强度顺序：色散力 < 偶极-偶极 < 氢键 < 共价键。
- Melting and boiling break intermolecular forces only, never covalent bonds in simple molecular substances. | 熔化与沸腾只破坏分子间作用力，在简单分子物质中不会破坏共价键。
- Solubility follows the like-dissolves-like principle based on matching force types. | 溶解度遵循基于作用力类型匹配的相似相溶原则。
Master these six points and specification 2.18 becomes one of the most dependable sources of marks in the whole physical chemistry paper.
掌握这六点，考纲 2.18 就会成为整个物理化学试卷中最稳定的得分点之一。
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