📚 A-Level Chemistry: Intermolecular Forces Explained | A-Level 化学：分子间作用力详解
Intermolecular forces are the attractions between molecules. They explain why water is a liquid at room temperature while hydrogen sulfide is a gas, why oil does not mix with water, and why ice floats. For CIE A-Level Chemistry, you need to understand three main types: London (dispersion) forces, permanent dipole-dipole forces and hydrogen bonding.
分子间作用力是分子与分子之间的吸引力。它们可以解释为什么室温下水是液体而硫化氢是气体，为什么油不能与水混合，以及为什么冰能浮在水面上。在 CIE A-Level 化学中，你需要掌握三类主要作用力：伦敦色散力、永久偶极-偶极作用力和氢键。
1. Intramolecular vs Intermolecular Forces | 分子内与分子间作用力
Intramolecular forces are the strong forces that hold atoms together inside a molecule, such as covalent bonds. These determine chemical stability and reactivity. Intermolecular forces are the weaker attractions between separate molecules, and they determine physical properties such as melting point, boiling point, viscosity and solubility.
分子内作用力是分子内部将原子结合在一起的强作用力，例如共价键，它们决定了化学稳定性和反应活性。分子间作用力是不同分子之间较弱的吸引力，它们决定了熔点、沸点、粘度和溶解度等物理性质。
When a substance boils or melts, covalent bonds are not broken. Only intermolecular forces are overcome. That is why molecular substances can often boil at relatively low temperatures, even when their covalent bonds are strong.
物质沸腾或熔化时，共价键并没有断裂，被破坏的只是分子间作用力。这就是为什么分子型物质即使在共价键很强的情况下，往往也能在较低的温度下沸腾。
Strength order: hydrogen bonding > permanent dipole-dipole forces > London/dispersion forces
强度排序：氢键 > 永久偶极-偶极作用力 > 伦敦色散力
2. London Dispersion Forces | 伦敦色散力
London dispersion forces, also called instantaneous dipole-induced dipole forces, are present between all molecules, polar or non-polar. They arise because electrons are constantly moving. At any instant, the electron cloud may be unevenly distributed, creating a temporary dipole. This temporary dipole can induce a dipole in a neighbouring molecule, producing a weak attraction.
伦敦色散力又称瞬时偶极-诱导偶极作用力，存在于所有分子之间，无论是极性分子还是非极性分子。它们源于电子的不断运动：在某一瞬间，电子云分布可能不均匀，形成瞬时偶极；这个瞬时偶极会诱导相邻分子产生偶极，从而形成微弱吸引。
The strength of London forces increases with the number of electrons because a larger, more polarisable electron cloud can distort more easily. This explains why boiling points increase down Group 18: helium has only 2 electrons, while xenon has 54 electrons, so xenon has much stronger London forces.
伦敦力的大小随电子数目的增加而增大，因为更大、更易极化的电子云更容易发生变形。这解释了为什么稀有气体的沸点随周期表向下递增：氦只有 2 个电子，而氙有 54 个电子，因此氙的伦敦力强得多。
Molecular shape also matters. Straight-chain alkanes have a larger surface area than their branched isomers, so they have more effective contact between molecules and stronger London forces. For example, butane boils at -0.5°C, while 2-methylpropane boils at -11.7°C.
分子形状也很重要。直链烷烃比支链异构体具有更大的分子表面积，分子间有效接触更多，伦敦力更强。例如，正丁烷的沸点是 -0.5°C，而 2-甲基丙烷的沸点是 -11.7°C。
3. Permanent Dipole-Dipole Forces | 永久偶极-偶极作用力
Permanent dipole-dipole forces operate between polar molecules. A polar covalent bond forms when two atoms with different electronegativities share electrons unequally. If the bond dipoles in a molecule do not cancel out, the molecule has a permanent dipole. For example, in HCl, chlorine is more electronegative than hydrogen, so the molecule is Hδ⁺Clδ⁻.
永久偶极-偶极作用力存在于极性分子之间。当电负性不同的原子共用电子时，电子云分布不均，就形成极性共价键。如果分子中的键偶极不能相互抵消，分子就具有永久偶极。例如，在 HCl 分子中，氯的电负性大于氢，因此分子表示为 Hδ⁺Clδ⁻。
The partial positive end of one molecule attracts the partial negative end of another. This is an extra force on top of London forces, so polar molecules generally have stronger intermolecular attractions than non-polar molecules of similar size.
一个分子的部分正电端会吸引另一个分子的部分负电端。这是除伦敦力之外的额外作用力，因此在分子大小相近时，极性分子通常比非极性分子具有更强的分子间吸引力。
However, dipole-dipole forces are still relatively weak. They only become noticeable when molecules are properly aligned, and at high temperatures, molecular motion can easily overcome them.
不过，偶极-偶极作用力仍然相对较弱。只有分子排列方向合适时，这些力才比较明显；温度升高时，分子的热运动很容易超过它们。
4. Hydrogen Bonding | 氢键
Hydrogen bonding is a special, strong type of permanent dipole-dipole force. It occurs when a hydrogen atom is covalently bonded to fluorine, oxygen or nitrogen, and is attracted to a lone pair of electrons on a fluorine, oxygen or nitrogen atom in another molecule.
氢键是一种特殊的、较强的永久偶极-偶极作用力。当氢原子与氟、氧或氮形成共价键后，它还会被另一个分子中氟、氧或氮原子上的孤对电子所吸引。
Because F, O and N are highly electronegative, the H atom becomes very electron-poor. This concentrated δ⁺ charge is strongly attracted to a lone pair on a neighbouring molecule. The hydrogen bond can be represented as O-H···O or N-H···N.
由于 F、O、N 的电负性很强，氢原子变得非常缺电子。这种集中的 δ⁺ 电荷会强烈吸引邻近分子上的孤对电子。氢键可以表示为 O-H···O 或 N-H···N。
Individual hydrogen bonds have an energy of about 10 to 40 kJ/mol. This is stronger than ordinary dipole-dipole forces but much weaker than a covalent bond, which typically requires several hundred kJ/mol to break.
单个氢键的键能约为 10 到 40 kJ/mol。它比普通偶极-偶极作用力强，但比共价键弱得多——断裂共价键通常需要几百 kJ/mol 的能量。
Typical substances with hydrogen bonding include water (H₂O), ammonia (NH₃) and hydrogen fluoride (HF). These substances have unexpectedly high boiling points compared with similar hydrides in their groups.
典型的含氢键物质包括水（H₂O）、氨（NH₃）和氟化氢（HF）。与同族类似氢化物相比，这些物质的沸点异常地高。
5. Water, Ice and the Anomalous Properties of H₂O | 水与冰——H₂O 的异常性质
Water is one of the most important examples of hydrogen bonding. Each oxygen atom in water can form two hydrogen bonds as a donor and accept two as an acceptor? In liquid water, hydrogen bonds are constantly breaking and reforming, so the molecules are packed relatively closely.
水是氢键最重要的例子之一。液态水中，氢键不断断裂和重新形成，因此分子排列相对紧密。
In ice, each water molecule forms four hydrogen bonds to neighbouring water molecules. These bonds hold the molecules in a tetrahedral, open lattice. Because the lattice is open, ice is less dense than liquid water, which is why ice floats.
在冰中，每个水分子与相邻水分子形成四个氢键。这些氢键使分子排列成四面体状的开阔晶格。由于晶格比较空洞，冰的密度小于液态水，所以冰能浮在水面上。
Hydrogen bonding is also responsible for water’s high boiling point, high specific heat capacity and high latent heat of vaporisation. A lot of energy is needed to overcome the hydrogen bonds between water molecules.
氢键还造成了水的高沸点、高比热容和高汽化热。要破坏水分子之间的氢键，需要输入大量能量。
Compared with H₂S, which has no hydrogen bonding, water has a much higher boiling point: 100°C instead of about -60°C. This is a classic examination comparison.
与没有氢键的 H₂S 相比，水的沸点要高得多：100°C vs 约 -60°C。这是一个非常经典的考试对比题。
6. Boiling Point Trends Across Hydrides | 氢化物沸点变化规律
For hydrides in Group 14, such as CH₄ and SiH₄, there is no hydrogen bonding. Their boiling points rise steadily with relative molecular mass because London forces increase as the number of electrons increases.
对于第 14 族的氢化物，如 CH₄ 和 SiH₄，不存在氢键。它们的
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