分子间力——定义、类型、方程式、例子

当化学系统的特性代表物质的整体特性时，它们的特性是可以观察到的。例如，单个分子不沸腾，而整体分子沸腾。水分子的集合具有润湿特性，而单个分子则没有。水，就像所有物质一样，可以以不同的状态存在。它以固态形式存在，以液态形式存在，以蒸气或蒸汽形式以气态形式存在。所有三种物质状态的物理特性都非常不同。然而，即使物理性质发生变化，化学性质也不会改变。但我们必须注意，化学反应的速率在某种程度上取决于物理状态。

因此，要了解物质的不同状态及其结构，我们需要了解什么是分子间力、分子相互作用、它们的性质、热能的影响以及粒子的运动。所有这些参数的平衡定义了物质的物理状态。

The forces of attraction and repulsion between interacting atoms and molecules are called intermolecular forces.

这些分子间力是物质的大部分化学和物理性质的原因。例如，分子间作用力越大，沸点越高。我们可以有把握地得出结论，元素的沸点与其分子间作用力的强度成正比。

分子间力与原子间力

范德华力是有吸引力的分子间力。它们在量级上有很大的不同，并进行了相应的分类。它们包括色散力或伦敦力、偶极诱导偶极力和偶极-偶极力。氢键是偶极-偶极相互作用，但单独考虑，因为只有少数元素可以参与氢键的形成。

Note: Ion-dipole forces are also intermolecular forces, but are not considered under van der Waals forces. We can use the boiling point of an element as the parameter to categorize the intermolecular forces.

分散部队或伦敦部队

色散力或伦敦力是由于电子运动而产生的力，会产生暂时的正电荷和负电荷。它主要发生在电对称且没有偶极矩的原子和非极性分子中。

分散部队或伦敦部队

正如我们在上图中看到的，我们首先考虑两个没有偶极矩的原子 A 和 B。接下来，我们可以在右侧看到具有瞬时偶极子且电子密度更高的原子 A，而原子 B 具有感应偶极子。在最后一部分，我们可以观察到原子 A 如何在左侧有更多的电子密度。原子 B 具有诱导偶极子。

伦敦力作用的距离很短（~500 pm），它们的大小很大程度上取决于粒子的极化率。它是所有分子间力中最弱的力。

偶极-偶极相互作用

极性分子是那些由于两端具有相反电荷而具有净偶极子的分子。它们具有偶极-偶极相互作用作为吸引力。由于原子的不同电负性，它们的永久偶极子与共价键相关。极性分子的部分正电荷吸引另一个分子的部分负电荷部分。

例如，HCl 分子具有偶极-偶极相互作用。

偶极-偶极相互作用

与氯相比，氢的负电性较小。因此，氯获得部分负电荷，而氢获得部分正电荷。因此，HCl分子之间发生偶极-偶极相互作用。

氢键

如前所述，这是偶极-偶极相互作用的一个特例。这主要存在于存在高极性 OH、HF 或 NH 键的分子中。

氢键

氢键通常被认为仅限于氮、氧和氟，但在某些情况下，氯等物质也参与氢键。

离子偶极相互作用

这些就像偶极-偶极相互作用，唯一的例外是它们在极性分子和离子之间上升。离子-偶极相互作用的强度取决于以下因素：

• 极性分子的大小。
• 离子的电荷和大小。
• 偶极矩的大小。

例如，当 NaCl 与水混合时，H2O 分子（极性）被烧杯中的 Na（钠）和 Cl（氯）离子吸引。

离子诱导偶极相互作用

在离子诱导的偶极相互作用中，离子被非极性分子极化。非极性分子在获得电荷时表现为诱导偶极子。

离子诱导偶极相互作用

离子和诱导偶极子之间的这种相互作用是离子诱导偶极子相互作用。

偶极诱导偶极相互作用

偶极诱导的偶极相互作用类似于离子诱导的相互作用，不同之处在于由于极性分子的存在，非极性转化为诱导偶极。

偶极诱导偶极相互作用

这些力位于具有永久偶极子的极性分子和没有永久偶极子的分子之间。

示例问题

问题1：分子间作用力和热相互作用有什么区别？

回答：

The difference between Intermolecular Forces and Thermal Interactions are as follows:

Intermolecular Forces	Thermal Interactions
The forces of attraction and repulsion between interacting atoms and molecules are called intermolecular forces.	The total measure of the sum of the kinetic energy of all the atoms and molecules is called thermal energy.
These forces are due to the dipole of one or both the given molecules.	These forces are due to the motion of particles.
Greater intermolecular forces lead to the substance being in the solid state.	Greater thermal interactions will not allow the substance to remain in the solid state.
These forces hold particles together.	These forces keep particles apart.
They do not have any effect due to the temperature, but the boiling point of the substance is directly proportional to the strength of the forces.	Thermal energy is directly dependent/proportional to temperature.
Intermolecular forces are weak in the gaseous phase and strongest in solid state.	Thermal forces are weak in solid state and high in the gaseous phase.
The volume of the given matter is less when the intermolecular forces are high.	The volume of the given matter is more when the thermal interactions are high.
Gases can’t be liquified on compression only due to the strong intermolecular forces.	Gases can be easily liquefied by reducing the thermal energy by lowering the temperature.
Predominance of the Intermolecular Forces	Predominance of the Thermal Interactions

问题2：什么是氢键？

回答：

Hydrogen Bonding is a special unique case of dipole-dipole interaction. Dipole-dipole interactions are the attractive forces on polar molecules. Hydrogen Bonding is found in mostly molecules in which highly polar O-H, H-F, or N-H bonds are present. It is mostly regarded to be limited to Nitrogen, Oxygen and Fluorine, but in some cases, species such as Chlorine also participate in Hydrogen bonding.  

问题 3：解释色散力和偶极-偶极相互作用的相互作用能。

回答：

Dispersion Forces or London Forces are those which arise due to the movement of electrons, creating a temporary positive and negative charge. London forces operate for a short distance (~500 pm) and their magnitude depends largely on the polarisability of the particle. The interaction energy of the dispersion forces is inversely proportional to the sixth power of the distance between two particles.

Interaction Energy α 1/x⁶, where x is the distance between two given particles.

Dipole-dipole interactions are the attractive forces on polar molecules. They are between polar molecules that are mostly of two types. The first type is stationary, while the other is rotating. 

The interaction energy of dipole-dipole interactions is inversely proportional to the third power of the distance between two particles in the case of stationary polar molecules.  

Interaction Energy α 1/x³, where x is the distance between two given particles.

The interaction energy of dipole-dipole forces varies inversely to the sixth power of the distance between two particles in the case of rotating polar molecules.

Interaction Energy ∝ 1/x⁶, where x is the distance between two given particles.

问题4：为什么冰的密度比水低？

回答：

Hydrogen Bonding affects the physical properties of compounds. Ice has hydrogen bonding as intermolecular forces. Thus, it has a lower density than water because of hydrogen bonding and cage-like structure of ice.

Ice has hexagonal three-dimensional crystal structure (as per X-ray crystallographic data). This hexagonal crystal structure is formed  due to intermolecular hydrogen bonding.

When ice melts, most of the hydrogen bonds break and some of the empty spaces are occupied by water molecules. Liquid water molecules are thus more closely packed together than molecules in ice. Thus, ice has a lower density than water.

问题 5：水在^摄氏4 度时密度最大。为什么？

回答：

Water molecules in ice exist in a crystal lattice with a lot of empty space. 

When ice melts into liquid water, the density of the water increases as the structure starts to break and collapse. As we increase the temperature, the molecules start moving faster and get further apart. As the temperature increases, the density decreases. At temperatures nearing 0^oC, water still has several ice-like clusters. As the temperature of warm water decreases, the water molecules slow down and the density increases. At 4^oC, the clusters start forming. The molecules are still slowing down and coming closer together, but the formation of clusters makes the molecules be further apart. Cluster formation is the bigger effect, so the density starts to decrease. Thus, the density of water is a maximum at 4^oC. 

问题6：氢键的强度取决于什么？

回答：

Strength of Hydrogen Bonds depends on the coulombic interaction between the lone-pair electrons of one electronegative atom of a molecule and the hydrogen atom of another molecule.