Ammonia and Ammonium Compounds | 氨与铵化合物

📚 Ammonia and Ammonium Compounds | 氨与铵化合物

Ammonia, NH₃, is one of the most important nitrogen compounds in A-Level chemistry. Its lone pair, trigonal pyramidal shape and ability to form ammonium salts explain much of its behaviour. This article reviews the structure, preparation, reactions and applications of ammonia and ammonium compounds for the Cambridge A-Level Chemistry specification.

氨（NH₃）是 A-Level 化学中最重要的含氮化合物之一。它的孤对电子、三角锥形结构以及形成铵盐的能力解释了其大多数化学行为。本文围绕剑桥 A-Level 化学大纲，系统梳理氨与铵化合物的结构、制备、反应及应用。

1. Structure and Bonding in Ammonia | 氨的结构与成键

In NH₃, the nitrogen atom is sp³ hybridised. Three hybrid orbitals form sigma bonds with hydrogen 1s orbitals, and one hybrid orbital contains a lone pair.

在 NH₃ 中，氮原子采取 sp³ 杂化。三个杂化轨道分别与氢的 1s 轨道形成 σ 键，另一个杂化轨道容纳一对孤对电子。

The shape is trigonal pyramidal, not tetrahedral, because the lone pair occupies one of the four electron-pair positions. The H–N–H bond angle is about 107°, which is reduced from 109.5° by stronger lone-pair–bonding-pair repulsion.

由于孤对电子占据四个电子对位置之一，分子形状为三角锥形而非四面体形。H–N–H 键角约为 107°，低于 109.5°，因为孤对电子与键对电子之间的排斥力更强。

The N–H bonds are polar because nitrogen is more electronegative than hydrogen. Since the molecule is not symmetrical, ammonia is a polar molecule. The lone pair is also responsible for the basic and ligand behaviour discussed later.

由于氮的电负性大于氢，N–H 键具有极性。同时分子结构不对称，因此氨是极性分子。孤对电子还决定了后文将要讨论的碱性和配体行为。

2. Physical Properties and Hydrogen Bonding | 物理性质与氢键

Ammonia is a colourless gas with a sharp, choking smell. It is less dense than air and is easily liquefied under pressure, which is important for storage and transport.

氨是无色、有强烈刺激性气味的气体。它的密度小于空气，加压时容易液化，这对储存和运输非常重要。

Ammonia is very soluble in water because NH₃ and H₂O can both form hydrogen bonds. Aqueous ammonia is weakly alkaline, but the gas itself is a covalent molecular substance.

氨极易溶于水，因为 NH₃ 和 H₂O 都能形成氢键。氨水呈弱碱性，但氨气本身是共价分子物质。

Compared with PH₃, ammonia has a much higher boiling point. This is due to intermolecular hydrogen bonding between NH₃ molecules, whereas PH₃ molecules cannot form significant hydrogen bonds.

与 PH₃ 相比，氨的沸点高得多。这是因为 NH₃ 分子之间存在分子间氢键，而 PH₃ 分子之间不能形成明显的氢键。

In water, the main equilibrium is NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, so the solution contains ammonia molecules, water molecules, ammonium ions and hydroxide ions.

在水中主要存在平衡 NH₃ + H₂O ⇌ NH₄⁺ + OH⁻，因此氨水中含有氨分子、水分子、铵离子和氢氧根离子。

3. Laboratory Preparation and Collection | 实验室制备与收集

In the laboratory, ammonia is usually prepared by heating an ammonium salt with a strong base such as calcium hydroxide:

在实验室中，氨通常由铵盐与强碱（如氢氧化钙）共热制得：

2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃

The gas is dried by passing it through calcium oxide. Concentrated sulfuric acid and calcium chloride are not suitable drying agents because they react with or absorb ammonia.

氨气通过氧化钙进行干燥。浓硫酸和氯化钙不适合用作干燥剂，因为它们会与氨反应或吸收氨。

Since ammonia is less dense than air and very soluble in water, it is collected by downward delivery. It cannot be collected over water.

由于氨的密度小于空气且极易溶于水，因此采用向下排空气法收集，不能使用排水集气法。

In the laboratory, ammonia can also be generated by warming concentrated aqueous ammonia, but heating the solid salt with a base gives a steadier stream of gas.

实验室中也可以加热浓氨水制取少量氨，但用铵盐与碱共热可以得到更稳定的气流。

4. Industrial Manufacture: The Haber Process | 工业制备：哈伯法

Ammonia is manufactured industrially by the Haber process from nitrogen and hydrogen:

工业上通过哈伯法由氮气和氢气合成氨：

N₂ + 3H₂ ⇌ 2NH₃ ΔH = −92 kJ mol⁻¹

The forward reaction is exothermic, so a low temperature favours a higher equilibrium yield of ammonia. However, a very low temperature makes the reaction too slow to be economic.

正反应是放热反应，因此低温有利于提高氨的平衡产率。但温度过低会使反应速率太慢，不具经济性。

A compromise temperature of about 400–450 °C is used with an iron catalyst. A high pressure of about 200 atm increases the yield because there are fewer gas molecules on the product side.

实际采用约 400–450 °C 的折中温度，并使用铁催化剂。约 200 atm 的高压可提高产率，因为产物侧气体分子数更少。

The iron catalyst increases the rate of both forward and reverse reactions equally, so it does not change the equilibrium position. Its surface must be kept clean because sulfur and oxygen compounds can poison the catalyst.

铁催化剂同等程度地加快正、逆反应速率，因此不会改变平衡位置。催化剂表面必须保持洁净，因为含硫和含氧化合物会使催化剂中毒。

Unreacted N₂ and H₂ are recycled to improve overall conversion. Hydrogen is usually obtained from methane and steam, while nitrogen comes from fractional distillation of liquid air.

未反应的 N₂ 和 H₂ 循环使用以提高总转化率。氢气通常由甲烷与水蒸气反应制得，氮气来自液态空气的分馏。

5. Ammonia as a Bronsted–Lowry Base | 氨作为布朗斯特–劳里碱

Ammonia acts as a Bronsted–Lowry base because its lone pair can accept a proton. In water, the following equilibrium is established:

氨可作为布朗斯特–劳里碱，因为它的孤对电子能够接受质子。在水中建立如下平衡：

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

The solution is only weakly alkaline because the equilibrium lies mainly to the left. Ammonia is therefore a weak base. The conjugate acid is NH₄⁺ and the conjugate base is OH⁻.

该溶液只呈弱碱性，因为平衡主要向左移动。因此氨是弱碱。其共轭酸为 NH₄⁺，共轭碱为 OH⁻。

Ammonia also acts as a Lewis base and as a ligand by donating its lone pair to metal ions. For example, with aqueous copper(II) ions it forms the deep blue complex [Cu(NH₃)₄(H₂O)₂]²⁺.

氨还可以作为路易斯碱和配体，将孤对电子提供给金属离子。例如，与铜(II)离子形成深蓝色配合物 [Cu(NH₃)₄(H₂O)₂]²⁺。

This ligand behaviour is important in qualitative analysis and in the formation of many transition metal complexes.

这种配体行为在定性分析以及许多过渡金属配合物的形成中都很重要。

6. Reactions with Acids and Formation of Ammonium Salts | 与酸反应及铵盐的生成

Ammonia neutralises acids to form ammonium salts. With hydrogen chloride, dense white fumes of ammonium chloride are produced:

氨能中和酸生成铵盐。与氯化氢反应时产生浓密的白色氯化铵烟雾：

NH₃ + HCl → NH₄Cl

With sulfuric acid and nitric acid, ammonium sulfate and ammonium nitrate are formed respectively:

与硫酸和硝酸反应分别生成硫酸铵和硝酸铵：

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

NH₃ + HNO₃ → NH₄NO

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Ammonia and Ammonium Compounds | 氨与铵化合物