Properties of Ammonia and Ammonium Compounds | 氨与铵化合物的性质

📚 Properties of Ammonia and Ammonium Compounds | 氨与铵化合物的性质

Ammonia (NH₃) and ammonium compounds form a cornerstone of A-Level chemistry, bridging fundamental bonding theory with practical industrial applications. Understanding their structure, basicity, and coordination behaviour is essential for tackling examination questions on nitrogen chemistry, equilibria, and transition metal complexes.

氨（NH₃）及其铵化合物是 A-Level 化学的基石，将基础成键理论与实际工业应用紧密相连。理解其结构、碱性和配位行为，对于解答涉及氮化学、化学平衡及过渡金属配合物的考试题目至关重要。

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

Ammonia adopts a trigonal pyramidal shape with a nitrogen atom at the apex and three hydrogen atoms at the base. The bond angle is approximately 107°, slightly less than the ideal tetrahedral angle of 109.5° due to the greater repulsion exerted by the lone pair of electrons on nitrogen.

氨分子呈三角锥形，氮原子位于顶点，三个氢原子位于底部。键角约为107°，略小于理想四面体角109.5°，原因是氮原子上孤对电子产生的排斥力更大。

The nitrogen atom is sp³ hybridised. Three of the four hybrid orbitals form sigma bonds with hydrogen 1s orbitals, while the fourth orbital contains the lone pair. The molecule is polar because the bond dipoles do not cancel, giving ammonia a permanent dipole moment of 1.47 D.

氮原子采取 sp³ 杂化。四个杂化轨道中的三个与氢的 1s 轨道形成 σ 键，第四个轨道容纳孤对电子。由于键偶极矩不能相互抵消，氨分子具有极性，永久偶极矩约为 1.47 D。

The lone pair is the key to ammonia’s chemical personality: it makes ammonia a Lewis base, a Brønsted-Lowry base, and a ligand capable of donating electrons to transition metal ions.

孤对电子是氨化学性质的“灵魂”：它使氨成为路易斯碱、布朗斯特-劳里碱，以及能够向过渡金属离子提供电子的配体。

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2. Physical Properties of Ammonia | 氨的物理性质

Ammonia is a colourless gas at room temperature with a characteristic pungent, choking smell. It is significantly less dense than air, with a relative molecular mass of 17.0 compared to air’s average of approximately 29.

氨在室温下是无色气体，具有特征性的刺激性、呛人气味。其密度显著低于空气，相对分子质量为17.0，而空气的平均相对分子质量约为29。

Ammonia has a boiling point of -33.3°C, which is unusually high for a molecule of its size. This is a direct consequence of hydrogen bonding between the lone pair on nitrogen and hydrogen atoms of neighbouring ammonia molecules.

氨的沸点为 -33.3°C，对于如此小的分子而言，这个数值异常地高。这直接归因于氮上的孤对电子与相邻氨分子中氢原子之间形成的氢键。

• Hydrogen bonding gives ammonia its anomalously high boiling point compared to phosphine (PH₃, bp -87.7°C).

  氢键使氨的沸点相比于磷化氢（PH₃，沸点 -87.7°C）异常地高。

• Solubility: ammonia is extremely soluble in water (~700 dm³ per dm³ of water at STP); it forms hydrogen bonds with water molecules.

  溶解性：氨极易溶于水（标准状况下约 700 dm³ 氨溶于 1 dm³ 水），因为它能与水分子形成氢键。

• Liquefaction: ammonia liquefies easily under pressure at room temperature, making it convenient for storage and transport.

  液化：在室温下加压即可使氨液化，便于储存和运输。

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3. Laboratory Preparation of Ammonia | 氨的实验室制备

In the laboratory, ammonia is prepared by heating an ammonium salt with a strong alkali. The classic method uses ammonium chloride and calcium hydroxide (slaked lime):

实验室中，通过加热铵盐与强碱的混合物来制备氨。经典方法使用氯化铵与氢氧化钙（熟石灰）：

2NH₄Cl(s) + Ca(OH)₂(s) → CaCl₂(s) + 2NH₃(g) + 2H₂O(l)

The gas is dried by passing it through a tower packed with calcium oxide (quicklime), since ammonia is basic and would react with acidic drying agents such as concentrated sulfuric acid or anhydrous calcium chloride.

气体通过装有氧化钙（生石灰）的干燥塔进行干燥，因为氨呈碱性，若使用浓硫酸或无水氯化钙等酸性干燥剂，会与氨发生反应。

Ammonia is collected by upward delivery (downward displacement of air) because it is less dense than air. The collecting vessel is covered to prevent the gas from escaping.

由于氨比空气轻，采用向上排空气法收集（向下排空气法），收集容器需加盖以防气体逸出。

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4. Industrial Synthesis: The Haber Process | 工业合成：哈伯过程

The Haber process is the industrial route for manufacturing ammonia from its elements. Nitrogen is obtained from fractional distillation of liquid air, while hydrogen comes from steam reforming of natural gas.

哈伯过程是利用单质工业合成氨的路线。氮气来源于液态空气的分馏，氢气来自天然气的蒸汽重整。

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹

This is an exothermic, reversible reaction with a decrease in the number of gas moles from reactants to products. The conditions chosen represent a compromise that optimises yield and rate:

这是一个放热、可逆的反应，从反应物到产物，气体摩尔数减少。所选择的工艺条件是产率与速率之间折衷优化的结果：

• Temperature of 450°C: high enough for a reasonable rate but low enough to give an acceptable equilibrium yield (~15%). Lower temperatures favour the exothermic forward reaction but are too slow industrially.

  温度450°C：既足以保证较快的反应速率，又不会太低而失去可接受的平均产率（约15%）。温度越低越有利于放热正反应，但工业上速率过慢。

• Pressure of 200 atm: high pressure shifts equilibrium to the right (fewer gas molecules) and increases the rate; 200 atm is a practical compromise between yield and the cost of compression.

  压强200 atm：高压使平衡向右移动（气体分子数减少）并提高反应速率；200 atm 是产率与压缩成本之间的实际折衷。

• Iron catalyst: accelerates the attainment of equilibrium without affecting the equilibrium position, lowering the activation energy for the reaction.

  铁催化剂：加快达到平衡的速率而不影响平衡位置，降低了反应的活化能。

Unreacted nitrogen and hydrogen are recycled, and the ammonia is removed by liquefaction — a process that maximises overall conversion.

未反应的氮气和氢气循环使用，氨通过液化法分离——这一流程使总转化率最大化。

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

Ammonia is a classic Brønsted-Lowry base: it accepts a proton (H⁺) to form the ammonium ion, NH₄⁺. This behaviour is central to its reactions with acids:

氨是经典的布朗斯特-劳里碱：它接受质子（H⁺）形成铵离子（NH₄⁺）。这一行为是氨与酸反应的核心：

NH₃(g) + HCl(g) → NH₄Cl(s)

When concentrated hydrochloric acid and concentrated ammonia solution are placed near each other, a dense white smoke of ammonium chloride forms. This is a classic qualitative test for ammonia gas.

当浓盐酸与浓氨水彼此靠近时，会产生氯化铵的浓密白烟。这是检验氨气的经典定性实验。

Ammonia also reacts with other acids to form the corresponding ammonium salts, all of which contain the tetrahedral ammonium ion (NH₄⁺) with a bond angle close to 109.5°.

氨还能与其他酸反应生成相应的铵盐，所有铵盐都含有四面体形的铵离子（NH₄⁺），键角接近109.5°。

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6. Ammonia in Water: A Weak Base | 氨在水中的行为：弱碱

When dissolved in water, ammonia acts as a weak base by reacting partially with water to produce hydroxide ions:

氨溶于水时，作为弱碱与部分水反应生成氢氧根离子：

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

The equilibrium lies well to the left, with a base dissociation constant K_b of approximately 1.8 × 10⁻⁵ mol dm⁻³ at 25°C. This means that the pH of a 1.0 mol dm⁻³ ammonia solution is only about 11.6.

该平衡严重偏向左侧，25°C 时碱解离常数 K_b 约为 1.8 × 10⁻⁵ mol dm⁻³。这意味着 1.0 mol dm⁻³ 氨溶液的 pH 仅为11.6左右。

The equilibrium can be shifted right by adding acid (which removes OH⁻) or shifted left by adding ammonium ions — a demonstration of Le Chatelier’s principle that can appear in exam questions.

加入酸（消耗 OH⁻）可使平衡右移；加入铵离子则使平衡左移——这是勒夏特列原理的体现，常在考试题目中出现。

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7. Formation of Ammonium Salts | 铵盐的形成

Ammonium salts are formed when ammonia reacts with acids. The ammonium ion is formed by a dative (coordinate) covalent bond: the lone pair on nitrogen donates to a proton, which has no electrons of its own.

铵盐是氨与酸反应的产物。铵离子的形成基于配位共价键：氮上的孤对电子向没有电子的质子提供电子。

• Ammonium chloride (NH₄Cl) from NH₃ and HCl — used in dry cells and soldering fluxes.

  氯化铵（NH₄Cl）由 NH₃ 与 HCl 反应生成——用于干电池和焊接助焊剂。

• Ammonium sulfate ((NH₄)₂SO₄) from NH₃ and sulfuric acid — a major nitrogenous fertiliser.

  硫酸铵（(NH₄)₂SO₄）由 NH₃ 与硫酸反应生成——重要的氮肥。

• Ammonium nitrate (NH₄NO₃) from NH₃ and nitric acid — used both as a fertiliser and in explosives.

  硝酸铵（NH₄NO₃）由 NH₃ 与硝酸反应生成——既是肥料也用于炸药。

• Ammonium carbonate ((NH₄)₂CO₃) from NH₃, CO₂ and water — a leavening agent in baking.

  碳酸铵（(NH₄)₂CO₃）由 NH₃、CO₂ 和水反应生成——用于烘焙膨松剂。

All ammonium salts are white crystalline solids, highly soluble in water. They share the common property of decomposing upon heating, and they all release ammonia when treated with a strong base.

所有铵盐均为白色结晶固体，易溶于水。它们的共同特性是受热分解，且与强碱共热时都能释放氨气。

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8. Thermal Decomposition of Ammonium Salts | 铵盐的热分解

Ammonium salts decompose on heating, but the products depend on the nature of the anion. This is a frequently examined topic requiring careful attention to product identification.

铵盐受热分解，但产物取决于阴离子的性质。这是常见的考点，需要仔细辨析产物。

Case 1: Ammonium chloride undergoes sublimation-like decomposition:

案例一：氯化铵发生升华式分解：

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

On cooling, the gases recombine at the cooler part of the tube to reform ammonium chloride — this is why heating NH₄Cl gives the appearance of sublimation.

冷却后，两种气体在试管较冷部位重新结合生成氯化铵——这就是加热 NH₄Cl 时看似升华的原因。

Case 2: Ammonium carbonate decomposes to give ammonia, carbon dioxide and water:

案例二：碳酸铵分解为氨、二氧化碳和水：

(NH₄)₂CO₃(s) → 2NH₃(g) + CO₂(g) + H₂O(g)

Case 3: Ammonium nitrate decomposes differently depending on temperature. At around 250°C, it produces nitrous oxide and water:

案例三：硝酸铵依温度不同而分解方式不同。约 250°C 时，生成一氧化二氮和水：

NH₄NO₃(s) → N₂O(g) + 2H₂O(g)

At temperatures above 300°C, or when detonated, it decomposes explosively to nitrogen, oxygen and water, releasing a huge volume of gas. This makes ammonium nitrate dangerous if handled carelessly.

温度超过 300°C 或被引爆时，它爆炸性分解为氮气、氧气和水，释放出大量气体。这使得硝酸铵若处置不当非常危险。

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9. Ammonia as a Ligand | 氨作为配体

Ammonia is a neutral monodentate ligand. The lone pair on nitrogen forms dative covalent bonds with transition metal ions, creating complex ions. This is a key part of transition metal chemistry in the CIE curriculum.

氨是中性单齿配体。氮上的孤对电子与过渡金属离子形成配位共价键，生成配合离子。这是CIE课程中过渡金属化学的关键部分。

When ammonia is added to aqueous solutions of transition metal ions, precipitation is often followed by dissolution of the precipitate in excess ammonia, forming deep-coloured complex ions:

向过渡金属离子水溶液中加入氨水时，常先生成沉淀，继而沉淀在过量氨中溶解，形成深色配离子：

• Copper(II) ions: first form a pale blue precipitate of Cu(OH)₂, which dissolves in excess ammonia to give a deep blue solution containing [Cu(NH₃)₄(H₂O)₂]²⁺.

  铜(II)离子：先形成淡蓝色 Cu(OH)₂ 沉淀，在过量氨中溶解，得到含 [Cu(NH₃)₄(H₂O)₂]²⁺ 的深蓝色溶液。

• Silver ions: form Ag₂O(s), a brown precipitate, which dissolves in dilute ammonia; with concentrated ammonia, [Ag(NH₃)₂]⁺ forms — this is the reagent used in Tollens’ test.

  银离子：形成棕色 Ag₂O 沉淀，溶于稀氨水；浓氨作用下生成 [Ag(NH₃)₂]⁺——即托伦试剂的有效成分。

• Zinc ions: form a white gelatinous Zn(OH)₂ precipitate, soluble in excess ammonia to give a colourless [Zn(NH₃)₄]²⁺ complex.

  锌离子：形成白色胶状 Zn(OH)₂ 沉淀，溶于过量氨生成无色的 [Zn(NH₃)₄]²⁺ 配离子。

Ammonia acts as a BETTER ligand than water for these metals because it is a stronger electron-pair donor, so replacement of water ligands by ammonia occurs readily.

对上述金属而言，氨是比水更好的配体，因为它是更强的电子对给予体，因此水配体被氨置换的过程较易发生。

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10. Test for the Ammonium Ion | 铵离子的检验

The standard test for ammonium ions uses strong alkali and heat. This is a practical technique that students must be able to describe precisely in examinations.

检验铵离子的标准方法使用强碱并加热。这是一项学生必须能准确描述的操作技能，考试中常被考查。

NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Procedure: a few drops of sodium hydroxide solution are added to the test solution in a test tube, which is then warmed gently. Ammonia gas is evolved and detected by its pungent smell and by holding a piece of damp red litmus paper at the mouth of the test tube — it turns blue.

操作步骤：在盛有待测液的试管中加入少量氢氧化钠溶液，然后微微加热。逸出的氨气通过刺激性气味以及湿润的红色石蕊试纸变蓝来判断。

For a more rigorous identification, the gas can be tested with concentrated hydrochloric acid on a glass rod, producing white fumes of ammonium chloride.

更严谨的确认方式是用蘸有浓盐酸的玻璃棒靠近，如产生氯化铵白烟，即可确认气体为氨。

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11. Key Applications of Ammonia and Ammonium Compounds | 氨及铵化合物的主要应用

The importance of ammonia chemistry extends far beyond the laboratory. Exam questions often link chemical properties to real-world applications, especially in agriculture and industry.

氨化学的重要性远超实验室范畴。考试题目常将化学性质与实际应用相联系，特别是在农业和工业领域。

• Fertilisers: ammonium nitrate, ammonium sulfate and ammonium phosphate are the primary nitrogen sources in agriculture. Nitrogen is essential for amino acids, proteins and nucleic acids in plants.

  化肥：硝酸铵、硫酸铵和磷酸铵是农业中主要的氮源。氮是植物合成氨基酸、蛋白质和核酸所必需的营养元素。

• Nitric acid manufacture: ammonia is the raw material in the Ostwald process, where it is catalytically oxidised to nitrogen monoxide, then nitrogen dioxide, and finally absorbed in water to produce HNO₃.

  硝酸工业：氨是奥斯特瓦尔德法的原料，先催化氧化为一氧化氮，再氧化为二氧化氮，最后用水吸收制得 HNO₃。

• Household cleaners: dilute ammonia solution is an effective grease remover and glass cleaner because of its alkaline nature and complex-forming ability.

  家用清洁剂：稀氨水因碱性和配位能力，是有效的除油剂和玻璃清洁剂。

• Refrigeration: ammonia’s high enthalpy of vaporisation and easy liquefaction make it an efficient refrigerant historically and in large-scale industrial plants.

  制冷：氨的汽化焓高且易于液化，使其在历史上及大型工业制冷设备中都是高效的制冷剂。

The delicate balance between ammonia’s benefits and its hazards — it is toxic, corrosive to eyes and respiratory tissue, and contributes to eutrophication when washed into waterways — must be understood by responsible chemists.

氨既有益处也有危险——它具有毒性、对眼睛和呼吸道组织有腐蚀性，且流入水体后会导致富营养化——负责任的化学家必须清醒认识这一平衡。

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