📚 Nitrogen and Sulfur: Reactions, Industrial Processes and Environmental Chemistry | 氮与硫：反应、工业过程与环境化学
Nitrogen and sulfur are the two most heavily examined non-metals in A-Level and IB chemistry. Between them they carry the Haber process, the Ostwald process, the Contact process, ammonia and nitric acid chemistry, sulfuric acid as an industrial reagent, the chemistry of acid rain, and the whole redox story of nitrogen from -3 to +5 and sulfur from -2 to +6. This revision article works through the position, bonding and occurrence of both elements, explains why nitrogen gas is so unreactive, then builds up the industrial processes and their conditions step by step before finishing with pollution control, fertilisers and the redox patterns you should be able to recall instantly in an exam.
氮和硫是 A-Level 与 IB 化学中考查最频繁的两种非金属。哈伯法、奥斯特瓦尔德法、接触法、氨与硝酸的化学、硫酸的工业应用、酸雨的化学，以及氮从 -3 到 +5、硫从 -2 到 +6 的整套氧化还原规律，都离不开它们。本文将从两种元素的位置、成键与存在形式讲起，解释氮气为何如此不活泼，再逐步构建各工业过程的反应与条件，最后落到污染治理、肥料以及考试中必须瞬间回忆起来的氧化还原规律。
1. Position, Bonding and Occurrence | 元素位置、成键与存在
Nitrogen is in Group 15 and has the electron configuration 1s² 2s² 2p³, so it needs three more electrons to complete its outer shell. Sulfur is in Group 16 with the configuration 1s² 2s² 2p⁶ 3s² 3p⁴. Both are non-metals, but their bonding behaviour is quite different.
氮位于第15族，电子构型为 1s² 2s² 2p³，因此还需要三个电子才能填满最外层。硫位于第16族，电子构型为 1s² 2s² 2p⁶ 3s² 3p⁴。两者都是非金属，但成键行为差别很大。
Nitrogen has five valence electrons: two in the 2s orbital and three unpaired electrons in the 2p orbitals. It therefore forms three covalent bonds and keeps one lone pair, giving compounds such as NH₃ and NF₃. Its common oxidation states run from -3 in ammonia and nitrides, through 0 in N₂, +2 in NO, +4 in NO₂, to +5 in nitric acid and nitrates. Nitrogen cannot expand its octet because there are no available 2d orbitals, which is why it never forms five or six bonds.
氮有五个价电子：2s 轨道两个，2p 轨道三个未成对电子。因此它形成三个共价键并保留一对孤对电子，得到 NH₃、NF₃ 等化合物。其常见氧化态从氨和氮化物中的 -3，经 N₂ 中的 0、NO 中的 +2、NO₂ 中的 +4，一直到硝酸和硝酸盐中的 +5。氮无法扩展八隅体，因为不存在可用的 2d 轨道，因此它绝不会形成五键或六键。
Sulfur has six valence electrons and forms two covalent bonds in simple compounds such as H₂S. Because it is in Period 3, it has empty 3d orbitals available and can expand its octet, giving four bonds (SO₂) or six bonds (SO₃, H₂SO₄). Its oxidation states are -2 in sulfides, +4 in SO₂ and sulfites, and +6 in SO₃, sulfuric acid and sulfates. The +6 state is the most stable in aqueous, oxidising conditions.
硫有六个价电子，在 H₂S 等简单化合物中形成两个共价键。由于位于第三周期，它有空的 3d 轨道可用，可以扩展八隅体，形成四键（SO₂）或六键（SO₃、H₂SO₄）。其氧化态为硫化物中的 -2、SO₂ 与亚硫酸盐中的 +4、SO₃ 与硫酸及硫酸盐中的 +6。在水溶液和氧化性条件下，+6 是最稳定的状态。
Nitrogen occurs mainly as N₂ in the atmosphere, while sulfur is found in elemental deposits and in sulfide and sulfate ores. Air is about 78% nitrogen by volume and 21% oxygen; the remainder includes argon, carbon dioxide and variable water vapour. Nitrogen is also locked up in proteins, nucleic acids, soil nitrates and mineral deposits such as sodium nitrate. Sulfur appears as volcanic and salt-dome deposits of the free element, as sulfide ores including iron pyrite (FeS₂), galena (PbS), sphalerite (ZnS) and chalcopyrite (CuFeS₂), and as sulfate ores such as gypsum (CaSO₄·2H₂O) and barytes (BaSO₄). Hydrogen sulfide is also present in natural gas and crude oil and must be removed before fuel is burned.
氮主要以大气中的 N₂ 形式存在，而硫存在于天然硫矿以及硫化物和硫酸盐矿石中。空气按体积约含 78% 氮气和 21% 氧气，其余为氩气、二氧化碳以及含量不定的水蒸气。氮还被固定在蛋白质、核酸、土壤硝酸盐和硝酸钠等矿物中。硫则以火山与盐丘中的单质硫、包括黄铁矿（FeS₂）、方铅矿（PbS）、闪锌矿（ZnS）和黄铜矿（CuFeS₂）在内的硫化物矿石，以及石膏（CaSO₄·2H₂O）、重晶石（BaSO₄）等硫酸盐矿石形式存在。天然气和原油中也含有硫化氢，燃烧前必须脱除。
The N≡N triple bond has a bond enthalpy of about 994 kJ mol⁻¹, making N₂ very unreactive. Sulfur forms S₈ ring molecules in its common allotropes. The two stable allotropes, rhombic (α) sulfur and monoclinic (β) sulfur, both contain puckered eight-membered rings; the rhombic form is stable below 95.5 °C and the monoclinic form is stable between 95.5 °C and the melting point of 115 °C. Sulfur also exists as a range of larger Sₙ rings in the vapour, and as amorphous or plastic sulfur when molten sulfur is poured into cold water.
N≡N 三键的键焓约为 994 kJ mol⁻¹，使 N₂ 非常不活泼。硫的常见同素异形体由 S₈ 环状分子构成。两种稳定同素异形体——斜方硫（α）与单斜硫（β）——都含褶皱的八元环；斜方硫在 95.5 °C 以下稳定，单斜硫在 95.5 °C 至熔点 115 °C 之间稳定。硫蒸气中还含有更大的 Sₙ 环，而熔融硫倒入冷水则得到无定形硫或弹性硫。
| Property / 性质 | Nitrogen / 氮 | Sulfur / 硫 |
| Group / 族 | 15 | 16 |
| Configuration / 电子构型 | 1s² 2s² 2p³ | 1s² 2s² 2p⁶ 3s² 3p⁴ |
| Typical oxidation states / 典型氧化态 | -3, 0, +2, +4, +5 | -2, 0, +4, +6 |
| Element form / 单质形态 | N₂ diatomic, N≡N | S₈ puckered rings |
| Octet expansion / 八隅体扩展 | Not possible / 不可能 | Possible using 3d orbitals |
2. Unreactivity of Nitrogen and Its Uses | 氮的惰性与用途
Because the N₂ molecule is non-polar and contains a very strong triple bond, nitrogen gas is inert under ordinary conditions. The bond enthalpy is about 994 kJ mol⁻¹, and breaking the triple bond also requires a large activation energy. As a result, nitrogen does not react with oxygen at room temperature, does not burn, and does not support combustion, even though it will react with a few very reactive metals.
由于 N₂ 分子是非极性的且含有非常强的三键，氮气在通常条件下是惰性的。其键焓约为 994 kJ mol⁻¹，断裂三键还需要很高的活化能。因此，氮在室温下不与氧反应、不燃烧、也不支持燃烧，尽管它能与少数极活泼的金属反应。
At very high temperatures, for example in lightning strikes or inside a car engine, nitrogen and oxygen do combine. This is an endothermic reaction and is reversible at high temperature.
在极高温度下，例如闪电或汽车发动机内部，氮与氧确实会化合。该反应为吸热反应，在高温下可逆。
N₂(g) + O₂(g) → 2NO(g) ΔH = +180 kJ mol⁻¹
Nitrogen also reacts with hot, strongly electropositive metals to form ionic nitrides, which are hydrolysed by water to give ammonia.
氮还与灼热的强正电性金属反应生成离子型氮化物，后者遇水水解生成氨。
3Mg(s) + N₂(g) → Mg₃N₂(s) Mg₃N₂(s) + 6H₂O(l) → 3Mg(OH)₂(s) + 2NH₃(g)
The industrial uses of nitrogen follow directly from its inertness. It is used as an inert blanketing atmosphere in food packaging, in the electronics industry and in the storage of reactive chemicals. Liquid nitrogen boils at -196 °C and is used as a coolant for cryogenic storage of biological samples, and in medicine for freezing tissue. Nitrogen is also the feedstock for the Haber process and for the manufacture of nitric acid and cyanides.
氮的工业用途直接源于其惰性：在食品包装、电子工业以及活泼化学品储存中用作惰性保护气氛。液氮沸点为 -196 °C，可用作生物样品低温储藏的冷却剂，并在医学中用于冷冻组织。氮气还是哈伯法以及硝酸、氰化物生产的原料。
In nature, nitrogen fixation converts atmospheric N₂ into usable compounds in three ways: biological fixation by nitrogen-fixing bacteria such as Rhizobium in root nodules, atmospheric fixation by lightning, and industrial fixation by the Haber process. Denitrifying bacteria later return nitrogen to the atmosphere, completing the nitrogen cycle.
在自然界中，固氮作用通过三种方式把大气中的 N₂ 转化为可用化合物：根瘤菌（如 Rhizobium）的生物固氮、闪电引起的大气固氮，以及哈伯法的工业固氮。反硝化细菌随后把氮元素送回大气，完成氮循环。
3. Ammonia: Structure, Bonding and Properties | 氨：结构、成键与性质
Ammonia is a trigonal pyramidal molecule with a bond angle of about 107°, slightly less than the tetrahedral 109.5° because the lone pair on nitrogen repels the bonding pairs more strongly. The molecule is polar, and the presence of N-H bonds allows strong hydrogen bonding between molecules, which explains the relatively high boiling point of -33 °C and the very high solubility in water, about 700 volumes of gas per volume of water at room temperature.
氨是三角锥形分子，键角约 107°，略小于四面体的 109.5°，因为氮上的孤对电子对成键电子对的排斥更强。该分子有极性，且 N-H 键的存在使分子间能形成强氢键，这解释了其相对较高的沸点 -33 °C 以及极高的水溶性——室温下约 1 体积水可溶解 700 体积氨气。
Ammonia is a weak Brønsted-Lowry base and a Lewis base. In water it accepts a proton; the resulting solution is alkaline because of the hydroxide ions produced by the equilibrium. Because the equilibrium lies well to the left, ammonia is only a weak base and the pH of a 0.1 mol dm⁻³ solution is around 11 rather than 13.
氨是弱布朗斯特-劳里碱，也是路易斯碱。在水中它接受质子；由于平衡产生的氢氧根离子，溶液呈碱性。因为平衡明显偏左，氨只是弱碱，0.1 mol dm⁻³ 溶液的 pH 约为 11 而不是 13。
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
As a Lewis base it donates its lone pair to form a dative covalent bond, giving the ammonium ion and many complex ions. Adding acid to ammonia gives ammonium salts, and adding alkali to an ammonium salt regenerates ammonia, which is the basis of the standard test for the ammonium ion.
作为路易斯碱，它提供孤对电子形成配位共价键，生成铵离子以及许多配离子。向氨中加酸得到铵盐，向铵盐中加强碱则重新释放氨，这正是检验铵离子的标准方法。
NH₃(aq) + HCl(aq) → NH₄Cl(aq) NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Ammonia is readily oxidised. In the presence of a hot platinum-rhodium catalyst it is oxidised to nitrogen monoxide, the first step of the Ostwald process. Burning ammonia in oxygen without a catalyst gives nitrogen and water instead.
氨容易被氧化。在灼热的铂铑催化剂存在下，氨被氧化为一氧化氮，这是奥斯特瓦尔德法的第一步；若不用催化剂直接在氧气中燃烧，产物则是氮气和水。
4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g) 4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(g)
Ammonia is used to make fertilisers such as ammonium nitrate, ammonium sulfate and urea, to make nitric acid, and in refrigeration and cleaning products. Its reactions with copper(II) ions in solution also give the deep blue tetraamminecopper(II) ion, a useful qualitative observation.
氨用于制造硝酸铵、硫酸铵和尿素等肥料，用于生产硝酸，也用于制冷和清洁产品。它与溶液中铜(II)离子反应生成深蓝色的四氨合铜(II)配离子，是有用的定性观察现象。
4. The Haber Process | 哈伯法
The Haber process combines nitrogen from the air with hydrogen from natural gas to make ammonia. The reaction is reversible and exothermic, and the forward reaction reduces the number of gas molecules from four to two.
哈伯法把空气中的氮与来自天然气的氢结合制取氨。该反应可逆且放热，正向反应使气体分子数由四个减少为两个。
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹
A typical industrial set of conditions is a temperature of 400-450 °C, a pressure of about 200 atm (20 MPa), and an iron catalyst promoted with potassium oxide and aluminium oxide. These conditions are a compromise between thermodynamics, kinetics and economics.
典型的工业条件为温度 400-450 °C、压力约 200 atm（20 MPa），并使用以氧化钾和氧化铝为助催化剂的铁催化剂。这些条件是热力学、动力学与经济性之间的折中。
- Temperature: because the forward reaction is exothermic, Le Chatelier's principle predicts a higher yield at low temperature, but the rate would be far too slow, so a moderately high temperature is used with a catalyst.
- Pressure: four moles of gas give two, so high pressure increases the yield, but very high pressure requires expensive, thick-walled plant.
- Catalyst: iron increases the rate and allows equilibrium to be reached quickly without changing the position of equilibrium.
- Recycling: only about 15% of the gas is converted per pass, so unreacted nitrogen and hydrogen are recycled, and ammonia is condensed out because it liquefies at -33 °C.
温度：正向反应放热，根据勒夏特列原理低温有利于提高产率，但反应速率太慢，因此采用中等偏高的温度并配合催化剂。压力
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