Nitrogen Gas | 氮气 - aleveler.com

📚 Nitrogen Gas | 氮气

Nitrogen gas, N₂, is the most abundant gas in Earth’s atmosphere and a cornerstone of A-Level Chemistry. This article examines its structure, inertness, preparation, key reactions and environmental importance.

氮气 (N₂) 是地球大气中含量最丰富的气体，也是 A-Level 化学的重要考点。本文分析其结构、惰性、制备方法、主要反应及环境意义。

1. Occurrence and Discovery | 存在与发现

Nitrogen gas, N₂, makes up about 78% by volume of dry air and roughly 75.5% by mass. It is also found in proteins, nucleic acids and mineral nitrates. The gas was first isolated in 1772 by Daniel Rutherford, who removed oxygen and carbon dioxide from a sample of air. He called it ‘noxious air’ because it did not support breathing or combustion.

氮气 (N₂) 约占干燥空气体积的 78%、质量的 75.5%。它还存在于蛋白质、核酸和硝酸盐矿物中。1772 年，丹尼尔·卢瑟福首次分离出该气体：他除去空气中的氧气和二氧化碳后，发现剩余气体既不支持呼吸也不支持燃烧，因此称之为“有毒空气”。

2. Molecular Structure and Bonding | 分子结构与键合

An N₂ molecule consists of two nitrogen atoms joined by a triple covalent bond, written as N≡N. Each nitrogen atom has five valence electrons; three pairs are shared, and one lone pair remains on each atom. The triple bond has a bond enthalpy of about +945 kJ mol⁻¹ and a bond length of 0.109 nm, making it one of the strongest diatomic bonds known.

N₂ 分子由两个氮原子以三共价键 (N≡N) 结合而成。每个氮原子有 5 个价电子；3 对电子参与共用，每个原子上保留 1 对孤对电子。该三键的键焓约为 +945 kJ mol⁻¹，键长 0.109 nm，是已知最强的双原子键之一。

3. Physical Properties | 物理性质

Nitrogen is a colourless, odourless and tasteless diatomic gas at room temperature. It condenses to a liquid at -196 °C (77 K) and freezes at -210 °C. Because its molar mass (28.0 g mol⁻¹) is very close to the average molar mass of air, its density is only slightly lower than air. The molecule is non-polar, so nitrogen has very low solubility in water.

常温下氮气是无色、无味、无臭的双原子气体。它在 -196 °C (77 K) 冷凝为液体，在 -210 °C 凝固。由于摩尔质量 (28.0 g mol⁻¹) 与空气平均摩尔质量非常接近，其密度仅略低于空气。该分子为非极性分子，因此氮气在水中的溶解度很低。

4. Inertness and Activation Energy | 惰性与活化能

Nitrogen is often described as inert at room temperature because the N≡N bond is very strong and the molecule is non-polar. Reactions require a large activation energy to break the triple bond or to distort the electron cloud. For example, the reaction N₂(g) + O₂(g) ⇌ 2NO(g) has ΔH = +180 kJ mol⁻¹ but only occurs at very high temperatures or in lightning strikes.

氮气在常温下通常表现为惰性，因为 N≡N 键极强且分子无极性。反应需要很高的活化能来断裂三键或使电子云变形。例如 N₂(g) + O₂(g) ⇌ 2NO(g) 的 ΔH = +180 kJ mol⁻¹，但只有在极高温度或闪电时才能发生。

5. Laboratory and Industrial Preparation | 实验室与工业制备

In the laboratory, N₂ can be prepared by gently heating a mixture of ammonium chloride and sodium nitrite: NH₄Cl(aq) + NaNO₂(aq) → N₂(g) + 2H₂O(l) + NaCl(aq). Industrially, nitrogen is obtained by fractional distillation of liquid air. Air is cooled to about -200 °C; nitrogen boils off first at -196 °C, leaving liquid oxygen behind.

实验室中，可将氯化铵与亚硝酸钠混合溶液微热制取氮气：NH₄Cl(aq) + NaNO₂(aq) → N₂(g) + 2H₂O(l) + NaCl(aq)。工业上通过液态空气的分馏获得氮气：将空气冷却至约 -200 °C，氮气在 -196 °C 首先沸腾逸出，液氧则留在后面。

6. Reaction with Metals | 与金属的反应

At high temperatures, nitrogen reacts with reactive metals to form ionic nitrides. Lithium is the only Group 1 metal that reacts directly with N₂: 6Li(s) + N₂(g) → 2Li₃N(s). Magnesium burns in nitrogen to give magnesium nitride: 3Mg(s) + N₂(g) → Mg₃N₂(s). These nitrides contain the N³⁻ ion and are hydrolysed by water to release ammonia, e.g. Mg₃N₂(s) + 6H₂O(l) → 3Mg(OH)₂(s) + 2NH₃(g).

在高温下，氮气可与活泼金属反应生成离子型氮化物。锂是唯一能直接与 N₂ 反应的第 1 族金属：6Li(s) + N₂(g) → 2Li₃N(s)。镁在氮气中燃烧生成氮化镁：3Mg(s) + N₂(g) → Mg₃N₂(s)。这些氮化物含有 N³⁻ 离子，遇水水解释放出氨，例如 Mg₃N₂(s) + 6H₂O(l) → 3Mg(OH)₂(s) + 2NH₃(g)。

7. Reaction with Hydrogen: The Haber Process | 与氢的反应：哈伯法

Nitrogen reacts with hydrogen in the Haber process to manufacture ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹. Typical conditions are 400-450 °C, 200 atm and an iron catalyst. The reaction is exothermic, so high temperature reduces equilibrium yield; however, a moderate temperature is used to achieve a reasonable rate. High pressure favours the forward reaction because 4 moles of gas form 2 moles. Unreacted N₂ and H₂ are recycled.

在哈伯法中，氮气与氢气反应制取氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹。典型条件为 400-450 °C、200 atm 和铁催化剂。该反应放热，因此高温会降低平衡产率；但为了获得合理反应速率，采用中等温度。高压有利于正向反应，因为 4 mol 气体生成 2 mol 气体。未反应的 N₂ 和 H₂ 循环使用。

8. Reaction with Oxygen: Formation of NOx | 与氧的反应：氮氧化物的形成

Nitrogen reacts with oxygen only at very high temperatures, such as in internal combustion engines or lightning: N₂(g) + O₂(g) ⇌ 2NO(g). The nitrogen monoxide formed is readily oxidised to nitrogen dioxide: 2NO(g) + O₂(g) → 2NO₂(g). Nitrogen dioxide is a brown, toxic, acidic gas. At high altitude, aircraft engines also produce NO, which contributes to ozone depletion.

氮气只有在极高温度下才与氧气反应，例如在内燃机或闪电中：N₂(g) + O₂(g) ⇌ 2NO(g)。生成的一氧化氮很容易被氧化为二氧化氮：2NO(g) + O₂(g) → 2NO₂(g)。二氧化氮是一种棕色、有毒的酸性气体。在高空，飞机发动机也会产生 NO，从而加剧臭氧层破坏。

9. Nitrogen Fixation and the Nitrogen Cycle | 固氮与氮循环

The strong N≡N bond makes atmospheric nitrogen unavailable to most organisms. Nitrogen fixation converts N₂ into reactive nitrogen compounds. Natural fixation occurs by lightning and by nitrogen-fixing bacteria such as Rhizobium in legume root nodules. Industrial fixation is dominated by the Haber process. In the nitrogen cycle, nitrifying bacteria oxidise ammonium to nitrite and nitrate, while denitrifying bacteria return N₂ to the atmosphere.

强大的 N≡N 键使大多数生物无法直接利用大气中的氮。固氮作用将 N₂ 转化为活性含氮化合物。天然固氮通过闪电和固氮细菌（如豆科植物根瘤中的根瘤菌）进行。工业固氮则以哈伯法为主。在氮循环中，硝化细菌将铵氧化为亚硝酸盐和硝酸盐，反硝化细菌则将氮气返回大气。

10. Oxides of Nitrogen and Pollution | 氮氧化物与污染

NO and NO₂ are collectively called NOₓ. They are formed from N₂ and O₂ at high temperatures. NO₂ reacts with water and oxygen to form nitric acid, contributing to acid rain: 4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq). In strong sunlight, NOₓ reacts with unburnt hydrocarbons to form photochemical smog and ground-level ozone. Catalytic converters in cars reduce NOₓ back to N₂ and O₂.

NO 和 NO₂ 合称 NOₓ。它们由 N₂ 与 O₂ 在高温下生成。NO₂ 与水、氧气反应生成硝酸，导致酸雨：4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq)。在强烈阳光下，NOₓ 与未燃烧的烃类反应形成光化学烟雾和地面臭氧。汽车中的催化转化器将 NOₓ 还原回 N₂ 和 O₂。

11. Uses of Nitrogen Gas | 氮气的用途

The main industrial use of nitrogen is in the manufacture of ammonia via the Haber process, which is then converted into fertilisers, nitric acid and explosives. Liquid nitrogen is used as a coolant for food freezing, biological sample storage and cryotherapy. Gaseous nitrogen provides an inert atmosphere for food packaging, electronics manufacturing and preventing oxidation in chemical processes. It is also used to purge pipelines and tanks.

氮气的主要工业用途是通过哈伯法制造氨，氨随后转化为化肥、硝酸和炸药。液氮用作食品冷冻、生物样品保存和冷冻疗法的冷却剂。气态氮气可为食品包装、电子制造以及化工过程中防止氧化提供惰性气氛。它还用于吹扫管道和储罐。

12. Summary and Exam Tips | 总结与考试提示

Examiners expect you to recall the electronic structure of N₂, explain its low reactivity in terms of bond enthalpy and activation energy, write balanced equations for its reactions with Li, Mg, H₂ and O₂, and evaluate the Haber process conditions. Be careful with state symbols and the sign of ΔH. Remember that nitrogen gas is not ‘unreactive’ in all conditions: it forms nitrides with reactive metals at high temperatures.

考试要求你能够：写出 N₂ 的电子结构；从键焓和活化能角度解释其低反应活性；配平它与 Li、Mg、H₂ 和 O₂ 的反应方程式；评价哈伯法的反应条件。注意状态符号和 ΔH 的正负号。还要记住氮气并非在所有条件下都“不活泼”：高温下它能与活泼金属生成氮化物。

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Nitrogen Gas | 氮气