定义Definition
二氧化氮 NO₂ 是红棕色、有刺激性气味的有毒气体，氮为 +4 价。分子含一个未成对电子，是自由基，易二聚为无色的 N₂O₄。它是汽车尾气和燃烧产生的主要空气污染物之一，参与形成光化学烟雾和酸雨。
Nitrogen dioxide, NO₂, is a toxic red-brown gas with a sharp smell in which nitrogen is +4. The molecule has an unpaired electron and is a radical, readily dimerising to colourless N₂O₄. It is a major air pollutant from vehicles and combustion and helps form photochemical smog and acid rain.
二聚平衡The dimerisation equilibrium
2NO₂(g, 红棕色) ⇌ N₂O₄(g, 无色)，ΔH = −57 kJ mol⁻¹。升温平衡左移，颜色变深；加压平衡右移（气体分子数减少），但压缩瞬间颜色因浓度增大先变深，然后逐渐变浅。这是演示勒夏特列原理的经典实验。
2NO₂(g, red-brown) ⇌ N₂O₄(g, colourless), ΔH = −57 kJ mol⁻¹. Heating shifts it left and deepens the colour; raising pressure shifts it right (fewer gas molecules), although on compression the colour first darkens as concentration rises, then fades. It is a classic demonstration of Le Chatelier's principle.
形成与环境影响Formation and environmental effects
发动机高温下 N₂ 与 O₂ 化合：N₂ + O₂ → 2NO，NO 在空气中被氧化：2NO + O₂ → 2NO₂。NO₂ 溶于水：3NO₂ + H₂O → 2HNO₃ + NO，导致酸雨；在阳光下分解产生 O 原子，引发对流层臭氧和光化学烟雾的形成。汽车催化转化器把 NOₓ 还原为 N₂。
At the high temperatures in engines N₂ and O₂ combine, N₂ + O₂ → 2NO, and NO is oxidised in air, 2NO + O₂ → 2NO₂. NO₂ dissolves in water, 3NO₂ + H₂O → 2HNO₃ + NO, causing acid rain, and in sunlight it splits off O atoms that start the formation of low-level ozone and photochemical smog. Catalytic converters reduce NOₓ to N₂.
化学性质Chemical properties
二聚：2NO₂ ⇌ N₂O₄
Dimerises: 2NO₂ ⇌ N₂O₄
与水反应：3NO₂ + H₂O → 2HNO₃ + NO
With water: 3NO₂ + H₂O → 2HNO₃ + NO
催化 SO₂ 的大气氧化：SO₂ + NO₂ → SO₃ + NO
Catalyses atmospheric oxidation of SO₂: SO₂ + NO₂ → SO₃ + NO
年表Timeline
- 1772
普里斯特利研究一氧化氮及其在空气中生成的红棕色气体
Priestley studies nitric oxide and the red-brown gas it forms in air
- 1940年代
洛杉矶光化学烟雾引起对氮氧化物污染的研究
Los Angeles photochemical smog prompts research on nitrogen oxide pollution