定义Definition
碘化氢 HI 是无色气体，极易溶于水形成氢碘酸，是卤化氢中酸性最强的。H–I 键较弱，HI 受热易分解，是强还原剂，能把浓硫酸还原为硫化氢。
Hydrogen iodide, HI, is a colourless gas that dissolves readily in water to give hydriodic acid, the strongest of the hydrohalic acids. Its weak H–I bond makes it easy to decompose by heat, and it is a strong reducing agent that can reduce concentrated sulfuric acid to hydrogen sulfide.
热分解平衡Thermal decomposition
2HI(g) ⇌ H₂(g) + I₂(g)。加热时出现紫色碘蒸气。这是研究化学平衡和反应动力学的经典体系，因为反应前后气体分子数相同，压强变化不影响平衡位置。
2HI(g) ⇌ H₂(g) + I₂(g); purple iodine vapour appears on heating. It is a classic system for studying equilibrium and kinetics, and because the number of gas molecules is the same on both sides, changing the pressure does not shift the equilibrium.
还原性Reducing power
碘化钠与浓硫酸反应时，生成的 HI 把硫酸一直还原到 H₂S：8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O，可闻到臭鸡蛋气味并析出紫黑色碘。这说明还原性 HI > HBr > HCl。
When sodium iodide reacts with concentrated sulfuric acid, the HI formed reduces the acid all the way to H₂S: 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O, with a bad-egg smell and violet-black iodine. This shows reducing power HI > HBr > HCl.
化学性质Chemical properties
受热分解：2HI ⇌ H₂ + I₂
Decomposes on heating: 2HI ⇌ H₂ + I₂
还原浓硫酸：8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O
Reduces concentrated sulfuric acid: 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O