还原剂Reducing agent

活泼金属：Na、Mg、Al、Zn、Fe。非金属单质与化合物：H₂、C、CO。低价离子与化合物：Fe²⁺、I⁻、S²⁻、SO₂、SO₃²⁻、H₂S。有机化学中常用 NaBH₄ 和 LiAlH₄ 把醛、酮还原为醇，LiAlH₄ 还能还原羧酸和酯。

Reactive metals: Na, Mg, Al, Zn, Fe. Non-metals and their compounds: H₂, C, CO. Ions and compounds in low oxidation states: Fe²⁺, I⁻, S²⁻, SO₂, SO₃²⁻, H₂S. In organic chemistry NaBH₄ and LiAlH₄ reduce aldehydes and ketones to alcohols, and LiAlH₄ also reduces carboxylic acids and esters.

标准电极电势 E° 越负，电对中的还原型越容易失电子，还原性越强：Li（−3.04 V）> K > Na > Mg > Al > Zn（−0.76 V）> Fe > H₂（0）> Cu。较强的还原剂能把较弱还原剂的氧化型还原，例如 Zn + Cu²⁺ → Zn²⁺ + Cu。卤离子的还原性 I⁻ > Br⁻ > Cl⁻ > F⁻，所以浓硫酸能把 HI 和 HBr 氧化，却不能氧化 HCl。

The more negative E°, the more readily the reduced form of a couple loses electrons and the stronger it is as a reducing agent: Li (−3.04 V) > K > Na > Mg > Al > Zn (−0.76 V) > Fe > H₂ (0) > Cu. A stronger reducing agent reduces the oxidised form of a weaker one, e.g. Zn + Cu²⁺ → Zn²⁺ + Cu. Among halide ions reducing power is I⁻ > Br⁻ > Cl⁻ > F⁻, which is why concentrated sulfuric acid oxidises HI and HBr but not HCl.

金属冶炼的本质是把金属化合物还原为金属。炼铁用 CO 作还原剂：Fe₂O₃ + 3CO → 2Fe + 3CO₂；铝热反应用铝还原金属氧化物：Fe₂O₃ + 2Al → Al₂O₃ + 2Fe；钛的冶炼用镁还原 TiCl₄。最活泼的金属如 Na、Al 只能用电解法（阴极上的电子作还原剂）制取。

Extracting a metal means reducing its compounds. Iron is made with CO as the reducing agent, Fe₂O₃ + 3CO → 2Fe + 3CO₂; the thermite reaction uses aluminium, Fe₂O₃ + 2Al → Al₂O₃ + 2Fe; titanium is made by reducing TiCl₄ with magnesium. The most reactive metals such as Na and Al can only be obtained by electrolysis, where electrons at the cathode do the reducing.