定义Definition
吸热反应是从环境吸收热量的反应，焓变 ΔH 为正值，反应时周围温度降低（或需要持续加热）。热分解、光合作用、硝酸铵溶于水以及氢氧化钡晶体与氯化铵的反应都是吸热的。
An endothermic reaction takes in heat from the surroundings, has a positive enthalpy change (ΔH > 0) and cools its surroundings, or needs continual heating. Thermal decomposition, photosynthesis, dissolving ammonium nitrate in water and the reaction of barium hydroxide crystals with ammonium chloride are endothermic.
例Examples
CaCO₃ → CaO + CO₂，ΔH 约 +178 kJ mol⁻¹，需持续强热；Ba(OH)₂·8H₂O 与 NH₄Cl 固体混合搅拌，温度明显下降，烧杯底部的水甚至会结冰。
CaCO₃ → CaO + CO₂, ΔH about +178 kJ mol⁻¹, needs continual strong heating; stirring solid Ba(OH)₂·8H₂O with NH₄Cl makes the temperature drop so far that water under the beaker can freeze.
为何吸热反应也能自发How endothermic reactions can be spontaneous
反应能否自发由吉布斯自由能 ΔG = ΔH − TΔS 决定。吸热反应若熵显著增加（如生成气体或溶解成离子），在足够高的温度下 ΔG 可为负，因而自发进行。
Whether a reaction is feasible depends on the Gibbs energy, ΔG = ΔH − TΔS. An endothermic reaction with a large entropy increase (forming gas, or dissolving into ions) has a negative ΔG at a high enough temperature and so proceeds spontaneously.