定义Definition
中和热是在稀溶液中，酸与碱发生中和反应生成 1 mol 水时的焓变。强酸与强碱的中和热基本恒定，约为 −57 kJ mol⁻¹，因为其实质都是 H⁺(aq) + OH⁻(aq) → H₂O(l)。
The enthalpy of neutralisation is the enthalpy change when an acid and a base react in dilute solution to form one mole of water. For strong acids with strong bases it is nearly constant at about −57 kJ mol⁻¹, because the reaction is always H⁺(aq) + OH⁻(aq) → H₂O(l).
弱酸弱碱的情况Weak acids and bases
弱酸或弱碱参与时，中和热的绝对值通常略小，如乙酸与 NaOH 约为 −55 kJ mol⁻¹，因为弱电解质的电离需要吸收一部分能量。氢氟酸是例外，F⁻ 的水合放热很多，中和热约为 −68 kJ mol⁻¹。
With a weak acid or base the value is usually a little less exothermic, about −55 kJ mol⁻¹ for ethanoic acid with NaOH, because ionising the weak electrolyte absorbs some energy. Hydrofluoric acid is an exception: hydration of F⁻ releases a lot of heat and the value is about −68 kJ mol⁻¹.
测定Measurement
在保温杯中混合已知体积和浓度的酸、碱溶液，测量最高温度，由 q = mcΔT 计算放出的热量，再除以生成水的物质的量。通常近似认为溶液的密度和比热容与水相同（1.00 g cm⁻³，4.18 J g⁻¹ K⁻¹）。
Mix known volumes and concentrations of acid and alkali in an insulated cup, record the highest temperature, find the heat released from q = mcΔT and divide by the amount of water formed. The solution is usually taken to have the density and specific heat capacity of water (1.00 g cm⁻³, 4.18 J g⁻¹ K⁻¹).
年表Timeline
- 1840
盖斯测定多种酸碱的中和热，发现强酸强碱数值相近
Hess measures heats of neutralisation and finds similar values for strong acids and bases
- 1887
阿伦尼乌斯电离理论解释中和热恒定的原因
Arrhenius's ionic theory explains why the value is constant