定义Definition
吉布斯自由能是热力学状态函数，G = H − TS。恒温恒压下，反应的吉布斯自由能变 ΔG = ΔH − TΔS 决定反应能否自发进行：ΔG < 0 可自发，ΔG > 0 不能自发，ΔG = 0 处于平衡。
Gibbs free energy is the state function G = H − TS. At constant temperature and pressure, the Gibbs energy change of a reaction, ΔG = ΔH − TΔS, decides feasibility: ΔG < 0 feasible, ΔG > 0 not feasible, ΔG = 0 at equilibrium.
温度的影响Effect of temperature
ΔH < 0、ΔS > 0：任何温度都自发。ΔH > 0、ΔS < 0：任何温度都不自发。ΔH < 0、ΔS < 0：低温自发。ΔH > 0、ΔS > 0：高温自发，转折温度 T = ΔH/ΔS，如 CaCO₃ 分解约在 1100 K 以上才自发。计算时注意 ΔS 的单位要从 J K⁻¹ mol⁻¹ 换算成 kJ K⁻¹ mol⁻¹。
ΔH < 0, ΔS > 0: feasible at all temperatures. ΔH > 0, ΔS < 0: never feasible. ΔH < 0, ΔS < 0: feasible at low temperature. ΔH > 0, ΔS > 0: feasible at high temperature, above T = ΔH/ΔS; CaCO₃ decomposition becomes feasible above about 1100 K. Convert ΔS from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹ when calculating.
与平衡常数和电动势的关系Links to K and E°
ΔG° = −RT ln K，ΔG° 越负，平衡常数越大；ΔG° = −nFE°cell，电池电动势为正对应反应自发。ΔG < 0 只说明反应在热力学上可行，不说明反应速率，速率还取决于活化能。
ΔG° = −RT ln K: the more negative ΔG°, the larger K; and ΔG° = −nFE°cell, so a positive cell potential means a feasible reaction. ΔG < 0 shows only that a reaction is thermodynamically possible, not how fast it goes; rate depends on activation energy.
年表Timeline
- 1873
吉布斯发表关于热力学曲面的论文
Gibbs publishes his papers on thermodynamic surfaces
- 1876
吉布斯发表《论非均相物质的平衡》
Gibbs publishes On the Equilibrium of Heterogeneous Substances
- 1882
亥姆霍兹提出自由能概念
Helmholtz introduces the concept of free energy
相关公式Formulas
例题Worked example
CaCO₃ → CaO + CO₂：ΔH° = +178 kJ/mol，ΔS° = +161 J/(mol·K)，T_转向 ≈ 1106 K（约 833 °C）以上自发。
CaCO₃ decomposition becomes spontaneous above ~1106 K.