U9.4 · Thermodynamic and Kinetic ControlPractice · 热力学控制与动力学控制

Thermodynamics says graphite is more stable (ΔG° < 0); kinetics says the conversion is extremely slow, because rearranging the whole covalent network has an enormous activation energy. That is kinetic control. Diamonds last because the change is slow, not because they are more stable.

A catalyst provides a pathway with a lower activation energy, speeding up the forward and reverse reactions equally so equilibrium is reached sooner. ΔG° depends only on the initial and final states, and K is set by ΔG°, so neither changes.

Both reactions are thermodynamically favorable, and making N₂ (①) is even more so (−1306 kJ versus −955 kJ). NO forms because the catalyst greatly speeds up ②, so in the brief contact time NO is the main product. A catalyst doesn’t change any reaction’s ΔG°.

ΔG° > 0 corresponds to K < 1 (here about 10⁻⁶): little product at equilibrium, but not none. A catalyst changes only the rate, not ΔG° or K. And ΔG° says nothing about speed: a reaction with a small K can still reach equilibrium quickly.

Raising the temperature gives more molecules enough energy to clear the activation barrier, so any reaction speeds up. But for an exothermic reaction, heating shifts the equilibrium backward and lowers K (and ΔG° = ΔH° − TΔS° changes with temperature too). Industry often has to balance speed against yield.