Thermodynamic and Kinetic Control
Introduced| English |
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| metastable/ˌmetəˈsteɪbl/ |
Allowed but not always happening
- Diamond should slowly turn to graphite -- yet it never seems to.
- The reaction is favourable, but unimaginably slow.
- Being allowed is one thing; happening in time is another.
- Two different questions govern every reaction.
Whether versus how fast
- Thermodynamics ($\Delta G$) asks whether a reaction can happen.
- Kinetics (activation energy) asks how fast it goes.
- A reaction needs a good answer to both to matter in practice.
Which quantity tells you whether a reaction is thermodynamically favourable?
$\Delta G$ decides favourability; activation energy decides speed.
To be useful in practice, a reaction should be both favourable and fast enough.
Both thermodynamics and kinetics must cooperate.
Favourable yet frozen
- A reaction with $\Delta G < 0$ but a huge activation energy is stuck.
- It is favourable, yet effectively never happens.
- Such a trapped state is called metastable 亚稳态.
A reaction with $\Delta G < 0$ can still be far too slow to observe.
A large activation energy makes a favourable reaction slow.
A favourable but kinetically trapped state is called ____.
Metastable means favourable yet effectively frozen.
Kinetics can be helped
- A catalyst lowers the activation energy, unlocking a slow reaction.
- It changes the kinetics, not the thermodynamics.
- $\Delta G$ stays the same, but now the reaction actually proceeds.
Thermodynamics or kinetics?
Sort each idea by whether it is set by free energy or by rate.
A catalyst changes a reaction's...
Catalysts lower the barrier but leave $\Delta G$ unchanged.
Diamond turning to graphite has $\Delta G < 0$. Why does it not happen?
- The activation energy is enormous.
- Kinetics traps it, so diamond is metastable.
Diamond persists even though converting to graphite is favourable because it is...
A huge activation energy traps diamond in a metastable state.
Thermodynamic favourability ($\Delta G < 0$) does not guarantee a reaction is observable -- a big activation energy can make it immeasurably slow. A catalyst changes speed (kinetics), not $\Delta G$ (thermodynamics). And "metastable" means favourable but kinetically trapped.
Thermodynamics ($\Delta G$) says whether a reaction can happen; kinetics (activation energy) says how fast. A favourable reaction can be frozen by a huge barrier -- a metastable state like diamond. A catalyst unlocks the kinetics without changing $\Delta G$.