A negative ΔG says a reaction can happen; it does not say when. The rate depends on the activation barrier, which thermodynamics does not see.
Key idea
Thermodynamics (ΔG) tells you which direction is favored and how far a reaction can go. Kinetics ($E_a$) tells you how fast. A favored reaction with a high barrier can be so slow that it effectively never happens: it is under kinetic control. When two products can form, the one with the lower barrier may form first even if the other is more stable.
Why it works
$\Delta G$ depends only on the start and end states. The rate depends on the height of the hump between them, through $e^{-E_a/RT}$. Nothing links the two: a reaction can fall a long way downhill and still have a high wall to climb first.
Worked examples
Worked example: Diamonds are not forever
For $\mathrm{C(diamond) \to C(graphite)}$, $\Delta G^\circ = -2.9\ \text{kJ/mol}$ at 25 °C. Why do diamonds last?
- $\Delta G^\circ < 0$: graphite is the more stable form, so the change is favored.
- But turning diamond into graphite means breaking and rearranging strong C–C bonds throughout the crystal, a huge barrier. At room temperature the rate is immeasurably small.
Answer: The conversion is favored but kinetically blocked.
Worked example: Two products, two barriers
A reactant can form P1 (barrier 60 kJ/mol) or P2 (barrier 80 kJ/mol); P2 is more stable. At 298 K, roughly how much faster does P1 form, if the factors $A$ are equal?
- $$\dfrac{k_1}{k_2} = e^{(80\,000 - 60\,000)/RT} = e^{20\,000/(8.314 \times 298)} = e^{8.07}$$
- About 3200 times faster. If P1 cannot easily turn back, the mixture ends up mostly P1, the kinetic product, even though P2 is thermodynamically favored.
Answer: About 3200 times faster.
- Wood and paper are favored to burn in air, but need a flame to get over the barrier.
- Hydrogen and oxygen sit unchanged until a spark: $\Delta G^\circ = -237\ \text{kJ}$ per mole of water.
- Your body's molecules are not at equilibrium with oxygen; enzymes choose which favored reactions run, and when.
The reverse is true too: a fast reaction need not be favored. Thermodynamics and kinetics answer different questions, and a full description of a reaction needs both.
Common mistake
Thinking a thermodynamically favored reaction must be fast, as if $\Delta G$ set the rate. $\Delta G$ fixes the direction; $E_a$ sets the speed. Check: look for the activation energy (or a catalyst) before predicting how fast anything goes.