9.4 Kinetic Control

Thermodynamic vs Kinetic Control

A reaction-coordinate diagram and two gauges for Favorability (ΔG°) and Rate (Arrhenius k). Drag Eₐ and T to see when a favored reaction stalls.

ΔG° vs Eₐk = A·e^(−Eₐ/RT)6 PresetsKinetic Stall
Topic 9.4

Thermodynamic and Kinetic Control

Explain, in terms of kinetics, why a thermodynamically favored reaction might not occur at a measurable rate.

EK 9.4.A.1 is a single sentence with large consequences: many processes that are thermodynamically favored do not occur to any measurable extent, or they occur at extremely slow rates.

This is the formal separation of the two big questions in chemistry:

  • Thermodynamics asks: where will the system end up? Answered by ΔG° and K.
  • Kinetics asks: how fast will it get there? Answered by Ea and the mechanism.

They are independent. A reaction can be strongly favored (ΔG° ≪ 0, K enormous) and still have an activation energy so large that essentially no molecules can clear it at ordinary temperatures.

The canonical examples:

  • Diamond → graphite. ΔG° ≈ −2.9 kJ/mol, so graphite is the favored form of carbon at room conditions. The conversion requires breaking a rigid three-dimensional covalent network, an enormous activation barrier, so diamonds persist essentially indefinitely.
  • Combustion of gasoline. Hugely favored, yet gasoline sits stably in a tank. A spark supplies the activation energy.
  • Rusting. Strongly favored but slow enough to be tolerable, which is why bridges are inspected rather than replaced daily.

How to speed up a kinetically hindered but favored reaction: raise the temperature (more particles clear Ea) or add a catalyst (new pathway with lower Ea). Neither changes ΔG° or K — they only change how long you wait.

Kinetic control vs. thermodynamic control. When two products are possible, low temperature and short times favor the one that forms fastest (kinetic product); high temperature and long times allow the system to reach the most stable product (thermodynamic product).

Key points

  • Thermodynamically favored says nothing about rate.
  • A large Ea can make a favored reaction unobservably slow.
  • Temperature and catalysts change rate; they do not change ΔG° or K.
  • Use “thermodynamically favored”, not “spontaneous”, in explanations.

Common mistakes

  • "Spontaneous means it happens immediately." The exact misconception the CED renamed the term to avoid.
  • Explaining slowness with thermodynamics. The answer must invoke activation energy.
  • Claiming a catalyst makes a reaction favored. It cannot change ΔG°.
  • Assuming a large K means fast. Independent quantities.

Worked example

The conversion of diamond to graphite has ΔG° = −2.9 kJ/mol at 298 K. Explain why diamonds do not spontaneously turn into graphite, using both thermodynamic and kinetic arguments.

Thermodynamic argument. ΔG° is negative, so the conversion of diamond to graphite is thermodynamically favored at 298 K and 1 atm. Graphite is the more stable allotrope of carbon under these conditions, and at equilibrium essentially all carbon would be graphite. Thermodynamics therefore predicts that the conversion should occur.

Kinetic argument. Diamond is a rigid three-dimensional covalent network in which every carbon is bonded to four others with fixed bond angles. Converting it to graphite requires breaking a large number of very strong C–C covalent bonds simultaneously and rearranging the atoms into planar sheets. The activation energy for this rearrangement is enormous.

At 298 K, the Maxwell–Boltzmann distribution places an utterly negligible fraction of the atoms above that activation barrier, so the rate of conversion is immeasurably small — on the order of billions of years.

Conclusion. The reaction is thermodynamically favored but kinetically inaccessible. ΔG° determines the destination; the activation energy determines whether the system can get there in any reasonable time. This is exactly why the CED prefers "thermodynamically favored" to "spontaneous": nothing about ΔG° < 0 implies speed.

Full notes for topic 9.4 →