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Thermodynamic and kinetic control

A reaction can want to happen and still refuse to. Favorability says which way is downhill; kinetics says how fast you can get there. A diamond is thermodynamically doomed to become graphite — on a timescale of eternity.

§1

Favorability versus rate.

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A reaction's thermodynamic favorability is set by ΔG, but its rate is set by the activation-energy barrier (kinetics). These are separate questions: a strongly favored reaction (ΔG < 0) can be immeasurably slow if the barrier is high.

The classic case is diamond → graphite: favored (ΔG < 0) yet so slow it never appears to happen. A reaction under kinetic control is stalled by its barrier, not by thermodynamics.

A catalyst lowers the activation barrier and speeds the reaction, but it does not change ΔG or the position of equilibrium — it only helps the system reach the outcome thermodynamics already allows.

UNIT 9 TOPIC 9.4 • THERMODYNAMIC AND KINETIC CONTROL THERMO VS KINETIC ΔG predicts thermodynamic favorability, not rate — not how fast a reaction actually goes. A reaction can be thermodynamically favored (ΔG < 0) yet still slow when Eₐ is large. Reaction-coordinate diagram free energy, G reactants transition state products Eₐ ΔG < 0 reaction progress → Products lie below reactants (ΔG < 0) — yet the reaction must climb Eₐ first. THERMODYNAMICALLY FAVORED ΔG < 0 → reaction is spontaneous Tells you the reaction CAN happen and where equilibrium lies. Says nothing about how fast. KINETIC CONTROL (Eₐ) A large activation energy Eₐ makes the reaction SLOW — regardless of ΔG. Rate depends on Eₐ and temperature, not on ΔG. EXAMPLE: diamond → graphite C(diamond) → C(graphite) ΔG < 0 : graphite is more stable (thermodynamically favored). But Eₐ is enormous, so diamond persists for ages — favored, yet unreactive. BOTTOM LINE ΔG sets the destination; Eₐ sets the speed. Thermodynamically favored ≠ fast. AP Chemistry · Unit 9 · Applications of Thermodynamics
Fig. 9.4.1 Favorability (ΔG) and rate (activation energy) are independent. A favored reaction can be extremely slow if its barrier is high. A catalyst lowers the barrier and speeds it up without changing ΔG or the equilibrium.
§2

Telling the two apart.

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Separate 'will it?' from 'how fast?'

  1. Ask favorability first. The sign of ΔG says whether the reaction is thermodynamically favored.
  2. Ask rate separately. The activation-energy barrier sets how fast it proceeds.
  3. Spot kinetic control. A favored reaction that is stuck is stalled by a high barrier, not at equilibrium.
  4. Place the catalyst. A catalyst lowers the barrier (faster) but leaves ΔG and K unchanged.
§3

The pieces you'll meet.

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Two independent axes: favorability and speed.

ΔG
Favorability
Set by ΔG; negative is favored.
Ea
Activation energy
The barrier; sets the rate.
kinetic
Kinetic control
Stalled by a high barrier despite ΔG < 0.
catalyst
Catalyst
Lowers Ea; does not change ΔG or K.
stall
Stalled ≠ equilibrium
A frozen reaction is not at equilibrium.
independent
Independent
Speed and favorability are separate.
§4

Worked example: favored but frozen.

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Diamond. Diamond → graphite has ΔG < 0, so graphite is the favored form.

Yet stable. The activation barrier for rearranging the carbon lattice is enormous, so the rate is effectively zero — diamonds last indefinitely.

Not equilibrium. The diamond is not at equilibrium with graphite; it is kinetically trapped behind a high barrier.

Catalyst. A catalyst could speed such a conversion by lowering the barrier, but it would not make graphite any more favored — ΔG is unchanged.

§5

Mistakes that cost real points.

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Pitfall · 01

"A thermodynamically favored reaction must be fast."

Favorability (ΔG) and rate are independent. A favored reaction can be extremely slow if its activation barrier is high — diamond turning to graphite is favored but takes ages. Do not infer speed from ΔG.

Fix. Treat favorability and rate as separate: ΔG says whether, the barrier says how fast.

Pitfall · 02

"A slow, stalled reaction has reached equilibrium."

A reaction frozen by a high activation barrier is under kinetic control, not at equilibrium — it may be far from the equilibrium mixture. Equilibrium means forward and reverse rates are equal, not that nothing is moving because of a barrier.

Fix. Distinguish a kinetically stalled reaction (high barrier) from a true equilibrium.

Pitfall · 03

"A catalyst makes a reaction more favorable (more negative ΔG)."

A catalyst only lowers the activation barrier, speeding the reaction; it does not change ΔG or the equilibrium position. It helps you reach the same outcome faster, not a more favored one.

Fix. Remember a catalyst changes the rate, not ΔG or K.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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