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Catalysis

A catalyst is a shortcut, not a shove. It opens a lower-energy path over the barrier, speeding the reaction, but it never changes where the reaction ends up or gets used up in the process.

§1

A lower path, not a changed destination.

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A catalyst speeds a reaction by providing an alternate pathway with a lower activation energy. More collisions can clear the lower barrier, so the reaction goes faster.

A catalyst is not consumed overall — it is used in an early step and regenerated in a later one. It also lowers Eₐ for both the forward and reverse directions equally.

Crucially, a catalyst does not change the reactant or product energies, so it leaves ΔH unchanged and does not shift the equilibrium position. It changes only the path and the speed, never the reaction's energetics or endpoint.

UNIT 5 TOPIC 5.11 • CATALYSIS CATALYSIS potential energy reaction coordinate uncatalyzed pathway catalyzed pathway same reactants same products lower barrier = smaller Eₐ Catalyst facts A catalyst speeds up a reaction by giving an alternate pathway with a lower Eₐ. It does NOT change ΔH, K, or the final equilibrium amounts — only the rate. Catalyst cycle Step 1: A + Cat → A–Cat catalyst is consumed Step 2: A–Cat + B → P + Cat catalyst is regenerated → not used up overall Common modes Binding / orientation of reactants Acid–base catalysis Surface (heterogeneous) catalysis CED ANCHOR 5.11 — catalysts lower Eₐ A catalyst affects the kinetics (rate), not the thermodynamics: ΔH, K, and equilibrium are unchanged. AP Chemistry · Unit 5 · Kinetics
Fig. 5.11.1 A catalyst provides an alternate pathway with a lower activation energy, speeding the reaction. It is not consumed, it lowers Eₐ for both directions equally, and it leaves the reactant and product energies (and therefore ΔH and equilibrium) unchanged.
§2

Reasoning about a catalyst.

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It lowers the barrier; it does not touch the endpoints.

  1. Lower the activation energy. The catalyst offers a new path with a smaller Eₐ, so more collisions succeed.
  2. Keep the endpoints fixed. Reactant and product energies are unchanged, so ΔH is unchanged.
  3. Speed both directions. Lowering Eₐ speeds the forward and reverse reactions equally, so equilibrium is reached faster but not shifted.
  4. Regenerate the catalyst. It is consumed in one step and reformed in another, so it is not used up overall.
§3

The pieces you'll meet.

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What a catalyst does and does not do.

catalyst
Catalyst
Provides a lower-Eₐ pathway; not consumed overall.
lower Ea
Lowers Eₐ
Speeds the reaction by lowering the barrier.
not consumed
Not consumed
Regenerated by the end of the reaction.
both ways
Both directions
Lowers Eₐ for forward and reverse equally.
ΔH same
ΔH unchanged
Reactant and product energies are unaffected.
no shift
Equilibrium unshifted
Reaches equilibrium faster, but the position is unchanged.
§4

Worked example: what does a catalyst change?

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Question. A catalyst is added to a reaction. What changes and what does not?

Changes. The activation energy is lowered (a new pathway), so the reaction — both forward and reverse — goes faster.

Does not change. The reactant and product energies, and therefore ΔH, are unchanged; the equilibrium position is unchanged (equilibrium is just reached sooner).

The catalyst itself. It is consumed in an early step and regenerated later, so it is not used up. It alters the path and speed, never the destination.

§5

Mistakes that cost real points.

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Pitfall · 01

"A catalyst lowers the ΔH of the reaction."

A catalyst lowers the activation energy, not ΔH. Reactant and product energies are unchanged, so ΔH stays the same. Confusing the barrier (Eₐ) with the net energy change (ΔH) is the central error here.

Fix. Keep it straight: a catalyst lowers Eₐ, leaving ΔH (and the reactant/product energies) unchanged.

Pitfall · 02

"A catalyst shifts the equilibrium toward more product."

A catalyst speeds the forward and reverse reactions equally, so it reaches equilibrium faster but does not shift its position. The amounts at equilibrium are the same with or without the catalyst.

Fix. Treat a catalyst as changing the rate to equilibrium, never the equilibrium position or the final yield.

Pitfall · 03

"A catalyst is used up in the reaction."

A catalyst is regenerated — consumed in one step and reformed in a later one — so it is not consumed overall and does not appear in the overall equation. That is why a small amount can catalyze a large amount of reaction.

Fix. Remember a catalyst is regenerated and absent from the overall equation; it is not a reactant that gets used up.

§6

Skill Check.

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