Catalysis
Explain complete regenerated catalytic cycles and why a changed path leaves the endpoint enthalpy unchanged.
AP Chemistry Topic 5.11 · CED printed p. 105 / PDF p. 112. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations and stoichiometric ratios
- Molarity and concentration versus amount
- Bond rearrangement and energy conservation
- Slopes, logarithms and reciprocal graphs
A catalyst makes a route with more successful reaction events available. It can provide lower activation barriers or improve the orientation/accessibility of reacting partners. It does not supply a larger initial particle-energy distribution merely by being added at the same temperature. Initial and final species energies are unchanged, so the overall enthalpy change is unchanged. Forward and reverse processes can both be accelerated; a catalyst speeds approach to equilibrium rather than changing the equilibrium composition at fixed temperature.
A catalyst participates in elementary steps and is regenerated. For E + A ⇌ EA followed by EA → E + P, E is used to bind A and returned when product leaves. Its total amount, counting free and bound forms, is conserved in the ideal cycle. The free catalyst concentration can vary as binding changes, so “unchanged catalyst” does not mean that every catalyst particle sits idle or that its free concentration is always identical.
To see consumption in a slow step explicitly, use the supplied elementary model C + A → CA (slow), followed by CA → C + P (fast). C is temporarily consumed in the rate-determining first step and restored in the second. Adding the steps cancels C and CA, leaving A → P. At fixed volume, [C] + [CA] is the conserved total catalyst concentration in this ideal model; free [C] alone need not remain constant. This is one possible model, not a rule that every catalyst is consumed in the slowest step.
Binding can position reacting groups together and make a suitable reaction geometry more likely. Enzymes commonly form bound intermediates that lower the barriers for specific transformations. A sketch should show binding, conversion and release with enzyme regeneration. An enzyme does not change the net thermodynamic energy difference, and its activity can depend on temperature or pH because its structure and binding behavior matter. These are qualitative models rather than a required enzyme-rate derivation.
In acid–base catalysis, a proton can be donated to create a more reactive intermediate and later removed to regenerate the catalytic acid, or accepted and later returned by a catalytic base. Temporary covalent bonding changes which elementary steps are available. Include the proton-transfer steps in the mechanism; a drawn H+ that vanishes forever is a consumed reagent, not a regenerated catalyst.
In surface catalysis, reactants bind at accessible sites, can be brought near one another or have bonds weakened, react while bound, and products leave to free the sites. The bound species are intermediates. Increasing accessible surface can raise the number of working sites, but a surface can become saturated or blocked. Heterogeneous catalysis is not just collisions with any inert wall: binding and subsequent chemical steps give the surface its function.
Let's work through it
Model: E + A ⇌ EA; EA → EP; EP → E + P. Compare it with uncatalyzed A → P.
- Add the steps and cancel E, EA and EP to recover A → P. E is catalyst; EA and EP are bound intermediates.
- The catalyzed route can have several lower local barriers and better reactant orientation instead of one high barrier.
- Both routes connect the same initial A and final P, so ΔH is unchanged. Total enzyme E +EA +EP is conserved in the ideal cycle; free E need not stay constant during binding.
Two supplied cycles illustrate distinct catalytic mechanisms.
- Acid cycle: H⁺ protonates A to AH⁺; AH⁺ rearranges to PH⁺; PH⁺ gives P +H⁺. Cancel H⁺ and the protonated intermediates to recover A →P. Protonation creates a new bonded intermediate and the final deprotonation restores H⁺.
- Surface cycle: A binds at a free site *, giving A*; A* converts to P*; P desorbs, restoring *. Bound A* and P* are intermediates. Free sites, rather than the entire metal bulk, permit repeated cycles.
- Both can change barriers/orientation; neither changes the same A-to-P endpoint enthalpy difference.
Supplied model: C + A → CA (slow); CA → C + P (fast).
- The slow step consumes free C to make bound intermediate CA. The later fast step releases C.
- Cancel C and CA from the step sum to obtain A → P. C is present before and after a completed cycle, whereas CA is produced and then consumed.
- Count both catalyst forms: at fixed volume and without catalyst loss or deactivation, [C] + [CA] is constant. Temporary binding can lower free [C] without consuming the total catalyst supply.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
What must the mechanism show to support calling a species a catalyst?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
What must the mechanism show to support calling a species a catalyst?
Worked answer and reason
Its participation followed by regeneration, with no net consumption in the ideal overall cycle.
A catalyst changes the route and is restored.
Retrieval 2
Why can adding surface catalyst stop giving a proportional rate increase?
Worked answer and reason
Accessible sites can become limited, saturated or blocked; binding and available reactant matter.
The surface model has a finite population of working sites.
Catch the tempting mistake
- Catalyst lowers ΔH because it lowers Ea.
- It changes pathway barriers; the same reactant/product energy difference stays fixed.
- A catalyst never changes form during reaction.
- It commonly exists in bound or protonated intermediates before regeneration.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 5 problem 19 · ap synthesis · multiple
- Unit 5 problem 20 · ap synthesis · verbal
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For the teacher
- Require a complete cycle rather than a catalyst label beside an arrow.
- Use acid-transfer, bound-enzyme and surface-site examples to assess distinct catalytic models.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 5.11.A
- Explain the relationship between the effect of a catalyst on a reaction and changes in the reaction mechanism.
- 5.11.A.1
- In order for a catalyst to increase the rate of a reaction, the addition of the catalyst must increase the number of effective collisions and/or provide a reaction path with a lower activation energy relative to the original reaction coordinate.
- 5.11.A.2
- In a reaction mechanism containing a catalyst, the net concentration of the catalyst is constant. However, the catalyst will frequently be consumed in the rate-determining step of the reaction, only to be regenerated in a subsequent step in the mechanism.
- 5.11.A.3
- Some catalysts accelerate a reaction by binding to the reactant(s). The reactants are either oriented more favorably or react with lower activation energy. There is often a new reaction intermediate in which the catalyst is bound to the reactant(s). Many enzymes function in this manner.
- 5.11.A.4
- Some catalysts involve covalent bonding between the catalyst and the reactant(s). An example is acid-base catalysis, in which a reactant or intermediate either gains or loses a proton. This introduces a new reaction intermediate and new elementary reactions involving that intermediate.
- 5.11.A.5
- In surface catalysis, a reactant or intermediate binds to, or forms a covalent bond with, the surface. This introduces elementary reactions involving these new bound reaction intermediate(s).
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Reaction Mechanisms — Scientific check: intermediates, elementary molecularity, slow steps and rapid-equilibrium substitution.
- OpenStax Chemistry 2e: Catalysis — Scientific check: homogeneous/surface/enzyme pathways and catalyst regeneration.
Glossary and deeper reading · 9 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Stoichiometry
Stoichiometry uses a balanced chemical equation to relate amounts of reactants and products. Equation coefficients provide mole ratios rather than direct mass ratios. For example, forming water from hydrogen and oxygen consumes two moles of hydrogen molecules per mole of oxygen molecules.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Enthalpy
Enthalpy is a state function defined from internal energy, pressure, and volume. At constant pressure with only pressure–volume work, an enthalpy change equals the heat transferred to the system. For example, a negative reaction enthalpy indicates heat release for the written reaction under those conditions.
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Chemical equilibrium
Chemical equilibrium is a dynamic state in which forward and reverse reaction rates are equal. Macroscopic composition remains constant in a closed system under fixed conditions, but reactant and product concentrations need not be equal. For example, a reversible reaction can reach equilibrium with much more reactant than product.
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Elementary reaction
An elementary reaction is one step in a proposed mechanism rather than the sum of multiple steps. For a specified elementary step, the rate-law concentration factors follow its reacting-particle event. For example, a bimolecular elementary collision can have a rate proportional to the concentrations of its two reactants.
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Reaction intermediate
A reaction intermediate is produced in one mechanism step and consumed in another. It cancels from the overall equation and differs from a catalyst, which is present before it is consumed and then regenerated. For example, a short-lived species formed in step one and used in step two can be an intermediate.
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pH
pH is the negative base-ten logarithm of hydronium activity. In suitable dilute models, activity is approximated by molar concentration relative to the standard concentration. For example, lowering hydronium concentration by a factor of ten raises pH by one unit.
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Covalent bond
A covalent bond involves shared electron density between atoms. The sharing can be approximately equal or unequal depending on the atoms and their environment. For example, the bond in a hydrogen molecule shares electrons between two identical atoms.
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Brønsted acid and base
A Brønsted acid donates a proton, and a Brønsted base accepts a proton. These roles describe a particular reaction, so some substances can act in either role. For example, water accepts a proton from hydrogen chloride but donates one to ammonia.