How does a catalyst speed a reaction without being consumed, and what kinds of catalysis are there?
Topic 5.11 Catalysis: explain how a catalyst increases the rate by providing an alternative pathway with a lower activation energy, and distinguish homogeneous, heterogeneous and enzyme catalysis.
A focused answer to AP Chemistry Topic 5.11, covering how a catalyst lowers the activation energy by offering an alternative mechanism, the types of catalysis (homogeneous, heterogeneous, enzymatic), and why a catalyst leaves enthalpy and equilibrium unchanged, with full worked examples.
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What this topic is asking
The College Board (Topic 5.11) wants you to explain how a catalyst increases the rate by providing an alternative pathway with a lower activation energy, and to distinguish the main types of catalysis: homogeneous, heterogeneous and enzyme. The key conceptual points are that a catalyst is not consumed and changes neither the enthalpy of reaction nor the position of equilibrium.
How a catalyst works
On the energy profile, the catalyzed pathway has a lower peak. By collision theory, a lower means a larger fraction of collisions clear the barrier (a larger area beyond on the Maxwell-Boltzmann distribution), so the rate rises. The catalyst does this by binding the reactants in a way that stabilizes the transition state, opening a route the uncatalysed reaction does not have.
What a catalyst does not change
This is a favorite distinction on the exam. A catalyst is a kinetic device: it changes how fast equilibrium is reached, not where equilibrium lies. The equilibrium constant, the enthalpy of reaction and the amounts present at equilibrium are all unaffected. Only conditions that change the energy landscape of reactants or products (or the temperature) can move equilibrium.
Types of catalysis
A homogeneous catalyst is in the same phase as the reactants, for example an acid catalyst dissolved in the same solution. A heterogeneous catalyst is in a different phase, most often a solid surface on which gas or solution reactants adsorb, react and desorb; the catalytic converter in a car and many industrial metal catalysts work this way. An enzyme is a biological protein catalyst that binds substrates in an active site with great specificity, lowering the activation energy of a biochemical reaction enormously. All three work by the same principle of a lower-energy pathway.
Try this
Q1. State the effect of a catalyst on the rate constant and explain why. [2 points]
- Cue. It increases , because lowering increases the term in the Arrhenius equation.
Q2. Classify a solid platinum catalyst used for a gas-phase reaction. [1 point]
- Cue. Heterogeneous (catalyst in a different phase from the reactants).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2023 (style)4 marksSection II (long FRQ, part). A catalyst is added to a reaction. (a) Explain, in terms of the energy profile, how the catalyst increases the rate. (b) Determine whether the catalyst changes the enthalpy of reaction, and justify. (c) Determine whether the catalyst changes the position of equilibrium, and justify. (d) A proposed catalytic mechanism is Step 1: ; Step 2: . Justify that the catalyst is regenerated.Show worked answer β
A 4-point conceptual FRQ on catalysis.
(a) Energy profile (1 point): the catalyst provides an alternative pathway with a lower activation energy, so a larger fraction of collisions can clear the barrier and the rate increases.
(b) Enthalpy (1 point): the catalyst does not change the energies of the reactants or products, only the barrier between them, so the enthalpy of reaction is unchanged.
(c) Equilibrium (1 point): a catalyst lowers the forward and reverse activation energies equally, speeding both directions by the same factor, so the position of equilibrium is unchanged; it only shortens the time to reach equilibrium.
(d) Regeneration (1 point): the catalyst is consumed in step 1 (forming cat-A) and regenerated in step 2, so it is not net consumed and does not appear in the overall equation.
Markers reward the lower-activation-energy pathway, the unchanged enthalpy, the unchanged equilibrium position, and the regeneration reasoning.
AP 2021 (style)1 marksSection I (multiple choice). A catalyst increases the rate of a reaction by (A) increasing the enthalpy of reaction (B) lowering the activation energy via an alternative pathway (C) increasing the temperature (D) shifting the equilibrium toward products. Justify your choice.Show worked answer β
A 1-point conceptual MCQ. The answer is (B).
A catalyst offers an alternative reaction pathway with a lower activation energy, raising the fraction of effective collisions and so the rate. It does not change the enthalpy, temperature or equilibrium position. The trap is (D): a catalyst speeds the approach to equilibrium but does not move it.
Related dot points
- Topic 5.5 Collision Model: use collision theory and the Arrhenius equation to explain how activation energy, temperature, orientation and collision frequency control the rate constant.
A focused answer to AP Chemistry Topic 5.5, covering collision theory, activation energy, the Maxwell-Boltzmann distribution, molecular orientation, and the Arrhenius equation linking rate constant to temperature, with full worked examples.
- Topic 5.6 Reaction Energy Profile: interpret a potential-energy diagram to identify the activation energy of the forward and reverse reactions, the transition state and the enthalpy of reaction.
A focused answer to AP Chemistry Topic 5.6, covering the potential-energy diagram, the transition state, the activation energy of the forward and reverse reactions, the relationship to enthalpy of reaction, and the effect of a catalyst, with full worked examples.
- Topic 5.7 Introduction to Reaction Mechanisms: represent a reaction as a sequence of elementary steps, identify reaction intermediates and catalysts, and confirm that the steps sum to the overall equation.
A focused answer to AP Chemistry Topic 5.7, covering reaction mechanisms as sequences of elementary steps, identifying intermediates and catalysts, and checking that the steps add up to the overall equation, with full worked examples.
- Topic 5.8 Reaction Mechanism and Rate Law: identify the rate-determining (slow) step of a mechanism and use it to write the rate law, and check a proposed mechanism against the experimental rate law.
A focused answer to AP Chemistry Topic 5.8, covering the rate-determining step, writing the rate law from the slow step, the slow-step-first case, and how a proposed mechanism must agree with the experimental rate law, with full worked examples.
- Topic 7.9 Introduction to Le Chatelier's Principle: predict the direction a system at equilibrium shifts in response to a change in concentration, volume or pressure, or temperature, using Le Chatelier's principle.
A focused answer to AP Chemistry Topic 7.9, covering Le Chatelier's principle and how an equilibrium shifts in response to changes in concentration, volume or pressure, and temperature, including the effect on K of temperature, with full worked examples.
Sources & how we know this
- AP Chemistry Course and Exam Description β College Board (2020)