Unit 5 · Kinetics

AP Chemistry Catalysts

What a catalyst changes and what it leaves alone, the three types of catalysis the exam names, and how to explain a faster rate at the particle level, in four worked examples.

A catalyst is a substance that speeds up a reaction without being used up. Catalysis is the last topic in the kinetics unit, and it ties the earlier ideas together: a catalyst adds new steps to a reaction mechanism, lowers the barrier on an energy profile, and can even show up in the rate law. Exam questions mostly test whether you can say exactly what a catalyst changes and what it does not.

What a Catalyst Changes and What It Leaves Alone

A catalyst takes part in the reaction: it is consumed in one elementary step and regenerated in a later one, so its net amount does not change. Because it opens a different pathway, it changes how fast the reaction goes but not where the reaction ends up.

The equilibrium row follows from the activation energy row: lowering both barriers equally speeds up both directions equally, so the balance point does not move. That is why a catalyst is never a stress in a Le Chatelier’s principle question.

The Three Types of Catalysis on the AP Exam

• Surface catalysis: reactants bind to the surface of a solid, often a metal, where bonds weaken and new bonds form before the products leave. Because the catalyst is in a different phase from the reactants, this is also called heterogeneous catalysis.
• Acid-base catalysis: a proton is transferred to or from a reactant, creating a new intermediate that reacts more easily. The acid or base is regenerated later in the mechanism.
• Enzyme catalysis: an enzyme, a protein, binds the reactant at its active site, which can hold it in a favorable orientation or form a new intermediate.

In all three, the catalyst works the same way at the level the exam cares about: it creates a new pathway with a lower activation energy, sometimes also making more collisions effective by holding reactants in a better orientation. The specific organic or biological mechanisms are examples, not something to memorize.

Worked Example 1: Comparing a Catalyzed and an Uncatalyzed Energy Profile

On an energy profile, the reactants sit at 40 kJ/mol and the products at 10 kJ/mol. The uncatalyzed reaction is one step with a transition state at 140 kJ/mol. The catalyzed reaction takes two steps: a transition state at 90 kJ/mol, an intermediate at 60 kJ/mol, and a second transition state at 85 kJ/mol. Compare the two pathways.

Uncatalyzed barrier: 140 − 40 = 100 kJ/mol.
Catalyzed barriers: step 1 is 90 − 40 = 50 kJ/mol; step 2 is 85 − 60 = 25 kJ/mol. The highest point on the path is 90 kJ/mol, so the overall barrier is 50 kJ/mol, and step 1 is the rate-determining step.
Reverse barriers: 140 − 10 = 130 kJ/mol uncatalyzed and 90 − 10 = 80 kJ/mol catalyzed. Both directions drop by 50 kJ/mol.
ΔH: 10 − 40 = −30 kJ/mol for both pathways.

The catalyzed curve has an extra valley, which is the intermediate the catalyst creates. The start and end points do not move, which is the picture behind “a catalyst does not change ΔH.”

Worked Example 2: Finding the Catalyst and the Rate Law in the Iodide Mechanism

Iodide ions speed up the decomposition of hydrogen peroxide by this two-step mechanism:

Step 1 (slow): H₂O₂ + I⁻ → H₂O + IO⁻
Step 2 (fast): H₂O₂ + IO⁻ → H₂O + O₂ + I⁻

Find the overall reaction, identify the catalyst and the intermediate, and write the rate law.

Overall: add the steps and cancel species that appear on both sides (I⁻ and IO⁻): 2H₂O₂ → 2H₂O + O₂.
Catalyst: I⁻, used in step 1 and regenerated in step 2.
Intermediate: IO⁻, made in step 1 and used up in step 2.
Rate law: from the slow step, rate = k[H₂O₂][I⁻].

The catalyst never appears in the overall equation, yet it does appear in the rate law, so doubling the iodide concentration doubles the rate. A species can be absent from the balanced equation and still control how fast the reaction goes.

Worked Example 3: Classifying Four Real Catalysts by Type

Name the type of catalysis in each case.

1. Iron in the Haber process, N₂ + 3H₂ → 2NH₃: nitrogen and hydrogen bind to the solid iron, so this is surface catalysis.
2. Catalase in cells breaking down H₂O₂: a protein binds the reactant at its active site, so this is enzyme catalysis.
3. Aqueous H⁺ speeding up the hydrolysis of an ester: a proton is transferred to the ester and released again at the end, so this is acid-base catalysis.
4. Platinum, palladium, and rhodium in a car’s catalytic converter, 2CO + 2NO → 2CO₂ + N₂: the gases react on the metal surface, so this is surface catalysis.

The clue is usually in the wording: a solid metal points to surface catalysis, a proton transfer points to acid-base catalysis, and a protein with an active site points to an enzyme.

Worked Example 4: Explaining a Faster Rate at the Particle Level

A free-response question asks: “Explain why adding a catalyst increases the rate of the reaction at the same temperature.” Write a complete answer.

Full-credit answer: The catalyst provides an alternative pathway with a lower activation energy. The temperature is unchanged, so the distribution of molecular kinetic energies is unchanged, but a larger fraction of collisions now has energy greater than or equal to the lower activation energy. More collisions are successful per unit time, so the rate increases.

Fig. 1 shows the idea with illustrative numbers: halving the barrier from 6RT to 3RT raises the share of molecules above it from about 0.7% to about 11%. You will not calculate these fractions on the exam, but the chart explains the answer.

Answer that loses the point: “The catalyst makes the molecules move faster.” That describes raising the temperature, not adding a catalyst. For the temperature version of this explanation, see kinetic molecular theory.

Five Catalyst Practice Questions With Answers

1. Reactants are at 30 kJ/mol and products at 70 kJ/mol. The transition state is at 150 kJ/mol without a catalyst and 110 kJ/mol with one. Find both forward activation energies, ΔH, and the catalyzed reverse activation energy.
   Show answer

   Uncatalyzed E_a = 120 kJ/mol, catalyzed E_a = 80 kJ/mol, ΔH = +40 kJ/mol for both, catalyzed reverse E_a = 110 − 70 = 40 kJ/mol.
2. A mechanism is Step 1: A + C → AC; Step 2: AC + B → AB + C. Give the overall reaction and identify the catalyst and the intermediate.
   Show answer

   Overall A + B → AB. C is the catalyst (used, then regenerated); AC is the intermediate (made, then used).
3. If step 1 in question 2 is the slow step, what is the rate law?
   Show answer

   Rate = k[A][C]. The catalyst appears in the rate law because it is a reactant in the slow step.
4. A catalyst is added to a reaction mixture already at equilibrium. What happens to K and to the amounts of reactants and products?
   Show answer

   Nothing. Both directions speed up equally, so K and the equilibrium amounts stay the same.
5. In the Ostwald process, 4NH₃ + 5O₂ → 4NO + 6H₂O runs over a hot platinum gauze. What type of catalysis is this?
   Show answer

   Surface (heterogeneous) catalysis: the gases react on the solid platinum surface.

Common Catalyst Mistakes

• Saying a catalyst changes ΔH. It lowers the barrier; the reactant and product energies stay put.
• Saying a catalyst shifts the equilibrium or raises the yield. It only helps the system reach the same equilibrium sooner.
• Saying a catalyst gives molecules more energy. The energy distribution is set by temperature; the catalyst lowers the energy needed.
• Mixing up catalysts and intermediates. A catalyst is used first and regenerated; an intermediate is made first and then used up.
• Leaving the catalyst out of the rate law. If it is a reactant in the slow step, it belongs in the rate law.
• Saying a catalyst is not involved in the reaction. It takes part in the mechanism; it is just not consumed overall.

Catalysts are a small topic, but they show up inside mechanism, energy-profile, and equilibrium questions across the exam. To see how stronger kinetics answers would move your overall estimate, try the AP Chem Score Calculator.

Related Resources

• AP Chem Score Calculator
• Unit 5 Review: Kinetics
• AP Chemistry Activation Energy
• AP Chemistry Reaction Mechanisms
• AP Chemistry Rate Law
• AP Chemistry Le Chatelier’s Principle
• AP Chemistry Kinetic Molecular Theory
• AP Chemistry Study Guide

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