Chemistry · CED Unit 5 Kinetics · 14 min read · Updated 2026-05-11
AP Chemistry Catalysis — AP Chemistry
AP Chemistry · CED Unit 5 Kinetics · 14 min read
1. Core Definition of Catalysis★★☆☆☆⏱ 3 min
Catalysis is the process where a catalyst increases the rate of a chemical reaction without being consumed in the overall reaction. Catalysts are incorporated into intermediate elementary steps, then fully regenerated by the final step, so they do not appear in the overall balanced reaction equation.
Catalysts work by providing an **alternative reaction mechanism** (a new sequence of elementary steps) with a lower overall activation energy ($E_a$) than the uncatalyzed pathway. Lower $E_a$ leads to a larger rate constant $k$ (per the Arrhenius equation) and a faster reaction. The AP exam almost exclusively focuses on rate-increasing catalysts; inhibitors (rate-decreasing "negative catalysts") are rarely tested.
2. Homogeneous vs Heterogeneous Catalysis★★☆☆☆⏱ 3 min
Catalysis is classified by the relative physical phase (state of matter) of the catalyst and the reaction reactants.
Heterogeneous catalysis typically proceeds via adsorption: reactants bind to active sites on the solid catalyst surface, which weakens existing bonds and lowers activation energy. Products then desorb, freeing active sites for new reactants. Homogeneous catalysis forms temporary intermediate complexes in solution, with catalyst regenerated at the end.
Exam tip: Always check the physical phase of the catalyst, not just its chemical identity, when classifying catalysis type.
3. Catalyzed Reaction Energy Profiles★★★☆☆⏱ 3 min
Because catalysts provide a new reaction mechanism, the catalyzed energy profile has a different shape than the uncatalyzed profile. The uncatalyzed reaction has one activation energy peak per elementary step, with the highest peak equal to the overall activation energy for the rate-determining step. The catalyzed pathway has a peak for each elementary step in its new mechanism, and the highest (overall) peak is always lower than the highest peak of the uncatalyzed pathway.
A key AP-tested point: catalysts do not change the energy of reactants or products, only the pathway between them. Therefore, the overall enthalpy change $
Delta H$, equilibrium constant $K$, and final equilibrium yield are all identical for catalyzed and uncatalyzed reactions.
Exam tip: On FRQs comparing energy profiles, always explicitly state that $
Delta H$ is the same for both pathways to earn full credit.
4. Enzyme Catalysis★★★☆☆⏱ 3 min
Enzymes are biological catalysts (almost always globular proteins) that speed up cellular reactions by up to 10 orders of magnitude. They are highly specific to their reactants (called substrates), due to the 3D structure of their active site: the region where substrate binds to form an enzyme-substrate complex, which reacts to form product and regenerate free enzyme.
A key kinetic property of enzyme catalysis is saturation kinetics: when enzyme concentration is constant, rate depends on substrate concentration differently at low vs high concentrations. Enzyme activity is also highly dependent on temperature and pH: high temperatures or extreme pH cause denaturation, which destroys the 3D active site structure and eliminates catalytic activity.
Exam tip: If an AP question asks for the order of an enzyme-catalyzed reaction when substrate concentration is much higher than enzyme concentration, the answer is almost always zero order.
5. AP-Style Concept Check★★★★☆⏱ 2 min
Common Pitfalls
Why: Students confuse lowering overall activation energy with changing the original reaction pathway
Why: Students mix up effect on reaction rate with effect on thermodynamics
Why: Students forget catalysts are regenerated, so they leave them on the reactant side
Why: Students generalize the 'not consumed' rule to all conditions, forgetting enzymes are proteins
Why: Students confuse chemical identity with physical phase
Why: Students forget the alternative mechanism has multiple elementary steps