Catalysis Study Guide

📖 Core Concepts Catalysis – a substance (catalyst) that increases the reaction rate without being consumed. Active site – the specific atom or crystal face on a solid where reactants adsorb and react. Turnover Number (TON) – total product molecules formed per catalyst molecule; measures productivity. Turnover Frequency (TOF) – TON divided by time; indicates how fast a catalyst operates. Rate‑determining step (RDS) – the slowest elementary step that governs the overall catalytic rate. Thermodynamic equilibrium – unchanged by a catalyst; only the path to equilibrium is faster. Homogeneous vs. Heterogeneous – same phase as reactants (usually liquid) vs. different phase (solid). 📌 Must Remember Catalysts lower the activation energy → faster rate, do not alter the equilibrium constant. TON = total product molecules / catalyst molecules; TOF = TON / time. Induction period occurs when a precatalyst must be converted to the active catalyst. Surface area ∝ activity for heterogeneous catalysts; smaller particles = more active sites. Poisoning blocks active sites (e.g., coke formation, Lindlar catalyst). Promoters improve dispersion, prevent coke, or tweak selectivity without being catalysts themselves. 🔄 Key Processes Catalytic Cycle Reactant adsorbs on active site → forms intermediate → converts to product → catalyst regenerated. Heterogeneous Reaction on a Solid Diffusion of reactant → adsorption on active site → surface reaction → desorption of product. Induction Period Precatalyst → activation step (often slow) → formation of active catalyst → steady‑state turnover. Partial Poisoning (Lindlar catalyst) Add poison → decrease hydrogenation activity → stop at alkene stage. 🔍 Key Comparisons Homogeneous vs. Heterogeneous Phase: same vs. different. Contact: molecular‑level mixing vs. surface adsorption. Recovery: easier separation (heterogeneous) vs. often requires purification (homogeneous). Inhibitor vs. Poison Reversibility: inhibitor can be reversible; poison is usually irreversible. Organocatalysis vs. Metal Catalysis Composition: small organic molecules vs. transition‑metal complexes. Typical interactions: covalent (proline) or non‑covalent (thiourea) vs. coordination chemistry. ⚠️ Common Misunderstandings “Catalysts change equilibrium.” They only speed the approach to equilibrium; the ΔG° and K remain the same. “More catalyst always means faster reaction.” After active sites are saturated, additional catalyst gives no rate gain. “All poisons are bad.” Controlled poisoning (e.g., Lindlar) is a deliberate selectivity tool. 🧠 Mental Models / Intuition “Energy shortcut” – imagine the catalyst as a lower hill (activation barrier) that the reactants can climb more easily. “Assembly line” – each catalyst molecule is a worker that repeatedly processes reactants; TON counts how many jobs each worker completes. 🚩 Exceptions & Edge Cases Catalytic inhibition can be reversible (e.g., competitive adsorption) – the reaction can recover when inhibitor is removed. Partial poisoning can increase selectivity rather than just decrease activity. Nanostructured supports can introduce quantum‑size effects that alter activity beyond simple surface‑area arguments. 📍 When to Use Which Choose homogeneous when you need fine control of stereochemistry or when reactants are all in solution. Choose heterogeneous for large‑scale, easy separation processes (e.g., catalytic cracking, automotive converters). Use organocatalysts for metal‑sensitive substrates or when avoiding metal contamination is critical. Apply photocatalysis when light can provide the activation energy (e.g., water splitting). Deploy electrocatalysis for electrochemical transformations (fuel cells, electrolyzers). 👀 Patterns to Recognize Rate‑determining step often involves the highest activation barrier – look for the slowest elementary step in a mechanism diagram. Surface‑area dependence: reaction rate scales with particle size → smaller particles → higher rates. Selectivity changes when a poison is present: partial hydrogenations, selective oxidations. 🗂️ Exam Traps “Catalyst appears in the overall reaction equation.” Remember the catalyst is not a reactant or product; it cancels out. Confusing TOF with overall reaction rate. TOF is per‑catalyst‑site; the observed rate also depends on catalyst amount. Assuming a catalyst changes ΔG° – a common distractor; equilibrium constant stays the same. Choosing homogeneous over heterogeneous because “all catalysts are liquids.” Over‑generalization; many industrial catalysts are solids. Identifying any inhibitor as a poison. Only irreversible deactivation = poison; reversible inhibitors are distinct.