Kinetics
Catalysis
Speeding up reactions without being consumed — alternative pathways with lower activation energy
A catalyst is a substance that speeds up a reaction without being consumed. Provides alternative reaction pathway with lower activation energy. Doesn't change equilibrium constant K — only how fast equilibrium reached. Three types: (1) Homogeneous — catalyst in same phase as reactants. (2) Heterogeneous — different phase (typically solid + liquid/gas). (3) Enzyme — biological catalysts (proteins). Industrial: Haber process (NH₃, Fe catalyst), Contact process (H₂SO₄, V₂O₅), catalytic converters (CO/NO_x, Pt/Pd). Drives modern chemistry; >90% of chemical products use catalysts.
- DefinitionSpeeds up reaction; not consumed
- MechanismLowers activation energy (alternative pathway)
- Effect on KNone (only affects rate)
- TypesHomogeneous, heterogeneous, enzyme
- Industrial scale>90% of chemicals produced with catalysts
- ExamplesPt (auto), Fe (Haber), V₂O₅ (Contact)
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Why catalysis matters
- Industrial chemistry. Most products use catalysts.
- Energy. Fuel cells, hydrogen production.
- Environment. Catalytic converters reduce pollution.
- Pharmaceuticals. Asymmetric synthesis.
- Biology. All enzymes are catalysts.
- Materials. Polymerization control.
- Green chemistry. Reduce energy, waste.
Common misconceptions
- Catalyst consumed. Returns to original form.
- Catalyst increases yield. Same equilibrium yield; just faster.
- All catalysts are metals. Many types — acids, enzymes, gases.
- One catalyst works for all reactions. Highly specific.
- Catalyst always in solid form. Various phases.
- Catalysts solve all problems. Don't change thermodynamics.
Frequently asked questions
How does a catalyst work?
Provides lower-energy reaction pathway. Original reaction has Ea_1 (high). Catalyst forms intermediate complex with reactant; intermediate goes to products. Each step has Ea < Ea_1. Lower activation energy → more molecules have enough energy → faster reaction. Catalyst returns to original form at end — not consumed.
Does catalyst affect equilibrium?
No. Catalysts speed up forward AND reverse reactions equally. K (equilibrium constant) unchanged. They reduce time to reach equilibrium. Important: industrial implications — use catalyst to make reaction fast enough to be practical, but yield (and thus product) determined by ΔG of reaction.
What's heterogeneous catalysis?
Catalyst in different phase from reactants. Typically: solid catalyst + gas/liquid reactants. Reactants adsorb onto catalyst surface; reaction occurs there; products desorb. Surface chemistry critical. Examples: Pt + H₂ + O₂ (fuel cells); Fe + N₂ + H₂ (Haber); zeolites + petroleum (cracking). Surface area maximized (porous, supported particles).
What's homogeneous catalysis?
Catalyst in same phase as reactants (typically all in solution). Examples: H⁺ in esterification; transition metal complexes in industrial processes. Advantages: better kinetics; tunable. Disadvantages: separating catalyst from products. Used in: pharmaceuticals, fine chemicals, polymerization.
How do enzymes catalyze?
Enzymes are protein catalysts. Specific 3D structure binds substrate at active site (lock-and-key or induced fit). Lower Ea: orient substrates correctly, stabilize transition state, donate/accept protons. Highly specific and efficient: enzymes can speed up reactions by 10⁶-10¹⁷×. Often work at body T and physiological pH. Examples: catalase (H₂O₂ → H₂O + O₂), DNA polymerase, lactase.
What's a catalytic converter?
Heterogeneous catalyst in vehicle exhaust system. Converts: CO → CO₂; NO_x → N₂; unburnt hydrocarbons → CO₂ + H₂O. Catalyst: Pt, Pd, Rh on ceramic honeycomb. Operates at 400-800°C. Reduces auto pollutants by ~90%. Critical for air quality; mandatory in most countries since 1970s.
What about poisoning?
Catalysts can be deactivated. Causes: (1) Coking — carbon buildup on surface. (2) Sintering — small particles fuse, lose surface area. (3) Poisoning — irreversibly bound species (e.g., S poisoning Pt). Industrial: regular regeneration. Lead in gasoline poisons catalytic converters — banned partly for this reason.