Catalysts in Action (College Board AP® Chemistry): Revision Note

Catalysts & Binding

• Some catalysts speed up a reaction by binding to the reactant(s)

  • The bound reactants are oriented more favorably or react with a lower activation energy, and the catalyst–reactant complex is a new reaction intermediate

  • Many enzymes work this way

• Catalysts are sometimes described as:

  • Homogeneous (in the same phase as the reactants)

  • Heterogeneous (in a different phase from the reactants)

• The AP course groups catalysis by how it works:

  • Binding (for example, enzymes)

  • Acid–base catalysis

  • Surface catalysis

• Heterogeneous catalysts are in a different phase to the reaction

  • For example, the use of platinum or palladium metals (solid) in organic reactions such as hydrogenation reactions

• Homogeneous catalysts exist in the same phase as the reactants

  • Homogeneous catalysts most often catalyze gaseous and aqueous reactions

  • For example, aqueous zinc chloride, ZnCl₂, is used to catalyze the organic reaction between hydrogen chloride and alcohol

Enzyme catalysis

• Enzymes are examples of biological catalysts that can speed up the rate of biological reactions in living organisms

  • Most enzymes are large protein molecules with molecular weights ranging from about 10,000 to about 1 million amu

  • Enzymes are very selective in the reactions they catalyze

  • Some are absolutely specific, operating for only one substance in only one reaction

• Enzyme molecules possess an active site

  • This is part of the molecule with a shape that allows a specific reactant molecule (substrate) to bind

  • The binding between the substrate and active site involves dipole-dipole attractions, hydrogen bonds, and dispersion forces

• Two models currently exist to explain how an enzyme and its substrate interact

  • Model 1:

    • The substrate molecule fits into the active site on the enzyme molecule, somewhat in the way a key fits into a lock

    • This results in the formation of an enzyme-substrate complex as a reaction intermediate

    • On binding to the enzyme, the substrate may have bonds weakened or new bonds formed that help yield the products

  • Model 2:

    • This is often called the induced fit model

    • This suggests that the active site of an enzyme changes its shape to fit its substrate

Enzyme catalysis models

Catalysts & Covalent Bonding

• Some heterogeneous catalysts can speed up the reaction rate through the formation of weak covalent bonds with the reactant molecules

• One example is the decomposition of N₂O on gold, the solid catalyst

N₂O (g) → N₂ (g) + ½ O₂ (g)

• In the catalyzed decomposition, N₂O is chemically adsorbed on the surface of the solid gold

  • A weak covalent bond is formed between the oxygen atom of an N₂O molecule and a gold atom on the surface

  • This weakens the bond joining nitrogen to oxygen, making it easier for the N₂O molecule to break apart

  • The process can be represented as follows:

Decomposition of N₂O on a gold catalyst

N≡N–O (g) + Au (s) → N≡N···O···Au (s) → N≡N (g) + O···Au (s)

• The equation shows the catalytic action of gold in the decomposition of dinitrogen oxide, N₂O

• A weak covalent bond, represented by the dotted lines, is formed between oxygen and the solid gold catalyst

• The oxygen atoms left on the surface then combine and leave as O₂ (g), giving the overall equation:

N₂O (g) → N₂ (g) + ½O₂ (g)

• Another example of catalysis involving covalent bond formation between a catalyst and the reactants is acid-base catalysis

  • In acid–base catalysis, a reactant or intermediate gains or loses a proton (H⁺). For example, H₃O⁺ donates a proton to the reactant (protonation), forming a more reactive intermediate

  • A later step gives the proton back, so the acid catalyst is regenerated

  • The hydrolysis of an ester (ester + water → carboxylic acid + alcohol), catalyzed by H₃O⁺ from hydrochloric acid, is a good example

Acid-catalyzed hydrolysis of esters

Surface Catalysis

• In industry, many reactions are catalyzed by the surfaces of solids

  • These reactions often involve gaseous reactants being adsorbed on the surface of a solid catalyst

  • Adsorption refers to the collection of one substance on the surface of another substance

Examples of surface catalysis

• Some of the more important examples of surface catalysis include:

  • The Contact Process - a multistep reaction for making sulfuric acid

2SO₂ (g) + O₂ (g) $\stackrel{{\mathrm{V}}_2{\mathrm{O}}_5\left(\mathrm{s}\right)}{\to }$ 2SO₃ (g)

• The Haber Process - for making ammonia

N₂ (g) + 3H₂ (g) $\stackrel{\mathrm{Fe}\left(\mathrm{s}\right)}{\to }$ 2NH₃ (g)

• Catalytic converters - for the removal of toxic / poisonous gases from car exhausts

2CO (g) + 2NO (g) $\stackrel{\mathrm{Pt}/\mathrm{Pd}/\mathrm{Rh}\left(\mathrm{s}\right)}{\to }$ 2CO₂ (g) + N₂ (g)

• Hydrogenation - for the hardening of vegetable oils

C₂H₄ (g) + H₂ (g) $\stackrel{\mathrm{Ni}/\mathrm{Pt}/\mathrm{Pd}\left(\mathrm{s}\right)}{\to }$ C₂H₆ (g)

How surface catalysis works

• Surface catalysis happens in three main stages:

  • Adsorption: the reactants bind to the surface (physically, by weak forces, or chemically, by covalent bonds), which weakens bonds in the reactants

  • Reaction: the bound intermediates react with each other in new elementary steps on the surface

  • Desorption: the products leave the surface