Catalysis

Catalysis – Complete Study Notes

A catalyst is a substance that increases the rate of a chemical reaction without itself being permanently chemically changed or consumed in the process. The study of catalysis is essential for industrial chemistry — nearly 90% of all industrial chemical processes use catalysts. Catalysis was first explained systematically by Jöns Jacob Berzelius in 1835.

1. How Does a Catalyst Work?

A catalyst works by providing an alternative reaction pathway with a lower activation energy (Ea). It does NOT change the energy of the reactants or products, and it does NOT alter the enthalpy change (ΔH) or the equilibrium constant (K) of the reaction. It only affects the rate at which equilibrium is reached.

  • More molecules now have sufficient energy (≥ new lower Ea) to react.
  • The catalyst is regenerated at the end of the reaction (not consumed).
  • A catalyst lowers Ea for BOTH the forward and the reverse reactions equally.

2. Types of Catalysis

A. Homogeneous Catalysis

The catalyst and reactants are in the same phase (both liquid or both gas).

  • Example: Oxidation of SO₂ to SO₃ using NO gas as catalyst (both are gases): 2SO₂ + O₂ → 2SO₃ (in presence of NO).
  • Hydrolysis of esters in presence of H⁺ ions (both in aqueous phase).
  • Formation of intermediates: The catalyst reacts with the reactant to form an intermediate, which then breaks down to yield the product and regenerate the catalyst.

B. Heterogeneous Catalysis

The catalyst and reactants are in different phases. Most commonly, a solid catalyst is used with liquid or gaseous reactants.

  • Haber Process: N₂(g) + 3H₂(g) → 2NH₃(g) using solid iron (Fe) catalyst with Al₂O₃ as promoter and K₂O as activator.
  • Contact Process (H₂SO₄): 2SO₂(g) + O₂(g) → 2SO₃(g) using vanadium pentoxide (V₂O₅) catalyst.
  • Catalytic Converter (Cars): Platinum-Rhodium gauze converts CO, NOₓ, and unburned hydrocarbons into CO₂, N₂, and water.
  • Nickel in Hydrogenation: Vegetable oils (unsaturated) are converted to solid fats (saturated) using nickel catalyst: C=C + H₂ → C-C.
  • The mechanism involves Adsorption Theory: reactant molecules adsorb onto the catalyst surface, bonds weaken, reaction occurs, and products desorb.

C. Enzyme Catalysis (Biochemical Catalysis)

Enzymes are biological catalysts made of proteins. They are highly specific — each enzyme catalyzes only one specific reaction. They work best at an optimum temperature (around 37°C for most human enzymes) and an optimum pH.

  • Lock and Key Model: The substrate (reactant) fits exactly into the active site of the enzyme, like a key fits a lock. This explains enzyme specificity.
  • Induced Fit Model: The active site is flexible and adjusts its shape to fit the substrate upon binding.
  • Examples: Pepsin digests proteins in the stomach (works at pH 2). Amylase breaks down starch in saliva (pH 7.4). Urease decomposes urea: (NH₂)₂CO + H₂O → 2NH₃ + CO₂.

3. Promoters and Poisons

  • Promoter: A substance added along with the catalyst to enhance its activity. Example: Al₂O₃ and K₂O are promoters in the Haber process. They do not act as catalysts themselves.
  • Catalyst Poison: A substance that reduces the activity of a catalyst. Example: In Haber process, traces of CO, H₂S, or As₂O₃ poison the iron catalyst. In enzyme reactions, heavy metals (Pb²⁺, Hg²⁺) poison enzymes by binding to active sites.

4. Autocatalysis

A reaction is said to be autocatalytic when one of its own products acts as a catalyst. Example: Oxidation of oxalic acid by KMnO₄ — the Mn²⁺ ions produced catalyze the reaction. The reaction starts slowly and accelerates as more product is formed.

5. Shape-Selective Catalysis (Zeolites)

Zeolites are microporous aluminosilicate minerals that act as shape-selective catalysts. Their uniform pore sizes allow only molecules of specific sizes and shapes to enter and react. ZSM-5 zeolite is used industrially to convert alcohol into gasoline-range hydrocarbons.

Key Exam Tips

  • Catalyst does NOT change K (equilibrium constant), ΔH, or the final yield at equilibrium.
  • Catalyst provides an alternative path with LOWER Ea — it lowers Ea for both forward and reverse.
  • Finely divided catalysts are more effective (more surface area).
  • Haber: Fe catalyst | Contact: V₂O₅ | Ostwald (HNO₃): Pt-Rh gauze | Hydrogenation: Ni.