9701 · 26.2
Homogeneous and heterogeneous catalysts
Catalysts are chemical matchmakers that speed up reactions without being consumed. They do this by offering a new, easier reaction route, and are classified as either homogeneous (same phase) or heterogeneous (different phase).
Need to know
What you need to know
- **Haber Process:** Synthesis of ammonia. $N₂(g) + 3H₂(g) \rightleftharpoons 2NH₃(g)$. Catalyst: Iron (Fe) solid.
- **Contact Process:** Manufacture of sulfuric acid. $2SO₂(g) + O₂(g) \rightleftharpoons 2SO₃(g)$. Catalyst: Vanadium(V) oxide (V₂O₅) solid.
- **Catalytic Converters:** In car exhausts. $2CO(g) + 2NO(g) \rightarrow 2CO₂(g) + N₂(g)$. Catalysts: Platinum (Pt), Palladium (Pd), and Rhodium (Rh) solids coated on a ceramic honeycomb.
Explanation
Catalysts: The Reaction Shortcut
- A catalyst provides an alternative reaction mechanism with a lower activation energy ($E_a$), increasing the rate of reaction. It is not consumed in the overall process.
- Homogeneous catalysts exist in the same physical state (phase) as the reactants, for example, aqueous Fe²⁺ ions catalysing a reaction between aqueous ions.
- Heterogeneous catalysts are in a different phase to the reactants. Typically, a solid catalyst provides a surface with active sites for gaseous or liquid reactants to adsorb and react, as seen with iron in the Haber process or V₂O₅ in the Contact process.
- The efficiency of heterogeneous catalysts can be reduced over time. Poisoning occurs when impurities bind irreversibly to active sites, while sintering is the loss of surface area due to particle fusion at high temperatures.