Topic 17 · page 18 of 20
Two kinds of catalyst
Heterogeneous catalysts are in a different phase from the reactants and work on their surface. Homogeneous catalysts are in the same phase and work through an intermediate. Transition metals are good at both.
Phase decides the kind 17.2617.2717.30
A catalyst speeds up a reaction by giving it a different route with a lower activation energy. It is not used up, and it does not change the position of equilibrium (Topic 11).
The two kinds of catalyst are named by their phase relative to the reactants. A phase is a solid, a liquid, a gas or an aqueous solution. Two different solutions in water are the same phase.
| Heterogeneous | Homogeneous | |
|---|---|---|
| Phase | different from the reactants | same as the reactants |
| Where it works | on the surface | throughout the mixture |
| Mechanism | reactants adsorb, react, then products desorb | forms an intermediate, then is regenerated |
| Examples | V₂O₅(s) in the contact process; Pt/Rh(s) in a catalytic converter; Fe(s) in the Haber process; Ni(s) in the hydrogenation of alkenes | Fe²⁺(aq) with I⁻(aq) and S₂O₈²⁻(aq); Mn²⁺(aq) with MnO₄⁻(aq) and C₂O₄²⁻(aq) |
Worked exampleClassifying a catalyst
In the contact process SO₂(g) and O₂(g) react over solid V₂O₅.
- The reactants are gases. The catalyst is a solid. The phases are different.
- The catalyst is heterogeneous.
- The reaction takes place at the surface of the solid.
Why transition metals 17.26
Transition metals and their compounds are good catalysts for two reasons.
- Variable oxidation number. They can gain and lose electrons easily, so they can form an intermediate and then return to their original state. This is how homogeneous catalysts such as Fe²⁺ work, and how V₂O₅ works in the contact process.
- Surface binding. The metal atoms at a surface have 3d and 4s electrons and vacant orbitals that can bind molecules, which weakens their bonds. This is how heterogeneous catalysts such as Pt/Rh work.
Check yourself
- Define heterogeneous catalyst and give one example.
- Define homogeneous catalyst and give one example.
- Classify each catalyst: (a) solid iron in the Haber process, (b) aqueous Fe²⁺ in the reaction between I⁻(aq) and S₂O₈²⁻(aq), (c) solid platinum in a catalytic converter.
- Give two reasons why transition metals make good catalysts.
- State where a heterogeneous reaction happens.
Answers cover these until you have tried the questions
- A catalyst in a different phase from the reactants; the reaction occurs at its surface. Example: V₂O₅(s) in the contact process.
- A catalyst in the same phase as the reactants; the catalysed reaction proceeds via an intermediate. Example: Fe²⁺(aq) in the reaction of I⁻ with S₂O₈²⁻.
- (a) heterogeneous (solid catalyst, gaseous reactants); (b) homogeneous (all aqueous); (c) heterogeneous (solid catalyst, gaseous reactants).
- They have variable oxidation numbers, so they can form intermediates by gaining and losing electrons; and they can bind (adsorb) reactant molecules at their surface, weakening bonds.
- At the surface of the catalyst.