Catalysts
Catalysts (A2)
- Catalysts are homogeneous or heterogeneous.
- Each works in a characteristic way.
- Both come back unchanged at the end.
Heterogeneous catalysts
- In a different physical state from the reactants (usually a solid with gases). It works in three stages:
- adsorption: reactant molecules stick to the surface.
- their bonds are weakened, so they react more easily.
- desorption: the products leave the surface.
- Examples: iron in the Haber process; Pt/Pd/Rh in a catalytic converter.
Practice
A heterogeneous catalyst:
Heterogeneous = different state (e.g. solid + gases); reactants adsorb, react, then products desorb.
Practice
Put the stages of heterogeneous catalysis in order.
Reactants adsorb, their weakened bonds react, then the products desorb.
Homogeneous catalysts
- In the same physical state as the reactants.
- It is used up in one step and reformed in a later step, so it returns unchanged.
- Examples: NOₓ helping oxidise atmospheric $\text{SO}_2$; $\text{Fe}^{2+}/\text{Fe}^{3+}$ speeding the $\text{I}^-$ + $\text{S}_2\text{O}_8^{2-}$ reaction.
Practice
A homogeneous catalyst:
Homogeneous = same state; it reacts in one step and is regenerated in a later step, so it returns unchanged.
Practice
Which is a homogeneous catalyst?
Aqueous Fe²⁺/Fe³⁺ ions are in the same state as the reacting ions — homogeneous; solid Fe and Pt are heterogeneous.
You've got it
Key idea
- heterogeneous (different state): adsorption → weakened bonds → desorption (e.g. Fe in Haber, Pt/Pd/Rh converter)
- homogeneous (same state): used up then reformed, so unchanged (e.g. $\text{Fe}^{2+}/\text{Fe}^{3+}$)