Homogeneous & Heterogeneous Catalysts (Cambridge (CIE) A Level Chemistry): Flashcards

A homogeneous catalyst is a catalyst that is in the same phase as the reactants in the reaction mixture.

1. Adsorption — reactant molecules diffuse to and bond to the catalyst surface (chemisorption weakens bonds within reactants).

2. Reaction — reactant atoms react with each other on the surface.

3. Desorption — product molecules break away from the surface and diffuse away.

False.

Iron is a heterogeneous catalyst. It is in the solid phase while the reactants N₂ (g) and H₂ (g) are in the gas phase, so they are in different phases.

S₂O₈^2- and I^- are both negatively charged, so direct collision requires overcoming large repulsive forces. Fe³⁺ reacts with each ion separately in two redox steps, avoiding repulsion and significantly lowering the activation energy.

In homogeneous catalysis of the S₂O₈^2- / I^- reaction:

Step 1: 2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + I₂ (aq)

Step 2: 2Fe²⁺ (aq) + S₂O₈^2- (aq) → 2Fe³⁺ (aq) + 2SO₄^2- (aq)

True.

NO₂ catalyses the oxidation of SO₂ to SO₃ and is then regenerated as NO is re-oxidised back to NO₂ by O₂. It is in the same gas phase as the reactants.

Platinum, palladium and rhodium are used. They convert nitrogen oxides (NO_x) into harmless nitrogen gas (N₂) and carbon monoxide (CO) into carbon dioxide (CO₂) by adsorption on the metal surface.

A heterogeneous catalyst is a catalyst that is in a different phase to the reactants in the reaction mixture.

The transition metal ion is first reduced (e.g. Fe³⁺ → Fe²⁺) by one reactant, then oxidised back (Fe²⁺ → Fe³⁺) by the other reactant. This allows it to be regenerated at the end and used repeatedly as a catalyst.