Answers to questions on Catalysts

1. (a) They are able to reversibly adsorb large amounts of gases onto their surface.

(b) It increases the surface area so makes the catalyst more efficient.

(c) 2C₈H₁₈(g) + 25O₂(g) → 16CO₂(g) + 18H₂O(g)

(d) 2NO(g) → N₂(g) + O₂(g)
2NO₂(g) → N₂(g) + 2O₂(g)

(e) If there is an excess of air more nitrogen and oxygen combine to form NO_x. If there is an excess of fuel more carbon monoxide and particulate carbon is formed. In both cases the engine is less efficient.

2. Zeolites themselves are relatively inert and have melting points in the region of about 1000 ^oC. They do not dissolve in water or other polar solvents and do not undergo oxidation in the air so they are very stable substances. They are good catalysts (e.g. in drug mIB Docs (2) Teamfacture and catalytic cracking of hydrocarbons) due to their porous cage-like structure. Reactant atoms and molecules become trapped in the pores enabling reactions to occur more readily. Because the pores in particular zeolites have fixed shapes and sizes zeolites are particularly selective catalysts for specific reactions.

3. (a) (COOH)₂ → 2CO₂(g) + 2H⁺(aq) + 2e⁻

(b) MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l)

(c) 2MnO₄⁻(aq) + 5(COOH)₂ + 6H⁺(aq) → 2Mn²⁺(aq) + 5CO₂(g) + 8H₂O(l)

(d) The reactant MnO₄⁻(aq) does not act as a catalyst but the product, which is a transition metal ion, does acts as a catalyst. Once some of the product is formed it is no longer necessary to add heat as the catalytic pathway has a lower activation energy so the more of the reactant molecules will possess the minimum energy necessary to react at room temperature. (This is known as autocatalysis)

(e) Once all the oxalic acid has reacted the purple manganate(VII) ions will no longer be reduced to colourless manganese(II) ions and so the solution will remain coloured.

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