Catalysts
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State what is meant by a heterogeneous catalyst.
the catalyst is in a different phase from the reactants
State what is meant by a homogenous catalyst.
→ same phase from reactants → usually form intermediate species
Describe how a heterogenous catalyst works.
→ reactants bind to active sites on surface of catalyst (adsorption) → increase in frequency of collisions → bonds in reactants are weakened & break → new bonds are formed between reactants → catalyst releases product, bonds between catalyst & product becomes weak (desorption) → product leaves active site
Explain the effects of a poisoned catalyst.
→ impurities strongly adsorb to surface → active site is blocked → catalyst is less efficience
Give three examples of a heterogenous catalyst and the processes they’re involved in.
V₂O₅ → contact process Fe → Haber process Cr₂O₃ → manufacture of methanol
Give the two equations of the catalyst involved in the contact process.
V₂O₅ + SO₂ → V₂O₄ + SO₃ V₂O₄ + 0.5O₂ → V₂O₅
Explain why leaded petrol cannot be used in cars fitted with a catalytic converter.
Lead strongly adsorbs onto the surface of the catalyst.
Explain why the uncatalysed between iodide and persulfate ions is slow.
→ both ions are negative → very few collisions due to repulsion → high activation energy
Give the two equations for the catalysed reaction between iodide and persulfate ions. (catalysed by Fe2+)
2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻ 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
Give an example of an autocatalysis reaction.
Titration reaction between manganate and ethanedioate ions
Explain the concentration time graph for an autocatalysed reaction.
→ initially, rate is slow as reaction is uncatalysed (high Ea as both ions are negative) → rate increases as reaction is catalysed → rate decreases as concentration of reactants decrease
State which species acts as the catalyst during the autucatalysis of manganate and ethanedioate ions.
Mn²⁺
Give the two equations for the catalysed reaction of manganate with ethanedioate ions.
Step 1: 4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O Step 2: 2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂
Suggest how poisoning of a catalyst, used in an industrial process, can be minimised.
purify reactants
Give two reasons why the use of a spectrometer is the most appropriate method for measuring concentration of coloured ions.
→ rapid process → easy to get many readings
Explain how Mn2+ acts as an autocatalyst using equations.
Mn²⁺ + MnO₄⁻ → Mn³⁺ Mn³⁺ + C₂O₄²⁻ → Mn²⁺ Ea is lowered as opposite ions attract.