9701 · 8.3
Homogeneous and heterogeneous catalysts flashcards
Revision flashcards for Cambridge 9701 Homogeneous and heterogeneous catalysts (syllabus 8.3). Flip, recall, then mark a real past-paper question.
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What is a catalyst?
A substance that increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy, without being chemically changed at the end of the reaction.
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What is a homogeneous catalyst?
A catalyst that is in the same physical state (phase) as the reactants. For example, an aqueous catalyst for a reaction between aqueous ions.
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What is a heterogeneous catalyst?
A catalyst that is in a different physical state (phase) from the reactants. For example, a solid catalyst for a reaction between gases.
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How does a homogeneous catalyst typically work?
It forms an intermediate compound with one of the reactants. This intermediate then reacts to form the products and regenerate the catalyst. This provides a multi-step pathway where each step has a lower activation energy than the uncatalysed single step.
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Describe the mechanism of heterogeneous catalysis.
1. Adsorption: Reactant molecules bind to active sites on the catalyst surface. 2. Reaction: Bonds within the reactants are weakened and break, and new bonds form to make the product. 3. Desorption: The product molecules detach from the surface.
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Give an example of heterogeneous catalysis.
The Haber process: Iron (Fe) catalyst is used for the synthesis of ammonia from nitrogen and hydrogen. N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
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Give an example of homogeneous catalysis.
Fe²⁺(aq) ions catalysing the reaction between peroxodisulfate(VI) ions (S₂O₈²⁻) and iodide ions (I⁻). Both reactants and the catalyst are in the aqueous phase.
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What is the catalyst in the Contact process?
Vanadium(V) oxide, V₂O₅, a solid heterogeneous catalyst used for the oxidation of SO₂ to SO₃.
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How does a catalyst affect the enthalpy change ($\Delta H$) of a reaction?
A catalyst has no effect on the enthalpy change ($\Delta H$). The energy difference between reactants and products remains the same.
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How does a catalyst affect the position of equilibrium?
A catalyst does not affect the position of equilibrium or the value of the equilibrium constant ($K_c$). It increases the rate of the forward and reverse reactions equally, so equilibrium is reached faster.
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What is autocatalysis?
A type of catalysis where one of the products of the reaction acts as a catalyst for that same reaction. An example is Mn²⁺ ions in the titration of ethanedioate with manganate(VII).
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Why are transition metals and their compounds good catalysts?
They have variable oxidation states, which allows them to form unstable intermediates (homogeneous catalysis), and they can provide active sites on their surface for adsorption (heterogeneous catalysis).