9701 · 26.2
Homogeneous and heterogeneous catalysts flashcards
Revision flashcards for Cambridge 9701 Homogeneous and heterogeneous catalysts (syllabus 26.2). 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 reaction pathway with a lower activation energy, without being used up in the overall 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 in aqueous solution.
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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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Describe the general mechanism of homogeneous catalysis.
The catalyst reacts with one of the reactants to form an intermediate. This intermediate then reacts with the other reactant to form the products and regenerate the catalyst.
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Describe the mechanism of heterogeneous catalysis.
1. Adsorption: Reactant molecules bind to active sites on the catalyst's surface. 2. Reaction: The reaction occurs between adsorbed reactants. 3. Desorption: Product molecules leave the surface.
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Give an example of homogeneous catalysis with equations.
The reaction between S₂O₈²⁻ and I⁻ ions, catalysed by Fe²⁺(aq). Step 1: S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq) Step 2: 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)
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What is an active site?
A specific location on the surface of a heterogeneous catalyst where reactant molecules can bind (adsorb) and react.
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What is catalyst poisoning?
The deactivation of a catalyst by a substance (a poison) that binds strongly and irreversibly to the active sites, blocking them from reactants. Example: Sulfur poisoning the iron catalyst in the Haber process.
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What is sintering?
The process where small catalyst particles fuse into larger ones at high temperatures, causing a decrease in the total surface area and therefore a reduction in catalytic activity.
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Why are transition metals and their compounds effective catalysts?
They have variable oxidation states (allowing them to act as electron carriers in homogeneous catalysis) and can provide a surface for adsorption (in heterogeneous catalysis).
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What is autocatalysis?
A reaction in which one of the products acts as a catalyst for the reaction. The reaction starts slowly and speeds up as the concentration of the product/catalyst increases. Example: Mn²⁺ in the oxidation of ethanedioic acid by KMnO₄.