Covers what a catalyst is and how it speeds a reaction by lowering activation energy without changing ΔG or equilibrium, then distinguishes catalysts from promoters and inhibitors. It contrasts homogeneous and heterogeneous catalysis with examples, explains autocatalysis using the oxalic acid and permanganate reaction, and reviews factors such as temperature, pressure, solvent, dispersion and poisons. It closes with biocatalysts such as enzymes and ribozymes, plus formulas for turnover number, turnover frequency and Michaelis–Menten kinetics.
Topic 9 of 42 in the MU Sofia syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 12 sections, about 999 words (7 min of reading), with 14 flashcards.
In this note
Catalysts
Mechanism of Catalyst Operation:
Homogeneous and heterogeneous catalysis Mechanism of catalytic reactions
Autocatalysis
Factors affecting the catalytic process
General concept for biocatalysis
High-Yield Points (Catalysis & Biocatalysis)
General Formula (Catalytic Rate Enhancement)
Turnover Number (TON)
Turnover Frequency (TOF)
Michaelis–Menten (for Enzyme Catalysis)
Catalyst Types (extra)
Catalysts
Definition:
A substance that increases the rate of a chemical reaction.
Remains unchanged qualitatively and quantitatively after the reaction.
Does not affect the overall change in Gibbs energy (ΔG) of the reaction.
Function:
Lowers the activation energy of the reaction, making it easier for molecules to react.
Acts like a matchmaker for reacting molecules, accelerating the reaction.
Characteristics:
Not consumed in the reaction.
Can be biological (enzymes) or synthetic materials.
Plays a crucial role in many industrial processes and biological functions.
Examples:
Platinum catalyst in car catalytic converters.
Enzyme lactase for digesting lactose.
Mechanism of Catalyst Operation:
Catalysts vs. Promoters vs. Inhibitors:
Catalysts: Lower the activation energy of a reaction, increasing the rate without being consumed.
Promoters: Similar to catalysts, they accelerate reactions but are consumed during the process.
Inhibitors: Decrease the reaction rate by increasing the activation energy or hindering interactions between reactants.
Mechanism of Catalysis:
Catalysts provide an alternative reaction pathway with a lower activation energy compared to the uncatalyzed reaction.
This alternative pathway involves intermediate steps that are more energetically favourable, allowing the overall reaction to proceed faster.
The catalyst itself undergoes temporary changes during these intermediate steps but ultimately returns to its original form by the end of the reaction cycle.
Types of Catalysis:
Homogeneous: Catalyst and reactants are in the same phase (e.g., liquid or gas).
Heterogeneous: Catalyst and reactants are in different phases (e.g., solid catalyst and liquid reactants). The reaction occurs at the interface between phases.
Autocatalysis: In autocatalysis, the reaction is catalysed by one of its products. One of the simplest examples of this is in the oxidation of a solution of ethanedioic acid (oxalic acid) by an acidified solution of potassium manganate(VII) (potassium permanganate).
Intramolecular: A functional group within the reactant molecule itself facilitates the reaction by bringing reacting atoms closer or stabilizing intermediates.
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