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Catalysts

Catalysts. Rates of Reactions. A Catalyst is… . A substance that alters (speeds up or slows down) the rate of a chemical reaction but is not consumed in the reaction. Positive & Negative. Positive catalysts = speed up a reaction Manganese dioxide, amylase

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Catalysts

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  1. Catalysts Rates of Reactions

  2. A Catalyst is… • A substance that alters (speeds up or slows down) the rate of a chemical reaction but is not consumed in the reaction

  3. Positive & Negative • Positive catalysts = speed up a reaction • Manganese dioxide, amylase • Negative catalysts (inhibitors) = slow down a reaction • Glycerine in H2O2, Calcium propionate to preserve bread

  4. A. Properties of Catalysts • Chemically unchanged • Specific • Lipase = fats • Pepsin = protein • Amylase = starch • Sucrase = sucrose • Small amounts effective • Equilibrium reached more quickly • Can be poisoned

  5. B. Types of Catalysis • Homogeneous Catalysis • Same Phase = no boundary between the reactants and catalyst • Eg. both liquids • Hetrogeneous Catalysis • Different phases = a boundary exists • Eg. A liquid and a solid • Autocatalysis • One of the products catalyses the reaction • Eg. Potassium permanganate & ammonium iron (II) sulfate (Mn2+)

  6. C. Mechanisms • 1. Intermediate Formation Theory • 2. The Surface Adsorption Theory

  7. C1. Intermediate Formation Theory • Reactant 1 + Catalyst → Intermediate compound • Intermediate compound + Reactant 2 → Product + Catalyst

  8. Collision Theory

  9. Activation Energy • Is the minimum amount of energy needed by colliding particles for a reaction to occur • Size of A.E. depends on the nature of reactants • A.E. determines the rate of reaction

  10. Maxwell – Boltzmann Distrubution

  11. Reaction Profile Diagram

  12. A = Reactants, B = Products

  13. Activation Energies & Rates • Low activation energy = fast rate of reaction • High activation energy = slow rate of reaction

  14. Effect of Catalyst on Reaction • Catalyst = lowers the activation energy (provides an alternative reaction pathway)

  15. Recap on Factors Affecting Rate of Reaction • Nature of Reactants : covalent = slow as bonds must be broken so have higher Activation Energy, ionic = fast • Particle Size : Smaller particle size = more surface area exposed = number of collisions increased = greater number of effective collisions = faster rate of reaction • Concentration : Higher concentration = number of collisions increased = greater number of effective collisions = faster rate of reaction • Temperature = number of high energy reactants increases = greater number of effective collisions = faster rate of reaction • Catalyst : Lowers Activation Energy by providing an alternative reaction pathway = faster rate of reaction

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