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Kinetics

Kinetics. What is Kinetics ?. Analysis of reaction mechanisms on the molecular scale Derivation of rate expressions Design and analysis of experiments to test rate equations and derive kinetic parameters Theoretical prediction of rate constants How can we improve it?. Elementary steps.

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Kinetics

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  1. Kinetics

  2. What is Kinetics ? • Analysis of reaction mechanisms on the molecular scale • Derivation of rate expressions • Design and analysis of experiments to test rate equations and derive kinetic parameters • Theoretical prediction of rate constants • How can we improve it?

  3. Elementary steps Discuss examples of elementary steps! A reaction is elementary if it cannot be split up in further steps. An elementary reaction proceeds exactly as expressed by the reaction equation. A reaction mechanism is a sequence of elementary steps.

  4. The Rate Equation

  5. Free Energy and Entropy Equilibrium: • G = 0 • Free energy: minimum • Entropy: maximum

  6. Free Energy and Entropy Reaction: G :  0 • free energy: > minimum • entropy: < maximum Thermodynamics Kinetics

  7. Chemical Equilibrium The Chemical Potential

  8. Thermodynamics Table 2.2 Thermodynamic Data for Important Catalytic Reactions Reaction Ho (kJ/mol) Go 298(K) NH3 ½ N2 + 3/2 H2 + 45.9 + 16.4 ½N2 + 3/2 H2 NH3 - 45.9 - 16.4 N2 + 3 H2 2 NH3* - 91.9* - 32.8 2 NO  N2 + O2* - 182.6 -175.2 CH4 + H2O  CO + 3 H2 +205.9 +142.0 CH4 + ½ O2 CO + 2 H2 - 35.9 - 86.8 CH4 + 2 O2 CO2 + 2 H2O - 802.6 - 801.0 CH4 + ½ O2 CH3OH - 275.6 - 111.77 CO2 + 3H2  CH3OH + H2O - 49.3 + 3.5 CO + 2H2 CH3OH - 90.5 - 25.1 CO + H2O  CO2 + H2 - 41.2 - 28.6 * this per two mole of NH3 or NO. Data taken from the JANAF Thermodynamic Tables.

  9. EX: Ammonia Synthesis N2 + 3 H2 = 2 NH3 G° = -32.8 kJ/mol H° = -91.9 kJ/mol

  10. inlet inlet C A D B outlet inlet The ammonia reactor N2+3 H2 = 2 NH3 G° = -32.8 kJ/mol H° = -91.9 kJ/mol

  11. Power Rate Laws Parametrization of the rate: Reaction Order Example:

  12. Equilibrium Constant and Free Energy - G° / RT - (H°-T S°) / RT K = e = e G° = standard Gibbs energy of reaction= H° - TS° Van 't Hoff equation: ` H° = standard reaction enthalpy S° = Standard reaction entropy

  13. Temperature dependence of the rate constant Arrhenius equation: k = v e - Eact / RT Eact = activation energy v = preexponential factor E Eact reactants products reaction parameter

  14. Potential Energy Representation:dissociation - association

  15. Temperature Dependence of the Rate d [AB] r = = k [A] [B] d t Arrhenius: - Eact /RT k = v e ln k = ln v Eact / RT ln k 1000 / T

  16. Apparent Activation Energy Example:

  17. Heterogeneous Catalysis Adsorption Reaction Desorption

  18. Basic surface interactions CO2 CO Molecular adsorption Dissociativ adsorption O Diffusion Reaction Desorption • Reactions take place on the metal surface

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