IC-1/18 Lecture Kinetics

IC-2/18 Lecture 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?

IC-3/18 Lecture 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.

IC-4/18 Lecture The Rate Equation

IC-5/18 Lecture Free Energy and Entropy Equilibrium:  G = 0 Free energy: minimum Entropy: maximum

IC-6/18 Lecture Reaction:  G :  0 free energy: > minimum entropy: < maximum Free Energy and Entropy Kinetics Thermodynamics

IC-7/18 Lecture Chemical Equilibrium The Chemical Potential

IC-8/18 Lecture Thermodynamics Table 2.2 Thermodynamic Data for Important Catalytic Reactions Reaction  H o (kJ/mol)  G o 298(K) NH 3  ½ N 2 + 3/2 H ½N 2 + 3/2 H 2  NH N H 2  2 NH 3 * * NO  N 2 + O 2 * CH 4 + H 2 O  CO + 3 H CH 4 + ½ O 2  CO + 2 H CH O 2  CO H 2 O CH 4 + ½ O 2  CH 3 OH CO 2 + 3H 2  CH 3 OH + H 2 O CO + 2H 2  CH 3 OH CO + H 2 O  CO 2 + H * this per two mole of NH 3 or NO. Data taken from the JANAF Thermodynamic Tables.

IC-9/18 Lecture EX: Ammonia Synthesis N H 2 = 2 NH 3 G° = kJ/mol H° = kJ/mol

IC-10/18 Lecture inlet outlet A B D C The ammonia reactor N 2 +3 H 2 = 2 NH 3  G° = kJ/mol  H° = kJ/mol

IC-11/18 Lecture Power Rate Laws Parametrization of the rate: Reaction Order Example:

IC-12/18 Lecture Equilibrium Constant and Free Energy Van 't Hoff equation: `  H° = standard reaction enthalpy  S° = Standard reaction entropy  G° = standard Gibbs energy of reaction=  H° - T  S° K = e -  G° / RT = e - (  H°-T  S°) / RT

IC-13/18 Lecture Temperature dependence of the rate constant - E act / RT E act reaction parameter E products reactants E act = activation energy v = preexponential factor Arrhenius equation: k = v e

IC-14/18 Lecture Potential Energy Representation: dissociation - association

IC-15/18 Lecture Temperature Dependence of the Rate r = = k [A] [B] d [AB] d t 1000 / T ln k ln k = ln v E act / RT k = v e - E act /RT Arrhenius:

IC-16/18 Lecture Apparent Activation Energy Example:

IC-17/18 Lecture Heterogeneous Catalysis Adsorption Reaction Desorption

IC-18/18 Lecture Basic surface interactions Reactions take place on the metal surface Desorption Reaction Molecular adsorption Dissociativ adsorption Diffusion CO CO 2 O