Lecture31A - Lecture 31 1 Kinetics, Rate Laws (Chapter...

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Unformatted text preview: Lecture 31 1 Kinetics, Rate Laws (Chapter 28.1-4) Kinetics (third branch of P Chem) isnt quite like Quantum (with its postulates) or Thermodynamics (with its 3 laws). There are no set laws or postulates that govern kinetics from first principles. Why? Reaction dynamics depend on: 1) Potential energy surface (get from solving the electronic Hamiltonian) 2) Thermal effects (Thermodynamics), i.e. the kinetic energy needed to surmount a barrier to reaction. 3) Quantum mechanical tunneling: even when barriers exist the reaction is still possible. Bottom line: we are still trying to understand from first principles simple reactions like F+H 2 ! This field is still going strong. The first step is to use phenomenological observations (somewhat similar to the start of Thermodynamics). Lecture 31 2 Lets turn to our standard reaction A B Y Z v A v B v Y v Z + + ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 A A A Y Y Y B B B Z Z Z n t n v t n t n v t n t n v t n t n v t =- = + =- = + ( 29 Again t is the extent of reaction (with explicit time dependence) ( 29 ( 29 products, reactants I I dn t d t v dt dt = + =- = [ ] [ ] 1 Now concentration of reactant or product I d I dn I V dt dt = = [ ] ( 29 ( 29 1 1 so rate of reaction I d I d t t v dt V dt = = products, reactants + =- = ( 29 Note: t is always positive (why?) Lecture 31 3 ( 29 ( 29 ( 29 ( 29 2 2 2 for 2 2 2 NO g...
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Lecture31A - Lecture 31 1 Kinetics, Rate Laws (Chapter...

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