Lecture5 - Rates Description of time dependence requires a...

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1 Physical Chemistry Lecture 5 Introduction to chemical kinetics Thermodynamics and kinetics Thermodynamics Relative stability of states Characterized by free-energy differences Static comparisons of states Kinetics Changes of state over time Several different topics Empirical description of the rates of reaction Determination of experimental parameters Microscopic theories of reaction Descriptions of reactions Chemical reaction often given by an equation of the type This description is incomplete Only gives beginning and ending states Sometimes does not even describe the final state correctly Gives stoichiometry of change Must describe time dependences of amounts of materials to describe the reaction more completely H gas O gas H O liquid 2 2 2 1 2 ( ) ( ) ( ) Rates Description of time dependence requires a statement of the time derivatives of the concentrations of reactants and products Rates Rate of change of [H 2 O]: d[H 2 O]/dt Rate of change of [H 2 ]: d[H 2 ]/dt Rate of change of [O 2 ]: d[O 2 ]/dt Rates are not independent because of mass conservation 1 oxygen molecule disappears for every 2 hydrogen molecules in forming 1 water molecule H gas O gas H O liquid 2 2 2 1 2 ( ) ( ) ( ) Reaction velocity The rates of appearance of products and disappearance of reactants are related by stoichiometry of the reaction Reaction velocity , v , “normalizes” rates of appearance of products and disappearance of reactants Stoichiometric coefficient, i Found from balanced equation Positive for products Negative for reactants .) ( ) ( 2 1 ) ( 2 2 2 liq O H gas O gas H dt i d v i ] [ 1 1 2 / 1 1 2 2 2 O H O H Rate law Description of how the reaction velocity depends on parameters such as concentrations, temperature, pressure, etc. Function may be complex Gives insight into the reaction’s progress Reactions do not necessarily occur as implied by the overall equation Attempts to describe the features of the underlying physical chemistry of reaction in detail v f A B T P react prod ([ ],[ ], , )
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