chem 211 13 notes

chem 211 13 notes - Chemical kinetics Reaction rates...

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Chemical kinetics Reaction rates Concentrations and reaction rate Reaction rate is change in concentration in one of the reactants or products divided by interval in which the change takes place Average reaction rate is the change in molar concentration during time interval [ ] average rate of disaperance of R R t = - [ ] average rate of appearance of P R t = rate of reaction is related to stoichiometric coefficients unique average rate of aA+bB cC+dD is any of: 1 [ ] 1 [ ] 1 [ ] 1 [ ] A B c D a t b t c t d t - = - = = spectroscopic techniques are used to study reaction rates, especially fast reactions instantaneous rate of reaction most reactions slow down as reactants are used up best approximation of rate at an instant is to draw tangent to plot of concentration vs. time slope of tangent is instantaneous rate rate laws and reaction orders initial rate of reaction is the instantaneous rate when the reaction begins rate law is an expression for instantaneous reaction rate in terms of concentration of a species at any given intstant rate constant is independent of concentrations, but depends on the temperature [ ] [ ] a b rate k A B = a, b determine order of reaction rate law must be determined experimentally zero order reactions are reactions for which the rate is independent of concentration orders can be negative, or non-integer overall order is the sum of the powers concentration and time integrated rate law gives concentrations of reactants or products at any time after start of reaction first order integrated rate laws 0 [ ] ln [ ] t A kt A = - or 0 [ ] [ ] kt t A A e - = exponential decay to show that reaction really is first order, plot ln of concentration vs. time straight line with slope of –k half lives for first order reactions
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