Presentation on theme: "Elementary Chemical Kinetics ( )"— Presentation transcript:
1 Elementary Chemical Kinetics (25.1-25.2) Kinetics is the study of how reactions occurSpeed of reaction depends on frequency of productive collisions between molecules (concentration, temperature, nature of productive collision)Many reactions involve more than one step, so a mechanism is used to explain how the reaction occursReaction rates are measured as the speed with which a reactant is consumed or a product is createdReaction rate is a differential equation since we are looking at a change in concentration in a given amount of timeOne typically monitors either decay of one reactant or the production of a single productAccomplished through absorbance, fluorescence, pH, etc.
2 Rate Laws and Reaction Mechanisms (25.3-25.4) We know from experience that reaction rates often depend on concentration of reactantsRate can be expressed as a product of reactant concentrations of certain ordersOrder for each reactant is not necessarily the stoichiometric coefficient (α ≠ a)Rate constant (k) must contain information about temperature and productive collisionsOverall order of the reaction is the sum of the orders for each reactant and must be determined experimentallyThe rate can be determined by measuring the change in concentration of a reactant/product over a short range of time (tangent to curve is rate)The order for each reactant can be obtained by changing the concentrations of a single species and monitoring the change in rate (isolation method, method of initial rates)Reaction mechanism is a set of elementary reactions that can be used to explain a rate lawOrder of reactants in an elementary rate law is the stoichiometric coefficientMechanism is only viable if the sum of elementary rate laws match overall rate law
3 Integrated Rate Law – First Order (25.5) Differential form of rate equation can be combined with rate law to give a relation between concentration and timeFirst-order reactions (elementary) only involve a single reactant to first-orderIntegrated rate law shows the concentration of A decays exponentially with timeLinearized plots can be used to determine orderNatural logarithm is used to get time out of the exponentIf a plot of ln[A] vs. time gives a line, then the reaction is first order and the slope of the line is related to the rate constantHalf-life is a measure of how long it takes for the concentration of reactant to decay to 50% and can also be used to indicate order of reactionHalf-life of first-order reaction is independent of concentration of reaction
4 Integrated Rate Law – Second Order (25.5) One type of second-order reaction (elementary) only involves one reactantType I second-order reactions show that time is related to the inverse of [A]The half-life of this reaction is dependent on the initial concentration of reactantIf a plot of 1/[A] vs. time is generated and gives a line, then the reaction is second-order and the slope is related to the rate constantAnother type of second-order reaction involves two reactants (Type II)This integrated rate law involves concentrations of both reactantsThis can be reduced to a Type I rate law if [A]0 = [B]0
5 Concentrations of Product and Reactant During Reaction