Study of Reaction Rates Chemical Kinetics Study of Reaction Rates

Reaction Rate [ ] = concentration in M Rate = [A]t2 – [A]t1 = D[A] t2 – t1 Dt Changes can be ______ or _______

Reaction Rate We will always work with rate as a positive quantity. Concentrations of reactants______ ____________, the rate expression will include a negative sign in order to keep the rate positive Instantaneous Rate – _______________ _________________________________ will be the slope of a line tangent to the curve at that point ____ and ___________ of the reaction (which species is being studied) affect the ____

Introduction to Rate Laws Chemical reactions are _____________. * After a period of time, enough _______ accumulate so that the ____________________________. At this point the concentration of the reactants depends on the difference in the rates of the forward and reverse reactions. This tends to complicate matters, so we will ______ _______________________, before the products have had time to build up to significant levels. If we choose conditions where the reverse reaction can be ________, the _______________________ ________________________________________. *reactants form products and products can form reactants

Rate = k [reactant]n Rate Law k = rate constant, determined ______________ n = ____ of the reactant, determined experimentally, can be an integer, including zero or a fraction [ ] = law only depends on concentration of _______

Rate Law Key Points: [_______] do not appear b/c conditions are set where reverse rxn is __________ n, k, We study rate laws in order to be able to _________________________________ ________________________________.

Types of Rate Laws Differential rate law Integrated rate law rate depends on ________________ aka __________ Integrated rate law Shows how the concentration of species in the rxn depend on _________ Either type provides the information to determine the individual steps of the rxn.

Determining the form of the Rate Law Form of rate law Determine the power to which each reactant concentration must be raised in the rate law Order of a particular reactant must be obtained by observing how the reaction rate depends on the concentration of that reactant

Method of Initial Rates Initial rate – ________________ rate determined just after the reaction _______ (before initial [reactants] have changed much) Several __________ are needed w/different [________], each with a initial rate, then this info is compared to determine the form * Overall reaction order, n+m+p = overall reaction order

Sample Exercise 12.1 pg. 570 We will determine the order of each reactant by comparing initial concentrations and initial rates from several experiments. Compare experiments where the reactant your studying is changing but the others are constant. Go to white screen for example problem.

Integrated Rate Law Reactions involving a single ________ all have the same rate law form: Integrated – concentration of a reactant as a function of _______

First Order Rate Laws Doubling the _______, doubles the ______ 2 N2O5  4 NO2 + O2 Rate = k [N2O5] When integrated with time the law becomes: ln[N2O5] = -kt + ln[N2O5]0 [N2O5] @t time [N2O5] @ t=0

First Order Rate Laws First Order Rate Law: Rate = k [A] Integrated First Order Rate Law: ln [A] = -kt + ln [A]0 If [A]0 and k are known, the [A] at any time can be calculated Uses the form ___________, straight line graph y = x = m = b = First order always gives a _________________, by plotting ____________ Can also be expressed as a ratio of [A] and [A]0 ln [A]0 = kt [A] Sample exercise 12.2 and 12.3

First Order Rate Law Half Life of a Reactant First Order Rxn Half Life Law*: t1/2 = .693 k First order t1/2 doesn’t depend on concentration Example 12.4

Second-Order and Zero Order Rate Laws These are similar to the first order. Each has rate law, integrated rate law, a plot for a straight line, a relationship of the slope to k and a half-life equation. In each law if you know k and [A]0 you can calculate the [A] at any time. A few special things - Second Order – doubling the conc of A, quadruples the rate, tripling increases 9x Zero Order – rate is constant, not affected by concentration Get your handout!!!

Integrated Rate Laws with more than one reactant If the concentration of one reactant is much smaller than the others, the amounts of the larger concentrations will not change very much and can be assumed to be constant. The change of the concentration over time of the reactant with the small concentration can used to determine the order. Rate = k [A]n ([B]m[C]p) – very large Rate = k’[A]n

Reaction Mechanisms ___________________, which only tells reactants, products and stoichiometry

NO2(g) + CO(g)  NO(g) + CO2(g) This reaction occurs by elementary steps, a reaction whose rate law can be written from its molecularity. NO2(g) + NO2(g)  NO3(g) + NO(g) slow NO3(g) + CO(g)  NO2(g) + CO2(g) fast These are the two elementary steps by which the above reaction occurs. Intermediate – a species that is neither a reactant or product, but is formed and consumed during the reaction.

Molecularity Unimolecular – ____ molecule Bimolecular – _____ molecules Termolecular – ______ molecules (very rare, _________________________________)

Reaction Mechanism – series of elementary steps that must: The rate law for an elementary step follows directly from the molecularity of that step. Reaction Mechanism – series of elementary steps that must: 1) Sum of the elementary steps must give the overall balance equation 2) The mechanism must agree w/ the experimentally determined rate law. Rate–Determining Step: slow step (highest Ea) A reaction is only as fast as its slowest step The slow step will be indicated to you. A mechanism can never be proven absolutely, it is possibly correct. Example 12.6

Material Not in Your Text Sometimes the rate expression obtained by the process involves a reactive intermediate ([intermediate] is to small to determine experimentally) The intermediate must be eliminated from the rate expression. See handwritten notes. Pg.5

Model of Chemical Kinetics Chemical rxns speed up when ___________________, all rate constants show an exponential increase w/ absolute temperature. Collision Model 1) 2) 3) How does temperature speed up the reaction? What is the energy being used for?

How does a catalyst speed up the reaction? What is one more way to speed up the reaction?