2Chemical Kinetics Study of rxn rates (how fast rxn progresses) Measured in [X]/tAlso deals with reaction mechanismsSteps rxn goes through to move from reactants to products
3Factors That Influence Reaction Rate Under a specific set of conditions, every reaction has its owncharacteristic rate, which depends upon the chemical nature ofthe reactants.Four factors can be controlled during the reaction:Concentration - molecules must collide to react;Physical state - molecules must mix to collide;Temperature - molecules must collide with enough energy to react;The use of a catalyst.
4Expressing Rxn RatesRates are expressed in some unit quantity per timeSI units for rates of distance (speed) are in m/sChemical rxn rate units are M/sConsider the rxn A→BRxn rates given can be initial, instantaneous, or average
6Expressing Rate aA + bB cC + dD rate = 1 a - = - [A] t b [B] c [C] = -[A]tb[B]c[C]= +d[D]The numerical value of the rate depends upon the substance thatserves as the reference. The rest is relative to the balancedchemical equation.
7PracticePROBLEM:Determine the average rxn rate over the course of the entire experiment for the data listed in Table 16.1PROBLEM:(a) Balance the following equation and express the reaction rate in terms of the change in concentration with time for each substance: NO(g) + O2(g) → N2O3(g). (b) How fast is [O2] decreasing when [NO] is decreasing at a rate of 1.60x10-4 M/s?
8The Rate Law The rate law governs the progress of a given rxn. For a general rxn: aA + bB → cC + dDThe rate law is given by the equation below:Rate = k[A]x[B]y,k – rate constant; x & y are rxn orders wrt A & BAll components of a rxn’s rate law must be determined experimentallyMeasure physical quantity that you can relate to the concentration of a reactant a specific instant (initial rate method) or over time (integrated rate law)
9Determining Rate Laws The Initial rate method Used to determine rxn orders experimentallyMeasure initial rate of different reactant concentrationsData is listed in a tableRatio data, in general rate law form, from 2 lines in the table to determine order of each reactant in rxnChoose lines where conc. reactant in question changes and conc. of all other reactants stays the same
10Initial Reactant Concentrations (mol/L) Practice2NO(g) + O2(g) NO2(g)Initial Reactant Concentrations (mol/L)Initial Rate (mol/L*s)ExperimentO2NO11.10x10-21.30x10-23.21x10-322.20x10-21.30x10-26.40x10-331.10x10-22.60x10-212.8x10-343.30x10-21.30x10-29.60x10-351.10x10-23.90x10-228.8x10-3Determine the general rate law for the rxn.Calculate the rate constant for experiment 2.
11The Rate ConstantSpecific for a particular rxn at a particular temperature, within experimental errorUnits for k tell you the overall rxn orderRemember units for rate are M/timeUnits for [A]x are MxFor a rxn with an overall order R, the unit for k can be found by
12The Integrated Rate Law Can be used for 2 reasonsDetermine reactant concentration after an elapsed time--- must know order of reactant, rate constant, correct formulaDetermine rxn order for a specific reactant--- must graph different quantities vs. time and see which gives most linear plotCan only be used for 0, 1st, and 2nd order rates
14PracticePROBLEM:At 250C, hydrogen iodide (HI) breaks down ver slowly to hydrogen and iodine: rate = k[HI]2.The rate 250C is 2.4x10-21 L/mols. It mol of HI is place in a 1.00L container, how long will it take for the concentration of HI to reach M (10% reacted)?PROBLEM:Determine the rxn order for N2O5 using the graphical data given
15Reaction Half-LifeTime required for reactant concentration to reach ½ its initial value
16The Arrhenius Equation Describes the relationship between temperature and rxn rateHigher T Larger k Increased/faster rateSmaller Ea Larger k Increased/faster rateLower Ea (or T) Smaller k Decreased/slower rateA is related to both the collision frequency an orientation probability factor (dependent on structural complexity)where k is the kinetic rate constant at TEa is the activation energyR is the energy gas constantT is the Kelvin temperatureA is the collision frequency factor
17Activation EnergyEnergy that must be overcome for reactants to form productsAll rxns regardless of initial and final energies have Ea > 0Some bonds must break and new bonds must formReactant molecules gain this energy through collisions with one anotherIncreasing temperature increases rate as # collisions and energy of collisions increase
19Reaction Energy Diagrams Used to depict changes reactant molecules undergo to form products
20Reaction MechanismSequence of single rxn steps that sum to the overall rxnIt is impossible to prove rxn mechanism experimentallyRxn energy diagrams can elucidate # steps in a mechanismSteps in a mechanism for an overall rxn are elementary steps in which the coefficients of each reactant denote the reaction rate order wrt the reactantThe sum of all reactant coefficients in an elementary step denote the molecularity of the stepThe higher the molecularity of an elementary step, the slower its rate
22Correlating Mechanism w/ Rate Law For a mechanism to be reasonable, its elementary steps must meet 3 criteria:Elementary steps must add up to overall balanced eqtnElementary steps must be physically reasonable (usu. bi- or lower molecularity)Mechanism must correlate with the rate lawOverall rate law is usually equivalent to the slowest step’s (the rate limiting step, RLS) rate lawRLS can be picked out in a rxn energy diagram and predicted in a mechanism
23PracticeThe rxn and rate law for the decomposition of dinitrogen pentoxide are2N2O5(g)→ 4NO2(g) + O2(g) rate = k[N2O5]2and the rxn energy diagram is given above. Which of the followingmechanisms is most likely?One-step collision C. 2N2O5(g) → N4O10(g) [fast] N4O10(g) → 4NO2(g) + O2(g) [slow]2N2O5(g) → N4O10(g) [slow]N4O10(g) → 4NO2(g) + O2(g) [fast]
24Catalysis Increasing rxn rate by adding a catalyst Catalyst function: Lowers Ea increases k without being consumed or changing product amountUsually lowers Ea by providing a different mechanism