Chemical
Kinetics
Chapter 14Chemical Kinetics
John D. Bookstaver
St. Charles Community College
St. Peters, MO
2006, Prentice Hall, Inc.
Modified by S.A. Green, 2006
Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.;and Bruce E. Bursten
Chemical
Kinetics
Kinetics
•Studies the rate at which a chemicalprocess occurs.
•Besides information about the speed atwhich reactions occur, kinetics alsosheds light on the reaction mechanism(exactly how the reaction occurs).
Chemical
Kinetics
Outline: Kinetics
Reaction Rates
How we measure rates.
Rate Laws
How the rate depends on amountsof reactants.
Reaction Mechanism
The various reactions that thesubstances go through to reachfinal products.
Chemical
Kinetics
Factors That Affect Reaction Rates
•Concentration of Reactants
As the concentration of reactants increases, so does thelikelihood that reactant molecules will collide, thus rateincreases
•Temperature
At higher temperatures, reactant molecules have more kineticenergy, move faster, and collide more often and with greaterenergy – faster Rx
•Catalysts
Speed up rxn by changingthe reaction mechanism.
Chemical
Kinetics
Chemical
Kinetics
Reaction Rates
In this reaction, theconcentration ofbutyl chloride,C4H9Cl, wasmeasured at varioustimes, t.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
[C4H9Cl] M
Chemical
Kinetics
Reaction Rates
The average rate ofthe reaction overeach interval is thechange inconcentration dividedby the change in time:
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Average Rate, M/s
Chemical
Kinetics
Reaction Rates
•Note that the averagerate decreases as thereaction proceeds.
•This is because as thereaction goes forward,there are fewercollisions betweenreactant molecules.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Chemical
Kinetics
Reaction Rates
•A plot of concentrationvs. time for this reactionyields a curve like this.
•The slope of a linetangent to the curve atany point is theinstantaneous rate atthat time.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Chemical
Kinetics
Reaction Rates
•The reaction slowsdown with time becausethe concentration of thereactants decreases.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Chemical
Kinetics
Reaction Rates and Stoichiometry
•In this reaction, the ratioof C4H9Cl to C4H9OH is1:1.
•Thus, the rate ofdisappearance ofC4H9Cl is the same asthe rate of appearanceof C4H9OH.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Rate =
-[C4H9Cl]
t
=
[C4H9OH]
t
Chemical
Kinetics
Reaction Rates and Stoichiometry
•What if the ratio is not 1:1?
H2(g) + I2(g) 2 HI(g)
• 2 HI are made for each H2 used.
•HI will be produced twice as fast as H2 disappears
•HI will be produced twice as fast as I2 disappears
Chemical
Kinetics
Reaction Rates and Stoichiometry
•To generalize, for the reaction
aA + bB
cC + dD
Reactants (decrease)
Products (increase)
Chemical
Kinetics
Concentration and Rate
According to the law of mass action, therate of a chemical reaction depends onthe concentrations of the chemicals thatinfluence the rate
Each reaction has its own equation thatgives its rate as a function of reactantconcentrations.
this is called its Rate Law
To determine the rate law we measure the rateat different starting concentrations.
Chemical
Kinetics
Concentration and Rate
Compare Experiments 1 and 2:when [NH4+] doubles, the initial rate doubles.
Chemical
Kinetics
Concentration and Rate
Likewise, compare Experiments 5 and 6:when [NO2-] doubles, the initial rate doubles.
Chemical
Kinetics
Concentration and Rate
This equation is called the rate law,and k is the rate constant.
Chemical
Kinetics
Rate Laws
•A rate law shows the relationship between the reactionrate and the concentrations of reactants.
•k is a constant that has a specific value for each reaction.
•The value of k is determined experimentally.
“Constant” is relative here-
k is unique for each rxn
k changes with T
Chemical
Kinetics
Rate Laws
•Exponents tell the order of the reaction withrespect to each reactant.
•This reaction is
First-order in [NH4+]
First-order in [NO2−]
•The overall reaction order can be found byadding the exponents on the reactants in therate law.
•This reaction is second-order overall.
Chemical
Kinetics
Temperature and Rate
•Generally, as temperatureincreases, so does thereaction rate.
•This is because k istemperature dependent.
Chemical
Kinetics
The Collision Model
•In a chemical reaction, bonds arebroken and new bonds are formed.
•Molecules can only react if they collidewith each other.
Chemical
Kinetics
The Collision Model
Furthermore, molecules must collide with thecorrect orientation and with enough energy tocause bond breakage and formation.
Chemical
Kinetics
Activation Energy
•In other words, there is a minimum amount of energyrequired for reaction: the activation energy, Ea.
•Just as a ball cannot get over a hill if it does not rollup the hill with enough energy, a reaction cannotoccur unless the molecules possess sufficient energyto get over the activation energy barrier.
Chemical
Kinetics
Reaction Coordinate Diagrams
It is helpful tovisualize energychangesthroughout aprocess on areaction coordinatediagram like thisone for therearrangement ofmethyl isonitrile.
Chemical
Kinetics
Reaction Coordinate Diagrams
•It shows the energy ofthe reactants andproducts (and,therefore, E).
•The high point on thediagram is the transitionstate.
•The species present at the transition state iscalled the activated complex.
•The energy gap between the reactants and theactivated complex is the activation energybarrier.
Chemical
Kinetics
Maxwell–Boltzmann Distributions
•Temperature isdefined as ameasure of theaverage kineticenergy of themolecules in asample.
•At any temperature there is a widedistribution of kinetic energies.
Chemical
Kinetics
Maxwell–Boltzmann Distributions
•As the temperatureincreases, the curveflattens andbroadens.
•Thus at highertemperatures, alarger population ofmolecules hashigher energy.
Chemical
Kinetics
Maxwell–Boltzmann Distributions
•If the dotted line represents the activationenergy, as the temperature increases, so doesthe fraction of molecules that can overcomethe activation energy barrier.
•As a result, thereaction rateincreases.
Chemical
Kinetics
Reaction Mechanisms
The sequence of events that describesthe actual process by which reactantsbecome products is called the reactionmechanism.
Chemical
Kinetics
Reaction Mechanisms
•Reactions may occur all at once orthrough several discrete steps.
•Each of these processes is known as anelementary reaction or elementaryprocess.
Chemical
Kinetics
Reaction Mechanisms
•The molecularity of a process tells how manymolecules are involved in the process.
•The rate law for an elementary step is writtendirectly from that step.
Chemical
Kinetics
Multistep Mechanisms
•In a multistep process, one of the steps willbe slower than all others.
•The overall reaction cannot occur faster thanthis slowest, rate-determining step.
Chemical
Kinetics
Slow Initial Step
•The rate law for this reaction is foundexperimentally to be
Rate = k [NO2]2
•CO is necessary for this reaction to occur, but therate of the reaction does not depend on itsconcentration.
•This suggests the reaction occurs in two steps.
NO2 (g) + CO (g) NO (g) + CO2 (g)
Chemical
Kinetics
Slow Initial Step
•A proposed mechanism for this reaction is
Step 1: NO2 + NO2 NO3 + NO (slow)
Step 2: NO3 + CO NO2 + CO2 (fast)
•The NO3 intermediate is consumed in the second step.
•As CO is not involved in the slow, rate-determining step, it doesnot appear in the rate law.
Chemical
Kinetics
Catalysts
•Catalysts increase the rate of a reaction bydecreasing the activation energy of thereaction.
•Catalysts change the mechanism by whichthe process occurs.
Chemical
Kinetics
Catalysts
One way acatalyst canspeed up areaction is byholding thereactants togetherand helping bondsto break.
Chemical
Kinetics
Enzymes
•Enzymes arecatalysts inbiological systems.
•The substrate fitsinto the active site ofthe enzyme muchlike a key fits into alock.
