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Chemical KineticsChemical Kinetics

The area of chemistry The area of chemistry that concerns that concerns reaction reaction rates.rates.

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Collision ModelCollision Model

Key IdeaKey Idea: Molecules must collide to : Molecules must collide to react and must be oriented the correct react and must be oriented the correct way.way.

However, only a small fraction of However, only a small fraction of collisions produces a reaction. Why?collisions produces a reaction. Why?

Arrhenius: An Arrhenius: An activation energyactivation energy must must be overcome.be overcome.

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Reaction RateReaction Rate

Change in concentrationChange in concentration (conc) of a (conc) of a reactant or product reactant or product per unit timeper unit time..

Rate = conc of A at time conc of A at time 2 1

2 1

t tt t

At Why does this formula

look so familiar?

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For exampleFor example

2NO2NO22 2NO + O 2NO + O22

As NOAs NO22 is used up more NO and O is used up more NO and O22 should appear.should appear.

How much OHow much O22 appears when 3.0 mol appears when 3.0 mol of NOof NO22 disappears? disappears?

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66

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Factors affecting rate of rxnFactors affecting rate of rxn

Temperature- for every 10Temperature- for every 10°°C increase, C increase, the rate of the reaction will double the rate of the reaction will double (approximation)(approximation)

[Reactants] – allows for more collisions.[Reactants] – allows for more collisions.

Catalyst – speeds up by lowering Catalyst – speeds up by lowering activation energy.activation energy.

Nature of reactants – Surface Area Nature of reactants – Surface Area mainly but also complexity of the mainly but also complexity of the molecule.molecule.

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Rate LawsRate Laws

Rate = Rate = kk[NO[NO22]]nn

kk = rate constant = rate constant

nn = rate order – affect of changing = rate order – affect of changing the [Reactant] on [Product]the [Reactant] on [Product]

k and n can only be found experimentally.k and n can only be found experimentally.

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Method of Initial RatesMethod of Initial Rates

Initial RateInitial Rate: the : the ““instantaneous instantaneous raterate”” just after the reaction begins. just after the reaction begins. This is a cheat so we don This is a cheat so we don’’t have to t have to worry about the reverse rxn.worry about the reverse rxn.

The initial rate is determined in The initial rate is determined in several experiments using several experiments using different initial concentrations.different initial concentrations.

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You can figure out order of each You can figure out order of each reactant and then figure out value of reactant and then figure out value of k. k.

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Overall Reaction OrderOverall Reaction Order

Sum of the order of each Sum of the order of each component in the rate law.component in the rate law.

rate = rate = kk[H[H22SeOSeO33][H][H++]]22[I[I]]33

The overall reaction order is 1 + 2 The overall reaction order is 1 + 2 + 3 = 6.+ 3 = 6.

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Types of Rate LawsTypes of Rate Laws

Differential Rate LawDifferential Rate Law: expresses : expresses how how raterate depends on depends on concentrationconcentration. . What we call the rate law.What we call the rate law.

Integrated Rate LawIntegrated Rate Law: expresses : expresses how how concentrationconcentration depends on depends on timetime. . Lets you figure out concentrations of Lets you figure out concentrations of stuff after a certain amount of time.stuff after a certain amount of time.

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First-Order Rate LawFirst-Order Rate LawFind it in the equation sheetFind it in the equation sheet

Integrated first-order rate law isIntegrated first-order rate law is

ln[A]ln[A]tt = = ktkt + ln[A] + ln[A]oo

Notice this is slope intercept formulaNotice this is slope intercept formula

Rate = A

A

t

k

For A For A Products in a 1st-order reaction, Products in a 1st-order reaction,

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Half-Life of a First-Order Half-Life of a First-Order ReactionReaction

tt1/21/2 = half-life of the reaction Half life of a = half-life of the reaction Half life of a reaction is the time required for a reactant reaction is the time required for a reactant to reach half its original concentrationto reach half its original concentration

kk = rate constant = rate constant

For a first-order reaction, the half-life For a first-order reaction, the half-life does not depend on concentration. (This is does not depend on concentration. (This is important)important)

tk1/2

0 693.

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Second-Order Rate LawSecond-Order Rate Law

For A For A products in a second-order products in a second-order reaction,reaction,

Integrated rate law is (slope Integrated rate law is (slope intercept again)intercept again)

Rate = A

A

t

k 2

1A

+ 1

A okt

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Half-Life of a Second-Order Half-Life of a Second-Order ReactionReaction

tt1/21/2 = half-life of the reaction= half-life of the reaction

kk = rate constant = rate constant

AAoo = initial concentration of A= initial concentration of A

The half-life is dependent upon the initial The half-life is dependent upon the initial concentration and changes over timeconcentration and changes over time

tk1/2

oA

1

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Zero order rxnsZero order rxns1. Rate is constant; it does not 1. Rate is constant; it does not change with changing concentrationchange with changing concentration2. Zero order sometimes happens 2. Zero order sometimes happens with catalysiswith catalysis3. The integrate rate law is NOT 3. The integrate rate law is NOT found on the formula sheet.found on the formula sheet.

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Reaction MechanismReaction Mechanism

The The series of stepsseries of steps by which a by which a chemical reaction occurs.chemical reaction occurs.

A chemical equation does not tell A chemical equation does not tell us us howhow reactants become reactants become products - it is a products - it is a summarysummary of the of the overalloverall process. process.

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Reaction Mechanism Reaction Mechanism (continued)(continued)

The reactionThe reaction

has many steps in the reaction has many steps in the reaction mechanism.mechanism.

6CO 6H O C H O O2 2light

6 12 6 2 6

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Often Used TermsOften Used TermsIntermediate: formed in one step and Intermediate: formed in one step and

used up in a subsequent step and so is used up in a subsequent step and so is never seen as a product.never seen as a product.

Molecularity: the number of species that Molecularity: the number of species that must collide to produce the reaction must collide to produce the reaction indicated by that step.indicated by that step.

Elementary Step: A reaction whose rate Elementary Step: A reaction whose rate law can be written from its molecularity.law can be written from its molecularity.

uni, bi and termolecularuni, bi and termolecular

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Elementary step exampleElementary step example

2 2 NONO22 + F + F22 2 NO2 NO22FF

follows the mechanism, follows the mechanism,

NONO22 + F + F22 NONO22F + F F + F

NONO22 + F + F NO NO22F F

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Rate-Determining StepRate-Determining Step

In a multistep reaction, it is In a multistep reaction, it is the slowest step. It therefore the slowest step. It therefore determines the rate of determines the rate of reaction.reaction.

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Writing a rate law from an Writing a rate law from an elementary stepelementary step

2 2 NONO22 + F + F22 2 NO 2 NO22FF

follows the mechanism, follows the mechanism,

NONO22 + F + F22 NO NO22F + F slowF + F slow

NONO22 + F + F NO NO22F fastF fast

The rate law can be written?????The rate law can be written?????

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TricksTricks

If it says fast equilibrium then you If it says fast equilibrium then you substitutesubstitute

Overall reaction 2NO + O2 Overall reaction 2NO + O2 2NO 2NO22

Step 1 2 NO Step 1 2 NO NN22OO22 (fast (fast equilibrium)equilibrium)

Step 2 NStep 2 N22OO22 + O + O22 2NO 2NO22 slowslow

Rate law = k[NO]Rate law = k[NO]22 [O [O22]]

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Tricks Part DosTricks Part Dos

Remember all the elementary steps Remember all the elementary steps must add up to the chemical must add up to the chemical reaction.reaction.

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Molecularity Elementary step Rate law

1 A -> products rate = k [A]

2 A + A -> products rate = k [A]2

2 A + B -> products rate = k [A] [B]

3 A + A + A -> products rate = k [A]3

3 A + 2 B -> products rate = k [A] [B]2

3 A + B + C -> products rate = k [A] [B] [C]

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Arrhenius EquationArrhenius Equation Collisions must have enough energy Collisions must have enough energy

to produce the reaction (must equal to produce the reaction (must equal or exceed the activation energy).or exceed the activation energy).

A graph showing two steps. A graph showing two steps. Blue is uncatalyzedBlue is uncatalyzed

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CatalysisCatalysis

Catalyst: A substance that speeds Catalyst: A substance that speeds up a reaction without being up a reaction without being consumed. It does so by lowering consumed. It does so by lowering the activation energy.the activation energy.

Enzyme: A large molecule (usually Enzyme: A large molecule (usually a protein) that catalyzes biological a protein) that catalyzes biological reactions.reactions.

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[NO[NO22] what order?] what order?

time (s)time (s) [][] lnln 1/[]1/[]

00 0.50.5 -0.69315-0.69315 22

12001200 0.4440.444 -0.81193-0.81193 2.2522522.252252

30003000 0.3810.381 -0.96496-0.96496 2.6246722.624672

45004500 0.340.34 -1.07881-1.07881 2.9411762.941176

90009000 0.250.25 -1.38629-1.38629 44

1800018000 0.1740.174 -1.7487-1.7487 5.7471265.747126

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[]

0

0.20.4

0.6

0 5000 10000 15000 200001/[]

0

2

4

6

8

0 5000 10000 15000 20000ln

-2

-1

0

0 5000 10000 15000 20000

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CatalystsCatalysts

Homogeneous (same phase) vs Homogeneous (same phase) vs Heterogenous (different phases)Heterogenous (different phases)

The difference between an The difference between an intermediate and a catalsyt is?intermediate and a catalsyt is?

Think about your areas of Think about your areas of strength and weakness…strength and weakness…

1.1. Figure out order from a set of data and Figure out order from a set of data and write a rate law.write a rate law.

2.2. Calculate the value of k including units.Calculate the value of k including units.

3.3. Use and understand integrated rate laws.Use and understand integrated rate laws.

4.4. Determine rate law from elementary Determine rate law from elementary steps.steps.

5.5. Know how catalysts workKnow how catalysts work

6.6. Understand misc. vocabulary.Understand misc. vocabulary.

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