CHEMICALCHEMICAL KINETICSKINETICS: :

THE RATES AND MECHANISMS THE RATES AND MECHANISMS OF CHEMICAL REACTIONSOF CHEMICAL REACTIONS

Chemical kinetics is the study of Chemical kinetics is the study of the speed or rate of a reaction the speed or rate of a reaction under various conditions. under various conditions.

Spontaneity is also important Spontaneity is also important AND a spontaneous reaction does AND a spontaneous reaction does NOT imply a rapid reaction. NOT imply a rapid reaction.

The changing of diamond into The changing of diamond into graphite is spontaneous but so graphite is spontaneous but so slow that it is not detectable even slow that it is not detectable even in a lifetime. in a lifetime.

A mechanism is a sequence of A mechanism is a sequence of events at the molecular level events at the molecular level that controls the speed and that controls the speed and outcome of the reaction.outcome of the reaction.

FACTORS THAT AFFECT FACTORS THAT AFFECT REACTION RATESREACTION RATES

The following conditions affect the The following conditions affect the speedspeed

of a chemical process:of a chemical process:

1. Nature of the reactants1. Nature of the reactants

Some reactant molecules Some reactant molecules react in a hurry, others react react in a hurry, others react very slowly. very slowly.

Pointers… Pointers…

Physical state Physical state - -

gasoline gasoline (l)(l) vs. gasoline vs. gasoline (g)(g)

KK22SOSO4(s)4(s) + Ba(NO + Ba(NO33))2(s)2(s) no rxn. no rxn. **while both of these in the **while both of these in the aqueousaqueous

state do react. state do react.

Chemical identityChemical identity What is What is reacting? reacting?

Usually, ions of opposite charge Usually, ions of opposite charge react very rapidly. react very rapidly.

Usually, the more bonds between Usually, the more bonds between reacting atoms in a molecule, the reacting atoms in a molecule, the slower the reaction rate. slower the reaction rate.

Substances with strong bonds Substances with strong bonds (larger bond energies) will react (larger bond energies) will react much more slowly. much more slowly.

Examples:Examples:

Metallic sodium reacts much faster Metallic sodium reacts much faster with water than metallic calcium. with water than metallic calcium.

Oxidation of methane can be Oxidation of methane can be increased with an increase in increased with an increase in temperature. temperature.

Photosynthesis is very slow and Photosynthesis is very slow and changes very little with an increase changes very little with an increase in temperature.in temperature.

2. Concentration of 2. Concentration of reactantsreactants

More molecules, More molecules,

More collisions.More collisions.

3. Temperature3. Temperature

heat ‘em up & speed ‘em up heat ‘em up & speed ‘em up

The faster they move, the more The faster they move, the more likely they are to collide. likely they are to collide.

An increase in temperature An increase in temperature produces more successful produces more successful collisions that are able to collisions that are able to overcome the needed activation overcome the needed activation energy, energy,

therefore, therefore,

a general increase in reaction a general increase in reaction rate with increasing temperature. rate with increasing temperature.

In fact,In fact,

A general rule of thumb is that a A general rule of thumb is that a 10°C increase in temperature will 10°C increase in temperature will double the reaction rate. double the reaction rate.

* This actually depends on the * This actually depends on the magnitude of the Ea* and the magnitude of the Ea* and the temperature range. temperature range.

4. Catalysts4. Catalysts

accelerate chemical reactions but accelerate chemical reactions but are not themselves transformed. are not themselves transformed.

Biological catalysts are proteins Biological catalysts are proteins called called

enzymes. enzymes.

A catalyst is a substance that A catalyst is a substance that changes the rate of reaction by changes the rate of reaction by altering the reaction pathway. altering the reaction pathway.

Most catalysts work by Most catalysts work by lowering the activation energy lowering the activation energy needed for the reaction to needed for the reaction to proceed; therefore, more proceed; therefore, more collisions are successful and the collisions are successful and the reaction rate is increased.reaction rate is increased.

Remember! Remember!

The catalyst is not part of the The catalyst is not part of the chemical reaction and is not used up chemical reaction and is not used up during the reaction.during the reaction.

* (May be homogeneous or * (May be homogeneous or heterogeneous catalysts.) heterogeneous catalysts.)

Example…Example…

HH22OO22 decomposes relatively slowly into decomposes relatively slowly into

HH22O and OO and O22

however… however…

exposure to light accelerates this exposure to light accelerates this

process AND with the help of MnOprocess AND with the help of MnO22, it , it goes extremely FAST!! goes extremely FAST!!

Note:Note:

A catalyst lowers the activation A catalyst lowers the activation energy barrier. Therefore, the energy barrier. Therefore, the forward and reverse reactions are forward and reverse reactions are both accelerated to the same both accelerated to the same degree. degree.

* (Some homogeneous catalysts * (Some homogeneous catalysts

actually appear in the rate law actually appear in the rate law because their concentration affects because their concentration affects the reaction. Ex. NO catalyzing Othe reaction. Ex. NO catalyzing O33 ) )

5. Surface area of 5. Surface area of reactantsreactants

exposed surfaces affect speed. exposed surfaces affect speed.

Except for substances in the gaseous Except for substances in the gaseous state or solution, reactions occur at state or solution, reactions occur at the boundary, or interface, between the boundary, or interface, between two phases. two phases.

The greater surface area exposed, The greater surface area exposed, the greater chance of collisions the greater chance of collisions between particles, hence, the between particles, hence, the reaction should proceed at a much reaction should proceed at a much faster rate. faster rate.

Ex. coal dust is very explosive as Ex. coal dust is very explosive as

opposed to a piece of charcoal. opposed to a piece of charcoal.

Solutions are ultimate exposure!Solutions are ultimate exposure!

THE COLLISION THEORY THE COLLISION THEORY OF REACTION RATES OF REACTION RATES

Particles must collide.Particles must collide.

Only two particles may collide at Only two particles may collide at one time. one time.

Proper orientation of colliding Proper orientation of colliding molecules so that atoms can molecules so that atoms can come in contact with each other come in contact with each other to become products. to become products.

The collision must occur with The collision must occur with enough energy to overcome the enough energy to overcome the electron/electron repulsion of electron/electron repulsion of the valence shell electrons of the valence shell electrons of the reacting species and must the reacting species and must have enough energy to have enough energy to transform translational energy transform translational energy into vibrational energy in order into vibrational energy in order to penetrate into each other so to penetrate into each other so that the electrons can rearrange that the electrons can rearrange and form new bonds.and form new bonds.

This new collision This new collision product is at the product is at the peak of the peak of the activation energy activation energy hump and is called hump and is called the the activated activated complexcomplex or the or the transition state. At transition state. At this point, the this point, the activated activated complexcomplex can still can still either fall to either fall to reactants or to reactants or to products.products.

With all of these criteria With all of these criteria met, the reaction may met, the reaction may proceed in the forward proceed in the forward direction. direction.

12.1 CHEMICAL REACTION 12.1 CHEMICAL REACTION RATESRATES

The speed of a reaction is The speed of a reaction is expressed in terms of its expressed in terms of its “rate”, some measurable “rate”, some measurable quantity is changing with quantity is changing with time.time.

The rate of a chemical reaction The rate of a chemical reaction is measured by the decrease in is measured by the decrease in concentration of a reactant or concentration of a reactant or an increase in concentration of an increase in concentration of a product in a unit of time.a product in a unit of time.

Rate =Rate =

change in concentration of a species change in concentration of a species

time interval time interval

When writing rate expressions, they When writing rate expressions, they can be written in terms of reactants can be written in terms of reactants disappearance or products disappearance or products appearance. appearance.

Rate is not constant, it changes Rate is not constant, it changes with time. Graphing the data of with time. Graphing the data of an experiment will show an an experiment will show an average rate of reaction. average rate of reaction.

You can find the instantaneous You can find the instantaneous rate by computing the slope of rate by computing the slope of a straight line tangent to the a straight line tangent to the curve at that time.curve at that time.

Graph of Experimental Graph of Experimental DataData

Reaction RateReaction Rate

Expressed as the Δ in concentration of a Expressed as the Δ in concentration of a

reagent per unit time or Δ[A]/Δtreagent per unit time or Δ[A]/Δt

Focus either on the disappearance ofFocus either on the disappearance of

reactants or the appearance of productsreactants or the appearance of products rate of Δ of a reactant is always rate of Δ of a reactant is always

negativenegative rate of Δ of a product is always rate of Δ of a product is always

positivepositive

Consider:Consider:

2 NO2 NO2(g) 2(g) →→ O O 2(g)2(g)+ 2 NO+ 2 NO(g)(g) Oxygen can appear only half as Oxygen can appear only half as

rapidly as the nitrogen dioxide rapidly as the nitrogen dioxide disappearsdisappears

NO appears twice as fast as oxygen NO appears twice as fast as oxygen appears.appears.

Calculate the Calculate the AVERAGEAVERAGE rate at which rate at which [NO[NO22] changes in the first 50.0 ] changes in the first 50.0 seconds:seconds:

RATE = - RATE = - Δ [NOΔ [NO22]] = - = - [.0079]-[0.0100][.0079]-[0.0100]

ΔtΔt 50.0 s 50.0 s

= -[-4.2 x 10= -[-4.2 x 10-5-5 mol/L • sec] mol/L • sec]

= 4.2 x 10= 4.2 x 10-5-5 mol/L • sec mol/L • sec

or, M • s-1or, M • s-1

Note that the rate is Note that the rate is NOTNOT constant but decreases with constant but decreases with time.time.

The rates given below are The rates given below are averageaverage rates.rates.

To find the value of the rate To find the value of the rate at a particular time, the at a particular time, the instantaneous rateinstantaneous rate, , compute the compute the slopeslope of a line of a line tangent to the curvetangent to the curve at that at that point. point.

Why the negative on NOWhy the negative on NO22??

RELATIVE RATESRELATIVE RATES

We can consider the appearance We can consider the appearance of products along with the of products along with the disappearance of reactants. disappearance of reactants.

The reactant’s concentration is The reactant’s concentration is declining, the product’s is declining, the product’s is increasing. increasing.

Respect the algebraic sign AND Respect the algebraic sign AND respect the stoichiometry. respect the stoichiometry.

Divide the rate of change in Divide the rate of change in concentration of each reactant concentration of each reactant by its stoichiometric coefficient by its stoichiometric coefficient in the balanced chem. eqn. and in the balanced chem. eqn. and this is foolproof and a breeze!this is foolproof and a breeze!

Thus... Thus... Rate of Reaction = Rate of Reaction =

- - 1Δ[NO 1Δ[NO22] ] = = 1 Δ[NO] 1 Δ[NO] = = Δ[O2] Δ[O2]

2 Δtime 2 Δtime 2 Δtime 2 Δtime Δtime Δtime

Of course you can change these once Of course you can change these once the ratio is set. You might prefer:the ratio is set. You might prefer:

-1 : +1 : +2-1 : +1 : +2

Relative Rates from the Relative Rates from the balanced equation:balanced equation:

Using the coefficients from the Using the coefficients from the

balanced equation, students should balanced equation, students should be be

able to give relative rates. able to give relative rates.

For example: For example:

4 PH4 PH3 (g)3 (g) P P4(g)4(g) + 6 H + 6 H2(g)2(g)

Initial rate rxn. Initial rate rxn. ==

Exercise Exercise

What are the What are the relativerelative rates of rates of change change

in concentration of the products in concentration of the products and and

reactant in the decomposition of reactant in the decomposition of

Nitrosyl chloride, NOCl?Nitrosyl chloride, NOCl?

2 NOCl 2 NOCl (g)(g) →→ 2 NO 2 NO(g)(g) + Cl + Cl2(g)2(g)

12.2 RATE LAWS: 12.2 RATE LAWS: AN INTRODUCTION AN INTRODUCTION

Reactions are reversible. So Reactions are reversible. So far, far,

we’ve only considered the we’ve only considered the forward forward

reaction. The reverse is equally reaction. The reverse is equally

important. important.

When the rate of the forward = When the rate of the forward = the rate of the reverse, we have the rate of the reverse, we have EQUILIBRIUM! EQUILIBRIUM!

To avoid this complication we To avoid this complication we will discuss reactions soon after will discuss reactions soon after mixing--initial reaction rates, mixing--initial reaction rates, and not worry and not worry about the buildup of products about the buildup of products and how that starts up the and how that starts up the reverse reaction.reverse reaction.

Initial Reaction RatesInitial Reaction Rates

Begin with pure reactants, mix thoroughly, then Begin with pure reactants, mix thoroughly, then

measure speed of rxn. Over time, the presence measure speed of rxn. Over time, the presence

of products can alter results dramatically and of products can alter results dramatically and lead lead

to confusing results. to confusing results.

We’ll be talking initial reaction rates throughout We’ll be talking initial reaction rates throughout

our discussions!our discussions!

Rate expression or rate Rate expression or rate law is the relation between law is the relation between reaction rate and the reaction rate and the concentrations of concentrations of reactants given by a reactants given by a mathematical equation.mathematical equation.

CONCENTRATION AND CONCENTRATION AND REACTION RATE: REACTION RATE:

THE RATE LAW OR RATE EXPRESSION:THE RATE LAW OR RATE EXPRESSION:

Rates generally depend on Rates generally depend on

reactant concentrations. reactant concentrations.

To find the exact relation between rate To find the exact relation between rate

and concentration, we must and concentration, we must do some do some

experimentsexperiments and collect information. and collect information.

Where C is a catalyst, the Where C is a catalyst, the rate expression will rate expression will alwaysalways have the form: have the form:

xXC

bB +aA

Initial rxn rate = Initial rxn rate = kk[A][A]mm[B][B]nn[C][C]pp

kk = rate constant = rate constant

[A] = concentration of reactant A[A] = concentration of reactant A

[B] = concentration of reactant B [B] = concentration of reactant B

[C] = concentration of the catalyst—won’t see [C] = concentration of the catalyst—won’t see

this too often in APthis too often in AP

m = order of reaction for reactant Am = order of reaction for reactant A

n = order of reaction for reactant Bn = order of reaction for reactant B

p = order of reaction for the catalyst Cp = order of reaction for the catalyst C

Exponents can be zero, whole Exponents can be zero, whole numbers, or fractions --numbers, or fractions --

AND MUST BE DETERMINED AND MUST BE DETERMINED BY EXPERIMENTATION!!BY EXPERIMENTATION!!

THE RATE CONSTANT, THE RATE CONSTANT, kk

Is temperature dependent & must be Is temperature dependent & must be

evaluated by experiment.evaluated by experiment.

Example: Example: rate = rate = kk[Pt(NH[Pt(NH33))22ClCl22]]

kk = 0.090/hr, therefore when [ion] = 0.018 mol/L = 0.090/hr, therefore when [ion] = 0.018 mol/L

rate = rate =

(.0090/hr)(0.018 mol/L) = 0.0016 mol/(L• hr)(.0090/hr)(0.018 mol/L) = 0.0016 mol/(L• hr)

ORDER OF A REACTIONORDER OF A REACTION

Order with respect to a certain reactant is Order with respect to a certain reactant is

the the exponentexponent on its concentration term in on its concentration term in

the rate expression.the rate expression.

Order of the reaction is the sum of all the Order of the reaction is the sum of all the

exponents on all the concentration terms exponents on all the concentration terms in in

the expression.the expression.

DETERMINATION OF THE DETERMINATION OF THE RATE EXPRESSIONRATE EXPRESSION

aA + bB aA + bB →→ xX xX

initial rate = initial rate = kk[A][A]oomm[B][B]oo

nn

the little subscript “o” means original.the little subscript “o” means original.

Zero orderZero order

The change in concentration of The change in concentration of reactant has no effect on the rate.reactant has no effect on the rate.

These are not very common. These are not very common.

General form of rate equation: General form of rate equation:

Rate = kRate = k

First orderFirst order

Rate is directly proportional to the Rate is directly proportional to the reactants concentration; doubling reactants concentration; doubling [rxt], doubles rate. These are very [rxt], doubles rate. These are very common! Nuclear decay reactions common! Nuclear decay reactions usually fit into this category. usually fit into this category.

General form of rate equation: General form of rate equation:

Rate = k [A]Rate = k [A]

Second orderSecond order

Rate is quadrupled when [rxt] is Rate is quadrupled when [rxt] is doubled and increases by a factor doubled and increases by a factor of 9 when [rxt] is tripled, etc. of 9 when [rxt] is tripled, etc. These are common, particularly in These are common, particularly in gas-phase reactions.gas-phase reactions.

General form of rate equation: General form of rate equation:

Rate = k [A]Rate = k [A]22

Fractional orders are Fractional orders are rare!rare!

Ex. Ex. rate = rate = kk[A][A]oomm[B][B]oo

nn

If m = 0 ; reaction is If m = 0 ; reaction is zero orderzero order with respect to A with respect to A If m = 1 ; reaction is If m = 1 ; reaction is 1st order1st order with respect to A with respect to AIf m = 2 ; reaction is If m = 2 ; reaction is 2nd order2nd order with respect to A with respect to AIf n = 0 ; reaction is If n = 0 ; reaction is zero orderzero order with respect to B with respect to BIf n = 1 ; reaction is If n = 1 ; reaction is 1st order1st order with respect to B with respect to BIf n = 2 ; reaction is If n = 2 ; reaction is 2nd order2nd order with respect to B with respect to B

Adding the orders of each Adding the orders of each reactant gives the reactant gives the overall overall orderorder of the reaction. of the reaction.

Since the rate stays the same Since the rate stays the same regardless of the concentration regardless of the concentration of [A], it is zero order with of [A], it is zero order with respect to A. respect to A.

However, the rate doubles with a However, the rate doubles with a doubling of [B] and triples with a doubling of [B] and triples with a tripling of [B]. tripling of [B].

This indicates the rate is first This indicates the rate is first order with respect to [B].order with respect to [B].

SummarySummary

Initial reaction rate = Initial reaction rate =

kk[A][A]oooo[B][B]oo1 = 1 = kk[B][B]oo

11

The overall reaction rate =The overall reaction rate =

1 + 0 = 11 + 0 = 1stst order overall. order overall.

Now. . . . .Now. . . . .

Use a set of the data to calculate Use a set of the data to calculate kk::

0.0050 mol/(L•hr) = 0.0050 mol/(L•hr) = kk[0.20 mol/L][0.20 mol/L]11

k =k = 2.5 x 10 2.5 x 10-2-2 /hr /hr

You should get the same value with You should get the same value with any any

set of data!set of data!

Ugly algebraic method Ugly algebraic method is sometimes is sometimes necessarynecessary

rate 1rate 1 = = k k [reactant][reactant]mm [reactant] [reactant]nn

rate 2 rate 2 k k [reactant][reactant]mm [reactant] [reactant]nn

Select a trial where one Select a trial where one reactant reactant

concentration is held constant concentration is held constant

SO THAT IT CANCELS; SO THAT IT CANCELS;

the the kk’s will also cancel.’s will also cancel.

Using trials 1 & 4:Using trials 1 & 4:

0.50 x 100.50 x 10--22 = = kk [0.50] [0.50]mm [0.20] [0.20]nn

1.00 x 101.00 x 10-2-2 kk [0.50] [0.50]mm [0.40] [0.40]nn

so…. so…. ½ = [ ½ ]½ = [ ½ ]nn

and and n must be ONE to make n must be ONE to make that true!that true!

ExerciseExercise

In the following reaction, a Co-Cl bond In the following reaction, a Co-Cl bond is is

replaced by a Co-OHreplaced by a Co-OH22 bond. bond.

[Co(NH[Co(NH33))55Cl]Cl]+2+2 + H + H22O O →→

[Co(NH[Co(NH33))55HH22O]O]+3+3 + Cl + Cl

Initial rate = Initial rate = k k {[Co(NH{[Co(NH33))55Cl]Cl]+2+2}}mm

Using the data below, find the Using the data below, find the value of value of mm in the rate expression in the rate expression and calculate the value of and calculate the value of kk..

Exp. Initial Concentration Initial rateExp. Initial Concentration Initial rate

of [Co(NHof [Co(NH33))55Cl]Cl]+2+2 mol/(L•min) mol/(L•min)

(mol/L) (mol/L)

1 1.0 x 101 1.0 x 10-3-3 1.3 x 10 1.3 x 10-7-7

2 2.0 x 102 2.0 x 10-3-3 2.6 x 10 2.6 x 10-7-7

3 3.0 x 103 3.0 x 10-3-3 3.9 x 10 3.9 x 10-7-7

4 1.0 x 104 1.0 x 10-3-3 1.3 x 10 1.3 x 10-7-7

Exercise 12.1Exercise 12.1

The reaction between bromate ions The reaction between bromate ions

and bromide ions in acidic aqueous and bromide ions in acidic aqueous

Solution is given by the equation:Solution is given by the equation:

BrOBrO3 3 – – (aq) + 5 Br (aq) + 5 Br –– (aq) + 6 H (aq) + 6 H++ (aq) (aq)

3 Br3 Br22 ( (ll) + 3 H) + 3 H22O (O (ll))

The table below gives the results of The table below gives the results of four four experimentsexperiments. Using these data, . Using these data, determine the orders for all three determine the orders for all three reactants, the overall reaction order, reactants, the overall reaction order, and the value of the rate constant. and the value of the rate constant.

What is the value of What is the value of kk? What are the ? What are the units of units of kk??

Trick for units for kTrick for units for k

Liters Liters (overall order – 1)(overall order – 1)__________________________

Moles Moles (overall order – 1)(overall order – 1) x (the unit of x (the unit of time)time)

TWO TYPES OF RATE TWO TYPES OF RATE LAWLAW

differential rate lawdifferential rate law----expresses how the rate depends expresses how the rate depends on on concentrationconcentration (most (most common & what we’ve been common & what we’ve been doing!)doing!)

integrated rate lawintegrated rate law----expresses how the expresses how the concentrations depend on concentrations depend on timetime

12.3 DETERMINING THE 12.3 DETERMINING THE FORM OF THE RATE LAWFORM OF THE RATE LAW

--experimental convenience--experimental convenience

Note the shape of this curve! It will Note the shape of this curve! It will save save

you time in the future!you time in the future!

Write the relative rate Write the relative rate expression:expression:

Write the differential rate Write the differential rate law [expression]:law [expression]:

12.4 INTEGRATED RATE LAW -12.4 INTEGRATED RATE LAW - CONCENTRATION/TIME CONCENTRATION/TIME RELATIONSHIPSRELATIONSHIPS

When we wish to know how When we wish to know how long a reaction must proceed to long a reaction must proceed to reach a predetermined reach a predetermined concentration of some reagent, concentration of some reagent, we can construct curves or we can construct curves or derive an equation that relates derive an equation that relates concentration and time.concentration and time.

GRAPHICAL METHODS FORGRAPHICAL METHODS FORDISTINGUISHING FIRST AND DISTINGUISHING FIRST AND SECOND ORDER REACTIONSSECOND ORDER REACTIONS

First order:First order:

ln[A] = -ln[A] = -kkt + ln[A]t + ln[A]oo y = ax + b y = ax + b

Second order:Second order:

1/[A] = 1/[A] = k kt + 1/[A]t + 1/[A]oo

y = ax + by = ax + b

ln[reactant] vs. time straight line for ln[reactant] vs. time straight line for

first order first order in that reactant & since a in that reactant & since a = -= -kk

the slope of the line is the slope of the line is negativenegative..

1/[reactant] vs. time straight line for 1/[reactant] vs. time straight line for

second ordersecond order in that reactant since a in that reactant since a = = kk

the slope is the slope is positivepositive..

Using the graphing Using the graphing calculatorcalculator

Set up your calculator so that Set up your calculator so that

time is always in L1 and the time is always in L1 and the y-list y-list

is alphabetical!is alphabetical!

L1 time (x variable throughout!)L1 time (x variable throughout!)

L2 concentration:L2 concentration: [A] [A]

straight line = zero orderstraight line = zero order

L3 ln concentration: ln [A] L3 ln concentration: ln [A]

straight line = first orderstraight line = first order

L4 reciprocal concentration: L4 reciprocal concentration: 1/[A] 1/[A]

straight line = second orderstraight line = second order

Run 3 linear regressions – one Run 3 linear regressions – one

each for:each for:

L1, L2 L1, L2

L1, L3 L1, L3

L1, L4 L1, L4 and see which has the best “r” and see which has the best “r”

[linear regression correlation [linear regression correlation coefficient in big people coefficient in big people language!]language!]

Paste the best one into y= Paste the best one into y= by hitting by hitting

22ndnd enter enter

to get the command back to get the command back on the screen, on the screen,

then “fix” it to read then “fix” it to read LinRegLinReg

{the combination that was {the combination that was the best regression}. the best regression}.

Next, hit Next, hit

, VARS > to Y-VARS , VARS > to Y-VARS then then 1 11 1

If you were successful, you’ll see If you were successful, you’ll see

LinReg(ax +b) LLinReg(ax +b) L11, L, Lwhichever you whichever you

chosechose, Y, Y11

displayed on your screen.displayed on your screen.

The order of the reaction is The order of the reaction is

0; 1; 2 0; 1; 2

respectively for each respectively for each combination. combination.

|slope| = k |slope| = k

Rate = k[rxt.]Rate = k[rxt.]orderorder

Next,Next,

since linear, since linear,

NEVER, EVER FORGET: NEVER, EVER FORGET:

y = mx + b y = mx + b

(TI uses an “a” instead of an “m”)(TI uses an “a” instead of an “m”)

If L1, L3 was your best “r”, the If L1, L3 was your best “r”, the reaction is first order andreaction is first order and

y = mx + b y = mx + b becomesbecomes ln [conc.] = k ln [conc.] = k

(DO use the proper sign for k (DO use the proper sign for k here!)here!)

t + ln [conc.t + ln [conc.oo]]

Do the same substitutions Do the same substitutions into into

y = mx + b y = mx + b

for the other formats!for the other formats!

Exercise 12.2Exercise 12.2

The decomposition of NThe decomposition of N22OO55 in the in the gas gas

phase was studied at constant phase was studied at constant

temperature.temperature.

2 N2 N22OO55 (g) (g) 4 NO 4 NO22 (g) + O (g) + O22 (g)(g)

The following results were collected:The following results were collected:

[N[N22OO55]] Time (s)Time (s)0.10000.1000 0 00.07070.0707 50 500.05000.0500 1001000.02500.0250 2002000.01250.0125 3003000.006250.00625 400400

Determine the rate law and Determine the rate law and calculate calculate

the value of the value of kk. .

Once you have the CORRECT Once you have the CORRECT equation for the reaction’s rate equation for the reaction’s rate law in your calculator so that it law in your calculator so that it can draw the CORRECT linear can draw the CORRECT linear regression line…regression line…

You can display the graph. You can display the graph.

Make sure your plot 1 is ON and Make sure your plot 1 is ON and then set it up to read the then set it up to read the CORRECT axes. CORRECT axes.

Check the max and min x-Check the max and min x-values that zoom 9 assigned to values that zoom 9 assigned to the window.the window.

You can now solve for any You can now solve for any concentration EXACTLY concentration EXACTLY between those max and min between those max and min values. values.

What if your window doesn’t What if your window doesn’t have have

the proper time range? the proper time range?

CHANGE IT!CHANGE IT!

To solve,To solve,

1.1. Display your graph by hitting “Graph”. Display your graph by hitting “Graph”.

2.2. Next hit “2Next hit “2ndnd” “trace” to get to calculate ” “trace” to get to calculate then choose “1” which is “value”. then choose “1” which is “value”.

3.3. Now your screen has the graph Now your screen has the graph displayed AND in the lower left corner displayed AND in the lower left corner an x= with a flashing cursor. an x= with a flashing cursor.

4.4. Just enter the time you want the Just enter the time you want the concentration for and voila! concentration for and voila!

Exercise 12.3Exercise 12.3

Using the data given in Ex. 12.2 (shown below), Using the data given in Ex. 12.2 (shown below), calculate calculate

[N[N22OO55] at 150 s after the start of the reaction.] at 150 s after the start of the reaction.

[N[N22OO55]] Time (s) Time (s)0.10000.1000 0 00.07070.0707 50 500.05000.0500 1001000.02500.0250 2002000.01250.0125 3003000.006250.00625 400400

Calculate the [NCalculate the [N22OO55] at the following times:] at the following times: 200 s200 s 400 s 400 s 600 s 600 s 1,000 s 1,000 s

HALF-LIFE AND REACTION RATE HALF-LIFE AND REACTION RATE FOR FIRST ORDER REACTIONS, FOR FIRST ORDER REACTIONS, tt1/21/2

The time required for one half of The time required for one half of one one

of the reactants to disappear.of the reactants to disappear.

[A] = ½[A][A] = ½[A]oo or or [A] [A] = ½ = ½ [A][A]oo

so... ln so... ln [A] [A] = = kk t t½½

[A][A]oo/2/2

and... ln 2 = tand... ln 2 = t½½

Rearrange, evaluate ln 2 and solve for Rearrange, evaluate ln 2 and solve for

tt½½ and you get and you get

tt½½ = = 0.693 0.693

kk

““Half life is INDEPENDENT OF Half life is INDEPENDENT OF

ORIGINALORIGINAL C CONCENTRATION for 1ONCENTRATION for 1stst

order!!!”order!!!”

Exercise 12.4Exercise 12.4

A certain first-order reaction has a A certain first-order reaction has a

half-life of 20.0 minutes.half-life of 20.0 minutes.

a. Calculate the rate constant for a. Calculate the rate constant for this this

reaction.reaction.

b. How much time is required for b. How much time is required for this reaction to be 75% complete?this reaction to be 75% complete?

HALF-LIFE AND REACTION RATE HALF-LIFE AND REACTION RATE FOR SECOND ORDER REACTIONS, FOR SECOND ORDER REACTIONS, t1/2t1/2

The time required for one half of one The time required for one half of one

of the reactants to disappear.of the reactants to disappear.

[A] = ½[A][A] = ½[A]oo or or [A] [A] = ½ = ½

[A][A]oo

so...so...

1 1 = = kk t t½½ + + 1 1 [A] [A]oo/2/2 [A][A]oo

Rearrange ,Rearrange ,

2 2 - - 1 1 = = kk t t½ ½

[A][A]oo [A] [A]oo

kk t t½½ = = 1 1 solve for t solve for t½½, ,

[A][A]oo

tt½½ = = 1 1 for a 2 for a 2ndnd order rxn. order rxn.

kk[A][A]oo

ExerciseExercise

The rate constant for the first order The rate constant for the first order transformation of cyclopropane to transformation of cyclopropane to propene is 5.40 x 10propene is 5.40 x 10-2-2/hr. /hr.

-What is the half-life of this reaction? -What is the half-life of this reaction?

-What fraction of the cyclopropane -What fraction of the cyclopropane remains after 51.2 hours? remains after 51.2 hours?

-What fraction remains after 18.0 -What fraction remains after 18.0 hours?hours?

ExerciseExercise

For the reaction of (CHFor the reaction of (CH33))33CBr with OH CBr with OH --,,

(CH(CH33))33CBr + OH CBr + OH -- →→ (CH (CH33))33COH + Br COH + Br --

The following data were obtained in The following data were obtained in

the laboratory:the laboratory:

TIME (s)TIME (s) [(CH [(CH33))33CBr]CBr]

00 0.1000.100

3030 0.0740.074

6060 0.0550.055

9090 0.0410.041

Plot these data as ln [(CHPlot these data as ln [(CH33))33CBr] CBr]

versus time. Sketch your graph.versus time. Sketch your graph.

Is the reaction first order or Is the reaction first order or second second

order? order?

What is the value of the rate What is the value of the rate

constant?constant?

Exercise 12.5Exercise 12.5

Butadiene reacts to form its Butadiene reacts to form its dimer dimer

according to the equation:according to the equation:

2 C2 C44HH66 (g) (g) C C88HH1212 (g) (g)

The following data were collected The following data were collected for this reaction at a given for this reaction at a given temperature:temperature:

[C[C44HH66]] Time ( Time ( 1 s) 1 s)0.010000.01000 0 00.006250.00625 100010000.004760.00476 180018000.003700.00370 280028000.003130.00313 360036000.002700.00270 440044000.002410.00241 520052000.002080.00208 62006200

What is the order of this reaction? What is the order of this reaction? Explain. Explain.

Sketch your graph as part of your Sketch your graph as part of your explanation. Write the rate law explanation. Write the rate law

expression:expression:

Continued:Continued: [C[C44HH66]] Time ( Time ( 1 s) 1 s)

0.010000.01000 0 00.006250.00625 100010000.004760.00476 180018000.003700.00370 280028000.003130.00313 360036000.002700.00270 440044000.002410.00241 520052000.002080.00208 62006200

-What is the value of the rate constant for this reaction?-What is the value of the rate constant for this reaction?-What is the half-life for the reaction under the -What is the half-life for the reaction under the

conditions conditions of this experiment? of this experiment?

HALF-LIFE AND REACTION RATE HALF-LIFE AND REACTION RATE FOR ZERO-ORDER REACTIONS, FOR ZERO-ORDER REACTIONS, tt1/21/2

The time required for one half of one of The time required for one half of one of

the reactants to disappear, BUT the reactants to disappear, BUT

Rate = Rate = kk[A][A]00 = = kk (a big fat 1) = (a big fat 1) = kk

Integrated rate law is: Integrated rate law is:

[A] = -[A] = -kkt + [A]t + [A]oo

[A] = ½[A][A] = ½[A]oo or or [A] [A] = ½ = ½

[A][A]oo

so... so...

[A][A]o o = - k t = - k t½½ + [A] + [A]oo

2 2

KK t t½½ = = [A] [A]oo 22kksolve for tsolve for t½½: :

tt½½ = = [A] [A]oo 22kk

for a ZERO order rxn. for a ZERO order rxn.

Zero-order reactions are most Zero-order reactions are most often encountered when a often encountered when a substance such as a metal substance such as a metal surface or an enzyme is required surface or an enzyme is required for the reaction to occur. for the reaction to occur.

The enzyme or catalyst may The enzyme or catalyst may become saturated and therefore become saturated and therefore an increase in the an increase in the [reactant/substrate] has no [reactant/substrate] has no effect on the rate.effect on the rate.

INTEGRATED RATE LAWS FOR INTEGRATED RATE LAWS FOR REACTIONS WITH MORE THAN REACTIONS WITH MORE THAN ONE REACTANTONE REACTANT

Must [still] be determined by Must [still] be determined by experiment! But we use a experiment! But we use a technique called technique called “swamping”. “swamping”.

Flood the reaction vessel with Flood the reaction vessel with high concentrations of all but high concentrations of all but one reactant and perform the one reactant and perform the experiment. experiment.

The reactants at high The reactants at high concentrations like say, 1.0 M concentrations like say, 1.0 M compared to the reactant compared to the reactant with a low concentration say, with a low concentration say, 1.0 x 101.0 x 10-3-3 M, stay the same. M, stay the same.

““In English”—the rate is now In English”—the rate is now dependent on the concentration dependent on the concentration of the little guy since the big of the little guy since the big guy’s aren’t changing, therefore:guy’s aren’t changing, therefore:

rate = rate = kk’ [little guy]’ [little guy]

We now re-write the rate as a We now re-write the rate as a pseudo-rate-lawpseudo-rate-law and and kk’ is a ’ is a pseudo-rate-constantpseudo-rate-constant

This is what is happening This is what is happening in in

the Crystal Violet lab!the Crystal Violet lab!

A SUMMARYA SUMMARY

12.6 REACTION 12.6 REACTION MECHANISMS MECHANISMS

The sequence of bond-making and The sequence of bond-making and bond-breaking steps that occurs bond-breaking steps that occurs during the conversion of reactants to during the conversion of reactants to products.products.

Must be determined by experiment! Must be determined by experiment!

Must agree with overall stoichiometry Must agree with overall stoichiometry AND the experimentally determined AND the experimentally determined rate law!rate law!

ELEMENTARY STEPSELEMENTARY STEPS

MolecularityMolecularity--number of molecules that --number of molecules that

participate in an atomic rearrangementparticipate in an atomic rearrangement unimolecular: involves one reactant unimolecular: involves one reactant

moleculemolecule bimolecular: involves a collision bimolecular: involves a collision

between two reactant moleculesbetween two reactant molecules termolecular: simultaneous collision termolecular: simultaneous collision

between three reactant molecules between three reactant molecules [very rare!]*[very rare!]*

RATE EXPRESSIONS FOR RATE EXPRESSIONS FOR ELEMENTARY STEPSELEMENTARY STEPS

The rate expression cannot be The rate expression cannot be predicted from overall predicted from overall stoichiometry. stoichiometry.

The rate expression of an The rate expression of an elementary step is given by the elementary step is given by the product of the rate constant and product of the rate constant and the concentrations of the reactants the concentrations of the reactants in the step.in the step.

THE PHYSICAL SIGNIFICANCE OF THE PHYSICAL SIGNIFICANCE OF RATE EXPRESSIONS FOR RATE EXPRESSIONS FOR ELEMENTARY STEPSELEMENTARY STEPS

the more molecules the more the more molecules the more collisions, the faster the ratecollisions, the faster the rate

the faster the molecules are the faster the molecules are moving, the more likely they moving, the more likely they will collide, the faster the ratewill collide, the faster the rate

MOLECULARITY AND MOLECULARITY AND ORDERORDER

an an elementary step elementary step is a is a reaction whose rate law can be reaction whose rate law can be written from its molecularitywritten from its molecularity

NOT true of the overall reaction NOT true of the overall reaction order!order!

ExerciseExercise

Nitrogen oxide is reduced by Nitrogen oxide is reduced by hydrogen hydrogen

to give water and nitrogen,to give water and nitrogen,

2 H2 H2(g) 2(g) + 2 NO+ 2 NO(g)(g) →→ N N2(g)2(g) + 2 H + 2 H22OO(g)(g)

One possible mechanism to One possible mechanism to

account for this reaction is:account for this reaction is:

2 NO2 NO(g)(g) N N22OO2(g)2(g)

NN22OO2(g)2(g) + H + H2(g)2(g) →→ N N22OO(g)(g) + H + H22OO(g)(g)

NN22OO(g)(g) + H + H2(g)2(g) →→ N N2(g)2(g) + H + H22OO(g)(g)

What is the molecularity of What is the molecularity of each of the three steps? each of the three steps?

Show that the sum of these Show that the sum of these elementary steps is the net elementary steps is the net reaction.reaction.

REACTION REACTION MECHANISMS AND MECHANISMS AND RATE EXPRESSIONSRATE EXPRESSIONS

determined by experimentdetermined by experiment the rate of the overall reaction the rate of the overall reaction

is limited by, and is exactly is limited by, and is exactly equal to, the combined rates equal to, the combined rates of all elementary steps of all elementary steps up to up to and including the slowest and including the slowest step in the mechanismstep in the mechanism

the slowest step is the the slowest step is the rate rate determining stepdetermining step

reaction intermediatereaction intermediate----produced in one step but produced in one step but consumed in another. consumed in another.

catalystcatalyst--goes in, comes out --goes in, comes out unharmed and DOES NOT unharmed and DOES NOT show up in the final rxn.show up in the final rxn.

Exercise 12.6Exercise 12.6

The balanced equation for the The balanced equation for the reaction reaction

of the gases nitrogen dioxide and of the gases nitrogen dioxide and

fluorine isfluorine is

2 NO2 NO22 (g) + F (g) + F22 (g) (g) 2 NO 2 NO22F (g)F (g)

The The experimentallyexperimentally determined rate determined rate

law islaw is

Rate = Rate = kk [NO [NO22][F][F22]]

Exercise 12.6, ContinuedExercise 12.6, Continued

A suggested mechanism for the reaction is A suggested mechanism for the reaction is

NONO22 + F + F22 NO NO22F + F SlowF + F Slow

F + NOF + NO22 NO NO22F FastF Fast

Is this an acceptable mechanism? That is, Is this an acceptable mechanism? That is, does it satisfy the two requirements? does it satisfy the two requirements?

Justify.Justify.

Given the following balanced equation:Given the following balanced equation:

2A + B 2A + B C + D + F C + D + F

The experimentally determined rate law isThe experimentally determined rate law is

rate = k[A][B]rate = k[A][B]

A suggested mechanism for the reaction isA suggested mechanism for the reaction is

B B M + F fast, equilibrium M + F fast, equilibrium

M + A M + A C + X slow C + X slow

A + X A + X D fast D fast

Is this an acceptable mechanism? Justify.Is this an acceptable mechanism? Justify.

ExampleExample

12.7 A MODEL FOR 12.7 A MODEL FOR KINETICSKINETICS

Generally reactions occur more Generally reactions occur more rapidly at higher temperatures rapidly at higher temperatures than at lower temperatures. than at lower temperatures.

The rate generally doubles for The rate generally doubles for every 10 K rise in temperature. It’s every 10 K rise in temperature. It’s an exponential increase!an exponential increase!

TRANSITION STATE TRANSITION STATE THEORYTHEORY

energy barrier must be overcomeenergy barrier must be overcome reaction energy diagramreaction energy diagram [humpy [humpy

diagrams]diagrams] transition state energy--max of rxn. E transition state energy--max of rxn. E

diagramdiagram activated complex--deformed molecules activated complex--deformed molecules

in their transition state, formed at the Ein their transition state, formed at the Etsts----unstable, can go either way!unstable, can go either way!

Activation energy, E*, Ea--energy a Activation energy, E*, Ea--energy a reacting molecule must absorb from its reacting molecule must absorb from its environment in order to react.environment in order to react.

Kinetics and net energy of Kinetics and net energy of reactionreaction

Potential Potential

Energy Energy

-relationship between kinetics and thermodynamics-relationship between kinetics and thermodynamics

-endo -endothermic—products end up higher in energy than thermic—products end up higher in energy than reactants reactants - heat is absorbed or taken into or added to the system and - heat is absorbed or taken into or added to the system and given given a positive sign. a positive sign.

-exo -exothermic—pictured above, products are of lower energy thermic—pictured above, products are of lower energy than than reactants - heat is lost to the surroundings and given a reactants - heat is lost to the surroundings and given a negative negative sign. sign.

Reaction Coordinate (time)

X2 + Y2

2 XY

Eactivation -- a kinetic quantity

E H--a thermodynamic quantity

APAP

Pick up handout and get a Pick up handout and get a calculator if you don’t have one. calculator if you don’t have one.

Get out your notes.Get out your notes.

COLLISION THEORYCOLLISION THEORY

Assumes molecules must collide Assumes molecules must collide in in

order to react!order to react!

Hindered by concentration, Hindered by concentration,

temperature and geometry--temperature and geometry--# of # of

effective collisionseffective collisions

THE EFFECT OF TEMPERATURE OF THE EFFECT OF TEMPERATURE OF REACTION RATE: ARRHENIUS REACTION RATE: ARRHENIUS EQUATIONEQUATION

kk = reaction rate constant = reaction rate constant

= Ae= Ae-E*/RT-E*/RT

R is the “energy” R R is the “energy” R

Or, 8.31 x 10Or, 8.31 x 10-3-3kJ/K•mol kJ/K•mol

A is the frequency factor units of A is the frequency factor units of

L/(mol • s) & depends on the L/(mol • s) & depends on the frequency of collisions and the frequency of collisions and the fraction of these that have the fraction of these that have the correct geometry--correct geometry--# of effective # of effective collisionscollisions

ee-E*/RT-E*/RT is always less than 1 and is is always less than 1 and is the fraction of molecules having the fraction of molecules having the minimum energy required the minimum energy required for reactionfor reaction

*Notice in the equation: As "Ea" *Notice in the equation: As "Ea" increases, "k" gets smaller and increases, "k" gets smaller and thus, the rate would decrease. thus, the rate would decrease.

Also, notice that as "T" goes up, Also, notice that as "T" goes up, "k" increases and so the rate "k" increases and so the rate would also increase. would also increase.

Applying the laws of logarithms, Applying the laws of logarithms,

taking thetaking the natural log of both natural log of both

sidessides, ln, we can rewrite the , ln, we can rewrite the

equation: equation:

ln k = ln A - ln k = ln A - Ea Ea

RTRT

Rewrite in the form of an equation Rewrite in the form of an equation for for

a a straight linestraight line, we get: , we get:

ln k = - ln k = - Ea Ea ( (1)1) + ln + ln [A][A]

R (T)R (T)

Taking this equation, plot 1/T vs. ln Taking this equation, plot 1/T vs. ln [k], [k],

and get a straight line. and get a straight line.

From the straight line, find the From the straight line, find the slopeslope

and then find the and then find the activation energy. activation energy.

slope = - slope = - EaEa

RR

so ... Ea = - (R) (slope)so ... Ea = - (R) (slope)

OR, OR, find Ea find Ea from data given from data given

mathematically:mathematically:

Used to calculateUsed to calculate

-value of activation energy -value of activation energy from temperature dependence from temperature dependence of the rate constantof the rate constant

-rate constant for a given -rate constant for a given temp - if the E* [also known as temp - if the E* [also known as EEaa] and A factor are known.] and A factor are known.

Example - ArrheniusExample - Arrhenius

Calculate the activation energy Calculate the activation energy for for

the following set of data: the following set of data:

T (°C) T (°C) k (l/mol- s)k (l/mol- s)

33 1.4 x 10 1.4 x 10-3-3

1313 2.9 x 10 2.9 x 10-3-3

2424 6.2 x 10 6.2 x 10-3-3

3333 1.2 x 10 1.2 x 10-2-2

Points to remember!!Points to remember!!

1. Ea is smaller; k is greater; 1. Ea is smaller; k is greater; the the

reaction is faster. reaction is faster.

2. Ea is greater; k is smaller; the 2. Ea is greater; k is smaller; the

reaction is slower. reaction is slower.

ExerciseExercise

The colorless gas dinitrogen tetroxide The colorless gas dinitrogen tetroxide decomposes to the brown gas NOdecomposes to the brown gas NO22 in in a first order reaction with a value of: a first order reaction with a value of:

kk = 4.5 x 10 = 4.5 x 1033/s at /s at 274K. 274K.

If If kk is 1.00 x 10 is 1.00 x 1044/s at 283K, what is the /s at 283K, what is the energy of activation?energy of activation?

Exercise 12.7Exercise 12.7The reaction: 2NThe reaction: 2N22OO33(g) (g) →→ 4 NO 4 NO22(g) + (g) +

OO22(g)(g)

Was studied at several temperatures and Was studied at several temperatures and

the following values of k were obtained:the following values of k were obtained:

Calculate the value of Ea for this reaction. Calculate the value of Ea for this reaction.

Sketch your graph.Sketch your graph.

Exercise 12.8Exercise 12.8

The gas-phase reaction between The gas-phase reaction between

methane and diatomic sulfur is methane and diatomic sulfur is given given

by the equation:by the equation:

CHCH44 (g) + 2S (g) + 2S22 (g) (g)

CSCS22 (g) + 2H (g) + 2H22S (g)S (g)

At 550° C the rate constant for this At 550° C the rate constant for this

reaction is 1.1 Mreaction is 1.1 M-1 -1 s s-1-1 and at 625° C and at 625° C

the rate constant is 6.4 Mthe rate constant is 6.4 M-1 -1 s s-1-1. .

Using these values, calculate EUsing these values, calculate Eaa for for

this reaction. this reaction.

12.8 CATALYSIS12.8 CATALYSIS

Alter the mechanism so Alter the mechanism so the activation energy the activation energy barrier can be lowered.barrier can be lowered.

Catalysts are not Catalysts are not

altered during the altered during the reaction--they serve to reaction--they serve to lower the activation lower the activation energy and speed up energy and speed up the reaction by offering the reaction by offering a different pathway for a different pathway for the reactionthe reaction

E is NOT changed for the processE is NOT changed for the process

biological catalysts are enzymes--biological catalysts are enzymes--proteins w/ specific shapes proteins w/ specific shapes

ATP synthetase is the most ATP synthetase is the most important enzyme in the human important enzyme in the human body!body!

HETEROGENEOUS HETEROGENEOUS CATALYSTCATALYST

different phase different phase than reactantsthan reactants

usually involves usually involves gaseous gaseous reactants reactants adsorbed on the adsorbed on the surface of a surface of a solid catalystsolid catalyst

adsorptionadsorption—refers to the —refers to the collection of one substance on collection of one substance on the surface of anotherthe surface of another

absorptionabsorption—refers to the —refers to the penetration of one substance penetration of one substance into another; water is absorbed into another; water is absorbed by a spongeby a sponge

hydrogenation of unsaturated hydrogenation of unsaturated hydrocarbons—especially important hydrocarbons—especially important in converting unsaturated fats [oils] in converting unsaturated fats [oils] into saturated fats [solids like into saturated fats [solids like Crisco]Crisco]

C=C bonds are converted into C-C C=C bonds are converted into C-C bonds by adding a pair of hydrogens bonds by adding a pair of hydrogens “across the double bond”“across the double bond”

H H H H C=C + H2 (g) H C C H H H H H ethene (ethylene) ethane

A simple example of A simple example of hydrogenation involves hydrogenation involves ethylene: ethylene:

This reaction uses a solid catalyst This reaction uses a solid catalyst in the form of Pt, Pd, or Ni. The in the form of Pt, Pd, or Ni. The hydrogen and ethylene adsorb on hydrogen and ethylene adsorb on the catalyst surface where the the catalyst surface where the reaction occurs. The catalyst reaction occurs. The catalyst allows for metal-hydrogen allows for metal-hydrogen interactions that weaken the strong interactions that weaken the strong H-H bonds and facilitate the H-H bonds and facilitate the reaction. reaction.

Typically involves 4 Typically involves 4 steps:steps:

1.1. Adsorption and activation of Adsorption and activation of the reactantsthe reactants

2.2. Migration of the adsorbed Migration of the adsorbed reactants on the surfacereactants on the surface

3.3. Reaction of the adsorbed Reaction of the adsorbed substances substances

4.4. Escape, or desorption, of the Escape, or desorption, of the productsproducts

Catalytic converters are Catalytic converters are also heterogeneous also heterogeneous catalysts.catalysts. They have been They have been

placed in placed in automobiles since automobiles since 1974. [I know! 1974. [I know! You weren’t born You weren’t born yet! Don’t rub it yet! Don’t rub it in.] Gasoline in.] Gasoline containing lead containing lead RUINS the RUINS the catalytic catalytic converter in your converter in your car! car!

HOMOGENEOUS HOMOGENEOUS CATALYSTCATALYST

exists in the same phase as the exists in the same phase as the reacting molecules.reacting molecules.

Freons or chlorofluorocarbons Freons or chlorofluorocarbons [CFC’s] were used until recently [CFC’s] were used until recently as refrigerants and as as refrigerants and as propellants in aerosol cans. propellants in aerosol cans.

Freon-12 (CClFreon-12 (CCl22FF22) is relatively ) is relatively inert and thus remains in the inert and thus remains in the environment for a long time. environment for a long time.

Eventually they migrate upward Eventually they migrate upward into the upper atmosphere and into the upper atmosphere and are decomposed by high-energy are decomposed by high-energy light.light.

Among the decomposition Among the decomposition

products are chlorine products are chlorine atoms:atoms:

CClCCl22FF22 (g)(g) →→ CClF CClF22 (g)(g) + Cl + Cl (g)(g)

These chlorine atoms can These chlorine atoms can catalyze catalyze

the decomposition of ozone:the decomposition of ozone:

Cl Cl (g)(g) + O + O3 (g)3 (g) ClO ClO (g)(g) + O + O2 (g)2 (g)

OO(g)(g) + + ClOClO(g)(g) ClCl(g)(g) + O + O2 (g)2 (g)

OO(g) (g) + O + O3(g)3(g) 2 O 2 O2(g)2(g)