Rates and reaction Rates and reaction mechanismmechanism

►The reaction mechanism is the The reaction mechanism is the sequence of individual reaction steps sequence of individual reaction steps that together complete the that together complete the transformation of reactants to transformation of reactants to productsproducts

►The single reaction described by The single reaction described by chemical equation may involve several chemical equation may involve several sequential elementary sequential elementary stepssteps

Reduction of NOReduction of NO22 by CO by CO

► Overall reaction is Overall reaction is

NONO22 + CO = NO + CO + CO = NO + CO22

► Two elementary stepsTwo elementary steps

NONO22 + NO + NO22 = NO + NO = NO + NO33

NONO33 + CO = NO + CO = NO22 + CO + CO22

Reaction intermediatesReaction intermediates

► Reaction intermediateReaction intermediate: an entity : an entity created during one of the elementary created during one of the elementary steps, but consumed during a steps, but consumed during a subsequent stepsubsequent step

► Intermediates are not transition Intermediates are not transition statesstates

► Intermediate does not appear in Intermediate does not appear in overall reaction equationoverall reaction equation

Intermediates are neither Intermediates are neither product nor reactantproduct nor reactant

►Overall reaction is Overall reaction is

NONO22 + CO = NO + CO + CO = NO + CO22

NONO33 is product of first step: is product of first step:NONO22 + NO + NO22 = NO + NO = NO + NO33

NONO3 3 is consumed in second stepis consumed in second stepNONO33 + CO = NO + CO = NO22 + CO + CO22

NONO33 is an intermediate is an intermediate

NONO22 and CO are reactants and CO are reactants

NO and CONO and CO22 are products are products

MolecularityMolecularity

►The number of molecules that The number of molecules that participate in the stepparticipate in the step Unimolecular involves one moleculeUnimolecular involves one molecule Bimolecular involves two moleculesBimolecular involves two molecules Termolecular (rare) involves three Termolecular (rare) involves three

moleculesmolecules

Laws and mechanismLaws and mechanism

► The rate law for an elementary (single) The rate law for an elementary (single) reaction step follows directly from the reaction step follows directly from the molecularitymolecularity

Rate laws for overall Rate laws for overall reactionsreactions

► If overall reaction occurs in single step, If overall reaction occurs in single step, rate law is obviously the same as for rate law is obviously the same as for that stepthat step

►For the single-step reactionFor the single-step reactionCHCH33Br + OHBr + OH-- = Br = Br-- + CH + CH33OHOH

]][[ 3

3

OHBrCHk

t

BrCHRate

Slow steps, bottlenecks and rate Slow steps, bottlenecks and rate equationsequations

► For two or more steps, one step will For two or more steps, one step will be slower than the rest. This is be slower than the rest. This is rate rate limiting.limiting. The reaction rate will not The reaction rate will not be faster than the slowest stepbe faster than the slowest step

► In the reactionIn the reaction

NONO22 + CO = NO + CO + CO = NO + CO22

1.1. NONO22 + NO + NO22 = NO + NO = NO + NO33 slowslow

2.2. NONO33 + CO = NO + CO = NO22 + CO + CO22 fastfastThe molecules in the fast step do The molecules in the fast step do notnot participate in participate in

the rate equationthe rate equation 22

2 ][NOkt

NORate

Rate-limiting step has Rate-limiting step has highest Ehighest Eaa

Rate laws inform about reaction Rate laws inform about reaction mechanismmechanism

► A logical sequence for determining reaction A logical sequence for determining reaction mechanisms from observed kinetic datamechanisms from observed kinetic data

Some like it hot: The Arrhenius Some like it hot: The Arrhenius equationequation

► It is well known that all chemical It is well known that all chemical processes go faster as temperature processes go faster as temperature increasesincreases

►This suggestsThis suggests Not all collisions result in productNot all collisions result in product Fraction of collisions that results in Fraction of collisions that results in

products increases with Tproducts increases with T

Collision theoryCollision theory

►Reactions only occur when molecules Reactions only occur when molecules collidecollide

►Not all collisions result in reactionsNot all collisions result in reactions Not all collisions have the right orientationNot all collisions have the right orientation Not all collisions have sufficient energyNot all collisions have sufficient energy

►Why is Why is thatthat??

Transition state and activation Transition state and activation energyenergy

► The approach of A towards BC increases The approach of A towards BC increases energy due to repulsions between electronsenergy due to repulsions between electrons

E

Maximum energy is transition Maximum energy is transition statestate

►Approach of A causes weakening of B – Approach of A causes weakening of B – C and formation of A – BC and formation of A – B

►At transition state, the energy is a At transition state, the energy is a maximummaximum

E

Over the barrierOver the barrier

►The B – C bond weakens and C leavesThe B – C bond weakens and C leaves►The A – B bond forms completelyThe A – B bond forms completely►The energy lowersThe energy lowers►Final state compared to initial state Final state compared to initial state

depends on relative strengths of the A depends on relative strengths of the A – B and B – C bonds– B and B – C bonds

E

The activation energy and The activation energy and transition statetransition state

The problem of energyThe problem of energy

► Not all molecules have same energyNot all molecules have same energy► Distribution in energy is broadDistribution in energy is broad► Distribution shifts to higher energy as T increasesDistribution shifts to higher energy as T increases► Only small fraction have sufficient energy to leap Only small fraction have sufficient energy to leap

barrierbarrier► Fraction increases with TFraction increases with T► Therefore rate increases with TTherefore rate increases with T

The exponentThe exponent► Fraction of molecules with sufficient Fraction of molecules with sufficient

energy is given byenergy is given by

► ff is very, very small: activation energy of is very, very small: activation energy of 50 kJ/mol yields value of 50 kJ/mol yields value of

ff = 10 = 10-9-9 ► Increasing T by 10Increasing T by 10ºC causes ºC causes ff to increase to increase

by a factor of 3by a factor of 3► Collision rate is much less sensitive to T Collision rate is much less sensitive to T

changechange► T-dependence of rate is determined by T-dependence of rate is determined by

fractionfraction of collisions with correct energy, of collisions with correct energy, not not totaltotal number of collisions number of collisions

RTEa

ef

The problem of orientationThe problem of orientation

►Not all (molecular) orientations are Not all (molecular) orientations are equalequal

►Fraction of collisions that has correct Fraction of collisions that has correct orientation is the orientation is the stericsteric factor – factor – pp

ReactionReaction

No reactionNo reaction

Putting it all together: Putting it all together: The Arrhenius equationThe Arrhenius equation

►Collision rateCollision rate

►Rate lawRate law►But But

]][[ BAZ

]][[]][[ BAkBApfZ

RTEa

ef

RTE

RTE aa

AepZek

Orientation factor

Collision factor

Energy factor

Experimental determination Experimental determination of Eof Eaa

►Plot of ln Plot of ln kk vs 1/T is linear vs 1/T is linear

EEaa = -R(slope) = -R(slope)

►From two temperatures:From two temperatures:

RT

EAk alnln

121

2 11ln

TTR

E

k

k a

Graphical determination of EGraphical determination of Eaa

►Exploring factors that affect Exploring factors that affect raterate

RT

EAk alnln

R

E

Td

kd a1ln

Ways over (or around) the Ways over (or around) the barrierbarrier

►Temperature increases reaction rate Temperature increases reaction rate by increasing fraction of molecules by increasing fraction of molecules with sufficient energy to jump barrierwith sufficient energy to jump barrier

►A catalyst lowers the barrier. A A catalyst lowers the barrier. A catalyst acts to increase the reaction catalyst acts to increase the reaction rate, but is not consumed itself during rate, but is not consumed itself during the reactionthe reaction

CatalysisCatalysis

► Catalysis is a fascinating field. It deals with Catalysis is a fascinating field. It deals with processes which may provide solutions for processes which may provide solutions for many of the key problems we face. It many of the key problems we face. It provides food through the ammonia provides food through the ammonia synthesis. It creates flexibility in the synthesis. It creates flexibility in the complex matrix of energy sources, energy complex matrix of energy sources, energy carriers and conversion and it contributes in carriers and conversion and it contributes in minimizing pollution.minimizing pollution.

J. Rostrup-Nielsen, Catalysis Today, 111 J. Rostrup-Nielsen, Catalysis Today, 111 (2006) 4 - 11(2006) 4 - 11

Catalysts modify the Catalysts modify the pathwaypathway

► Addition of chlorine catalyst increases rate Addition of chlorine catalyst increases rate of decomposition of ozone into Oof decomposition of ozone into O22 – reason – reason for the destruction of ozone layer by CFCsfor the destruction of ozone layer by CFCs

► Although two barriers are present, both are Although two barriers are present, both are smaller than the one without the catalyst, smaller than the one without the catalyst, and reaction rate is higherand reaction rate is higher

Without catalysts, there would Without catalysts, there would be no life at all, from microbes be no life at all, from microbes

to humans to humans ►ENZYMES are biological catalystsENZYMES are biological catalysts►Most enzymes are proteins – large Most enzymes are proteins – large

moleculesmolecules►Have correct shape to bring reactant Have correct shape to bring reactant

molecules together in correct molecules together in correct orientationorientation

Enzyme