CHEM 3310 CHEM 3310

Chemical KineticsChemical Kinetics

Reaction Mechanism

Reaction Mechanism

Reaction mechanism is a collection of steps that account for the way the reactants become products.

This is not the same as a balanced chemical equation.

A balanced chemical equation does not tell us how the reactants become the products. It tells us the before (reactants) and the after (products) states of the overall processoverall process.

Example: Photosynthesis

2612622 666 OOHCOHCO light

This reaction occurs in many steps!

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This reaction occurs in many steps!

Reaction Mechanism

Determine the rate law by experiment

Devise a mechanism

Predict the rate lawfor the mechanism

If the predicted and experimental rate laws

agree

If the predicted and experimental rate laws do

not agree

Look for additional supporting evidence

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Reaction Mechanism

Ozone is observed to be depleting in the Stratosphere

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The Chapman Cycle – the natural creation and destruction of ozone in the stratosphere

Reaction Mechanism

A UV photon breaks apart a O2 molecule to atomic O. An O atom collides

with a O2 molecule

The Chapman Cycle the natural creation and destruction of ozone in the stratosphere

with a O2 molecule forms O3.

O3 absorbs a UV photon and dissociates into an O

and O2 molecule.

O3 could react with an O atomand forms 2 O molecules

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and forms 2 O2 molecules.

A rate determining step!

Reaction Mechanism

The Chapman Cycle – the naturalReaction mechanism is a collection of steps that account for the way the reactants become products

creation and destruction of ozone in the Stratosphere

become products.

Reaction Mechanism:

Step 1a: O3 O2 + O (fast)

St 1b

Step 2

Step 1b: O2 + O O3 (fast) Step 1bp 2 3 ( )[reverse of Step 1]

Step 2: O3 + O 2 O2 (slow)

Step 1a

Step 2 is a slow step becausethere are relatively few

O atoms around.

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Step 1a

• Reaction mechanism is a collection of steps that account for the way

Reaction Mechanism

p ythe reactants become products. These steps are called elementarysteps.

• The reaction steps involve the breaking of chemical bonds and/or the

The Chapman Cycle

p gmaking of new bonds.

(The kinetics of the production and destruction of ozone in the stratosphere.)

Step 1a: O3 O2 + O (fast)O3 O2 + O Step 1:

Step 1b: O2 + O O3 (fast)[reverse of Step 1]

3 2p

Step 2: O3 + O 2 O2

Step 2: O3 + O 2 O2 (slow)

Overall reaction is

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2 O3 (g) 3 O2 (g)

Reaction Mechanism – Elementary Reaction

O3 O2 + OStep 1: (fast)O3 O2 O Step 1:

Step 2: O3 + O 2 O2 (slow)

(fast)

• Each step in a reaction mechanism is a description of thechemical reaction.

• Reactions may occur all at once or through several discrete steps.Each of these processes is known as an elementary reaction orelementary step.

• Elementary step could be reversible (i.e. equilibrium is reached).When this is present in a reaction mechanism, this is usually thefast step.fast step.

• Each elementary step occurs at different rates.

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Reaction Mechanism – Elementary Reaction

O3 O2 + OStep 1: (fast)

• The step(s) in a reaction mechanism must add to give the overall

O3 O2 O Step 1:

Step 2: O3 + O 2 O2 (slow)

(fast)

The step(s) in a reaction mechanism must add to give the overall balanced chemical reaction.

• The proposed mechanism must be consistent with the observed

• In predicting the rate law for an elementary step, the exponents for the concentration terms are the same as the stoichiometric coefficients

rate law of the reaction.

the concentration terms are the same as the stoichiometric coefficients.

Example of a one-step mechanism:

NO + O3 NO2 + O2

k ]O[]NO[]O[d]NO[d]O[d]NO[d 223

1 1

1 1

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k-- ]O[]NO[dtdtdtdt 3

223 1 1

Reaction Mechanism - Terminologies

M l l it Th b f l l t th t t id• Molecularity – The number of molecules or atoms on the reactant side that collides to produce the reaction of that elementarystep.

Uni-, Bi-, Termolecular

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Involving 1 species

Involving 2 species

Involving 3 species

Reaction Mechanism - Terminologies

O3 O2 + OStep 1: (fast)O3 O2 O Step 1:

Step 2: O3 + O 2 O2 (slow)

(fast)

• Intermediate – These are species that are formed in one elementarystep and consumed in the subsequent step.

At i O i i t di tAtomic O is an intermediate

• Rate determining step – Elementary steps do not occur at the samerate The slowest step is called therate. The slowest step is called the rate determining step. It determines the overall rate of the reaction.

Step 2 is the rate determining step

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Reaction Mechanism

Example: The following is a reaction mechanism:

Step 1: NO2 + NO2 NO3 + NO (slow)Step 1: NO2 + NO2 NO3 + NO (slow)Step 2: NO3 + CO NO2 + CO2 (fast)

1. Identify the intermediate.

NO3 is the intermediate.

2. What is the overall reaction?

NO2 + CO CO2 + NO2 2

3. Which is the rate determining step?

Step 1

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Step 1

Reaction Mechanism – Back to ozone depletion

Observe a decrease in the overall levels ofObserve a decrease in the overall levels of ozone in the stratosphere in the past decades.

We are losing protection from UV light.2 O3 (g) 3 O2 (g)

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Reaction MechanismDepletion of ozoneDepletion of ozone

Ozone depletion in thestratosphere is caused byh i l th t d ichemicals that are used in

refrigerators and air conditioners,

chlorofluorocarbons (CFCs).( )

Image credit: http://australia.jrn.msu.edu/2000/work/projects/environment/ozonepage.html

LASP (Laboratory for Atmospheric and Space Physics)

I dit

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Image credit:http://lasp.colorado.edu/home/blog/2010/09/30/lasp-researchers-launch-balloons-to-study-ozone-over-antarctica/

Reaction Mechanism

The catalytic destruction of ozone in the stratosphere involves chlorine as a catalyst.The catalytic destruction of ozone in the stratosphere involves chlorine as a catalyst.

Upon absorption of UV light, the CFCs initiate a catalyzed reaction mechanism.

Step 1: CF2Cl2 CF2Cl + Clh

Step 1 generates Cl, chlorine. The released Cl slowly migrate to the stratosphere.

Chl i ip 2 2 2

Production of chlorine monoxide

Chlorine is the catalyst for Step 2.

St 2 Cl O ClO OStep 2: Cl + O3 ClO + O2

S C O O C O

Overall reaction for Step 2 and Step 3

Step 3: ClO + O3 Cl + 2O2

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2 O3 (g) 3 O2 (g)

Reaction Mechanism

The rate law that governs the Chapman Cycle is

12

23

2

23 ]O[]O[

]O[]O[ kkRate2

Increase [O3 ] increases the reaction rate.Increase [O2] decreases the reaction rate. Negative exponent![ 2]

- 2nd order with respect to O3.- Negative 1st order with respect to O

g p

- Negative 1 order with respect to O2.- Overall reaction order = 1st order.

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Reaction Mechanism

Experimental evidence shows that k is large in the presence of UV.

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Reaction Mechanism

Determine the rate law by experiment

Devise a mechanism

Predict the rate lawfor the mechanism

If the predicted and experimental rate laws

agree

If the predicted and experimental rate laws do

not agree

Look for additional supporting evidence

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We will learn how to predict the rate law from

a proposed reaction mechanism.

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