CHEM 3310

Chemical Kinetics

Reaction Mechanism Reaction Mechanism

What is a 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!

What is a Reaction Mechanism?

Determine the rate law by experiment

Devise a reaction 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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What is a Reaction Mechanism?

• Reaction mechanism is a collection of steps that account for the waythe reactants become products. These steps are called elementary steps.

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

• 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 orp yelementary step.

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What is a Reaction Mechanism?

• Each elementary step occurs at different rates.

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

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

• The proposed mechanism must be consistent with the observedrate law of the reactionrate law of the reaction.

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l

What is a Reaction Mechanism?

Example:

2 O3 (g) 3 O2 (g)(balanced reaction) (balanced reaction)

This is NOT a reaction mechanism.

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Th Ch C l

Reaction Mechanism

The Chapman Cycle

the natural creation and destruction ofand destruction of

ozone in the stratosphere

Ozone in the stratosphere is good. It absorbs solar UV and

protects us.

Ozone in the troposphere is

smog, concentrated around urban areas.

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The Chapman Cycle is a set of reaction mechanism that describesthe natural creation and destruction of ozone in the stratosphere

Reaction Mechanism

1. A UV photon breaks apart a O2 molecule to atomic O. 2. An O atom collides

with a O2 molecule

the natural creation and destruction of ozone in the stratosphere.

with a O2 molecule forms O3.

Production of ozone

Destruction of ozone

3. O3 absorbs a UV photon and dissociates

into an O and O2molecule.

4. O3 could react with an O atom and forms 2 O

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atom and forms 2 O2molecules.

A rate determining step!

The Chapman CycleWhat is a Reaction Mechanism?

1.

1. 2 O2 4 O Source of O3

2. 2. 4 O + O2 2 O3

3. O3 O2 + O

of O3

Use up

4. O3 + O 2 O2

Add up the steps

of O3

Ch C l ll l

Nothing to add up; nothing really happens

34.

Chapman Cycle: null cycleA steady-state concentration

of ozone is produced.

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3.

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 – 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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Experimental evidence shows that k is large for the step

Reaction MechanismExperimental evidence shows that k is large for the step

O3 (g) + UV  O (g) + O2 (g)                (destruction stepin Chapman cycle)

in the presence of UV.

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Reaction Mechanism

fThe rate law that governs the depletion of ozone

2 O3 (g) 3 O2 (g)

is experimentally determined to be overall first order.

12

23

2

23 ]O[]O[

]O[]O[ kkRate

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

N t i l ti - 2nd order with respect to O3.- Negative 1st order with respect to O2.- Overall reaction order = 1st order

Not a simple reaction mechanism.

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- Overall reaction order = 1 order.

Reaction MechanismThe Chapman Cycle – the natural

Reaction 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.

A proposed reaction mechanism for ozone destruction:for ozone destruction:

Step 1a: O3 O2 + O (fast)

St 1b

Step 2

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

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

Proposed reaction mechanism for ozone destruction:

Step 1a: O3 O2 + O (fast)

Step 1b: O2 + O O3 (fast)

O3 O2 + O Step 1:

Step 2: O + O 2 Op 2 3 ( )[reverse of Step 1a]

Step 2: O3 + O 2 O2 (slow)

Step 2: O3 + O 2 O2

Add up the steps

Overall reaction is 2 O3 (g) 3 O2 (g)

Add up the steps

Overall reaction is 2 O3 (g) 3 O2 (g)

Is this mechanism consistent with the experimentally determined rate law?

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p y

Reaction Mechanism

Some terminologies:

O3 O2 + O Step 1: (fast)

g

• Intermediate – These are species that are formed in one elementary

Step 2: O3 + O 2 O2 (slow)

p ystep and consumed in the subsequent step.

Atomic O is an intermediate

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

Step 2 is the rate determining step

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p g p

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 Clslowly migrate to the stratosphere

Chlorine is the catalyst

Production of chlorine monoxide

stratosphere. yfor Step 2.

Step 2: Cl + O3 ClO + O2

reacts with ozone

Step 3: ClO + O3 Cl + 2O2

• Cl is a catalystOverall reaction for Step 2 & Step 3

2 O3 (g) 3 O2 (g)

• Cl is a catalyst

• ClO is an intermediate

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Reaction Mechanism

The catalytic destruction of ozone in the stratosphere involves nitrogen oxides as a

Step 1: NO + O3 NO2 + O2

catalyst.

Step 2: NO2 + O NO + O2NO is the Catalyst.

• NO is a catalyst

• NO2 is an intermediate

Overall reaction for Step 1 and Step 2

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

The high temperature reaction of N and O in supersonic airplane enginesThe high temperature reaction of N2 and O2 in supersonic airplane engines produces NO. The airplanes' exhaust injects additional nitrogen oxide into the stratosphere results in accelerating ozone depletion.

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Different destruction mechanisms can predominate under special circumstances.

Reaction Mechanism

Determine the rate law by experiment

Devise a reaction 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

Next, we will learn how to derive the rate law from

a proposed reaction h i

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mechanism.