Reaction Mechanisms Consider the following reaction: 4 HBr (g) +O 2(g) →2H 2 O (g) +2Br 2(g) There are five reactants… Typically more than 3 molecules

Reaction Mechanisms

Reaction Mechanisms Consider the following reaction: 4 HBr(g) + O2(g) → 2H2O(g) + 2Br2(g)

There are five reactants…Typically more than 3 molecules colliding at any one time is very improbable.

So reaction having 5 reactants in the bal. equation must occur in more than one step.

Even a reaction containing 2 or 3 molecules can occur in more than one step.

The actual sequence of steps which make up an overall reaction is called a reaction mechanism!

Mechanism 1 .

HBr(g) + O2(g) → HOOBr(g) slow

HBr(g) + HOOBr(g) → 2HOBr(g) fast

2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast

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Reaction Path

Mechanism 1 Cancel out identical formulas to get the overall equation.




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Reaction Path





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Reaction Path





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Reaction Path

Mechanism 1 Intermediates are produced and then consumed and cross out from right (first) and left (second). HBr(g) + O2(g) → HOOBr(g) slow



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Reaction Path

Mechanism 1 What is left is the overall reaction.




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Reaction Path

Mechanism 1 What is left is the overall reaction.




4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

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Reaction Path

Mechanism 1 Step 1, which is the slow step, is called the rate determining step and has the highest activation energy. HBr(g) + O2(g) → HOOBr(g) slow



4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

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Reaction Path

Mechanism 1 HBr(g) + O2(g) → HOOBr(g) slow



4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

A potential energy diagram for this reaction might look like this.

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Reaction Path

Mechanism 1 HBr(g) + O2(g) → HOOBr(g) slow



4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

A potential energy diagram for this reaction might look like this.

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Reaction Path

Mechanism 1




4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

highestA potential energy diagram for this Mechanism has three humpsThe slow step has the highest Ea

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Reaction Path

Ea

Ea(rev)

The Rate Determining Step

The slowest step in the reaction mechanism is called the rate-determining step. It has the highest Ea.

To increase the rate, you must increase the rate of this step. Increasing the rate of a fast step will not increase the rate of the overall reaction.

Identifying a Catalyst in a Mechanism How to find a catalyst in a reaction mechanism??

A catalyst is used in one step; it speeds up the reaction, and then is regenerated in a later step. Therefore, a catalyst will cancel out but will be first found on the left side (with reactants) in an earlier step then on the right side.

Identifying a Catalyst in a Mechanism A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step then on the right side.

C →→ C


C →→ C


C →→ C

An intermediate is produced and then consumed. It will be on the right side in an earlier step then on the left side.


C →→ C


→ II →


C →→ C


→ II →

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate:

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate:

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate: C

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate: C

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate: C CD

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: Intermediate: C CD

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: Catalyst: D Intermediate: C CD

Mechanism 2

1. A + B → C 2. C + D → CD 3. CD + E → ABE + D Overall Equation: A + B + E → ABE Catalyst: D Intermediate: C CD

Mechanism 3

Step 1 Br2 → 2Br

Step 2 Br + OCl2 → BrOCl + Cl

Step 3 Br + Cl → BrCl

Overall Reaction: Intermediates:

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Reaction Path

Mechanism 3

Step 1 Br2 → 2Br




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Reaction Path

Mechanism 3

Step 1 Br2 → 2Br



Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates:

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Reaction Path

Mechanism 3

Step 1 Br2 → 2Br



Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl

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Reaction Path

Mechanism 3

Step 1 Br2 → 2Br




ΔH =Ea (forward) = Ea (reverse) =The enthalpy of BrThe enthalpy of BrCl

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Reaction Path

800

200

400

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = Ea (reverse) =The enthalpy of Br The enthalpy of BrCl

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Reaction Path

800

200

400

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = Ea (reverse) =The enthalpy of Br The enthalpy of BrCl

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Reaction Path

800

200

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = 600 kJ Ea (reverse) =The enthalpy of Br The enthalpy of BrCl

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Reaction Path

800

200

400

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = 600 kJ Ea (reverse) = 400 kJThe enthalpy of Br The enthalpy of BrCl

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Reaction Path

800

200

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = 600 kJ Ea (reverse) = 400 kJThe enthalpy of Br 300 kJ The enthalpy of BrCl

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Reaction Path

800

200

400

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Mechanism 3

Step 1 Br2 → 2Br




ΔH = 200 kJEa (forward) = 600 kJ Ea (reverse) = 400 kJThe enthalpy of Br 300 kJ The enthalpy of BrCl 400 kJ

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Reaction Path

800

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Mechanism 4

The following mechanism shows how chlorine-containing fluorocarbons destroy the ozone layer. Ozone is O3 and O

1. CFCl3 → CFCl2 + Cl

2. Cl + O3 → ClO + O2

3. ClO + O → Cl + O2


Mechanism 4



2. Cl + O3 → ClO + O2

3. ClO + O → Cl + O2


Mechanism 4



2. Cl + O3 → ClO + O2

3. ClO + O → Cl + O2

Overall Reaction: CFCl3 + O3 + O → CFCl2 + 2O2 + Cl

Intermediates:

Mechanism 4



2. Cl + O3 → ClO + O2

3. ClO + O → Cl + O2

Overall Reaction: CFCl3 + O3 + O → CFCl2 + 2O2+ Cl

Intermediates: Cl ClO Formula of activated complex in step 2:

Determine the Missing Step of the Mechanism

Mechanism 5

A + B → C

C + D → B + AD

Overall: A + D + E → ADE


Mechanism 5

A + B → C

C + D → B + AD



Mechanism 5

A + B → C

C + D → B + AD


Put in step 3 so that you get the overall reaction


Mechanism 5

A + B → C

C + D → B + AD

AD Overall: A + D + E → ADE

Put in AD to cancel


Mechanism 5

A + B → C

C + D → B + AD

AD + → Overall: A + D + E → ADE

Put in AD to cancel


Mechanism 5

A + B → C

C + D → B + AD

AD + E → Overall: A + D + E → ADE

Put in E


Mechanism 5

A + B → C

C + D → B + AD

AD + E → ADE Overall: A + D + E → ADE

Put in ADE


Mechanism 6

C + D → CD

CD + E → ABE + D

Overall: A + B + E → ABE


Mechanism 6

C + D → CD

CD + E → ABE + D



Mechanism 6

C + D → CD

CD + E → ABE + D



Mechanism 6

C + D → CD

CD + E → ABE + D



Mechanism 6

→ C

C + D → CD

CD + E → ABE + D Overall: A + B + E → ABE


Mechanism 6

→ C

C + D → CD



Mechanism 6

A + B → C

C + D → CD



Mechanism 7

CFCl3 → CFCl2 + Cl

ClO + O → Cl + O2

Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl


Mechanism 7


ClO + O → Cl + O2



Mechanism 7


Cl

ClO + O → Cl + O2



Mechanism 7


Cl

ClO + O → Cl + O2



Mechanism 7


Cl + O3

ClO + O → Cl + O2



Mechanism 7


Cl + O3 → ClO + O2

ClO + O → Cl + O2



Mechanism 7


Cl + O3 → ClO + O2

ClO + O → Cl + O2



Mechanism 7


Cl + O3 → ClO + O2

ClO + O → Cl + O2



Mechanism 7


Cl + O3 → ClO + O2

ClO + O → Cl + O2



Mechanism 7


Cl + O3 → ClO + O2

ClO + O → Cl + O2


Reaction IntermediateNotes:• An intermediate doesn’t accumulate (like a

product) because as soon as it is formed, it gets used up again.

• Intermediates are not necessarily unstable (in other circumstances, they may last a while).

• An activated complex is very unstable and short-lived. It doesn’t usually obey bonding “rules.”

HOMEWORK

Hebden Textbook Page 28 Questions #47-53

Reaction Mechanism

Review Questions

Provincial Exam Questions



PE Diagram for a Reaction Mechanism

AC (Step 1) AC (Step 2)

AC (Step 3)

HOOBr

PE HBr + O2

HOBr

H2O + Br2

Reaction ProceedsLabel this diagram: RDS, Ea (Overall Rxn), Ea (Step 2), ∆H.


Notes:• each “hump” is a step• the higher the hump (greater Ea), the slower the

step• the highest hump (greatest Ea) is for the RDS• AC’s are found at top of humps, intermediates

in middle “valleys”, products in the final “valley”

• the Ea for the forward overall rxn is vertical distance from reactants to top of highest hump


• The Ea for the overall forward reaction is the difference in energy between the reactants and the top of the highest peak.


Example: Given the following Potential Energy Diagram for a reaction mechanism:

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Reaction Proceeds


1. This mechanism has steps

2. Ea for overall rxn = ______kJ

3. Ea (reverse rxn) = kJ

4. The overall rxn is thermic

5. H = kJ

6. H for reverse rxn = kJ

7. Ea (step1)=

8. RDS for forward rxn is step ________


If asked for Ea for a particular step, you use:

Ea= PE (activated complex for that step)-PE (reactants for THAT PARTICULAR step)

If asked for Ea for overall reaction, use:

Ea= PE (highest energy activated complex)-PE (original reactantsd)

Rate determining step is simply the one with the highest numerical value of Ea you find.

CatalystsCatalyst: an introduced substance which

produces an alternate mechanism with a lower activation energy.


HOMEWORK

• Hebden Textbook

• Page 30 Questions 54-55

• Page34 Exercises 56-61

WE ARE DONE UNIT 1!

Documents

Reaction Mechanisms Consider the following reaction: 4 HBr (g) +O 2(g) →2H 2 O (g) +2Br 2(g) There are five reactants… Typically more than 3 molecules