Kinetics Lesson 5 PE Diagrams Mechanisms

KineticsLesson 5

PE Diagrams Mechanisms

Potential and Kinetic Energy Changes during a CollisionEndothermic Reactants Activated Complex Products

Potential and Kinetic Energy Changes during a CollisionEndothermic Reactants Activated Complex Products

Potential Energy

Potential and Kinetic Energy Changes during a CollisionEndothermic Reactants Activated Complex Products max

Potential Energy high low


Potential Energy high low Kinetic Energy


Potential Energy high low Kinetic Energy

high min low


Potential Energy high low Uphill- endothermic! Kinetic Energy

high min low

Pick the slow and fast reaction


Ea Ea


Ea Ea

Slow Fast- low Ea

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

Watch Simulation- set both temperature and mass to the highest setting. Set red to 1 and blue to four and remove the barrier. How often do all five particles collide?

There are five reactant particles.If there are three or more reactant particles they will not likely react in one step.They will react in a series of steps, which is a mechanism.

http://mc2.cchem.berkeley.edu/Java/molecules/index.html

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




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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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Mechanism 1 What is left is the overall reaction.




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Mechanism 1 What is left is the overall reaction.




4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)

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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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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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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 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 than on the right side.


C →→ C


C →→ C

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


C →→ C

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

→ 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: 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: 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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Mechanism 3

Step 1 Br2 → 2Br



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

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Mechanism 3

Step 1 Br2 → 2Br



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

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Mechanism 3

Step 1 Br2 → 2Br




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

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800

200

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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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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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800

200

400

600

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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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

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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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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

Overall Reaction: Intermediates:

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

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

Overall: A + D + E → ADE

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

C + D → CD

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


Mechanism 6

A + B → C

C + D → CD

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


Mechanism 7

CFCl3 → CFCl2 + Cl

ClO + O → Cl + O2

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


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


Kinetics Lesson 5 PE Diagrams Mechanisms

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