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.
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.
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.
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 Intermediates are produced and then consumed and cross out from right (first) and left (second). 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 What is left is the overall reaction.
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 What is left is the overall reaction.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
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
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
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Reaction Path
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
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
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
A potential energy diagram for this reaction might look like this.
PE
Reaction Path
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
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
highestA potential energy diagram for this Mechanism has three humpsThe slow step has the highest Ea
PE
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
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
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
An intermediate is produced and then consumed. It will be on the right side in an earlier step then on the left 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
An intermediate is produced and then consumed. It will be on the right side in an earlier step then on the left side.
→ II →
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
An intermediate is produced and then consumed. It will be on the right side in an earlier step then 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:
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
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
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates:
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Reaction Path
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
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Reaction Path
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
ΔH =Ea (forward) = Ea (reverse) =The enthalpy of BrThe enthalpy of BrCl
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Reaction Path
800
200
400
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
ΔH = 200 kJEa (forward) = Ea (reverse) =The enthalpy of Br The enthalpy of BrCl
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Reaction Path
800
200
400
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
ΔH = 200 kJEa (forward) = Ea (reverse) =The enthalpy of Br The enthalpy of BrCl
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Reaction Path
800
200
400
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
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
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
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
400
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
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
600
Mechanism 3
Step 1 Br2 → 2Br
Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl Intermediates: Br Cl
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
200
400
600
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
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
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: CFCl3 + O3 + O → CFCl2 + 2O2 + Cl
Intermediates:
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: 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
Determine the Missing Step of the Mechanism
Mechanism 5
A + B → C
C + D → B + AD
Overall: A + D + E → ADE
Determine the Missing Step of the Mechanism
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
Determine the Missing Step of the Mechanism
Mechanism 5
A + B → C
C + D → B + AD
AD Overall: A + D + E → ADE
Put in AD to cancel
Determine the Missing Step of the Mechanism
Mechanism 5
A + B → C
C + D → B + AD
AD + → Overall: A + D + E → ADE
Put in AD to cancel
Determine the Missing Step of the Mechanism
Mechanism 5
A + B → C
C + D → B + AD
AD + E → Overall: A + D + E → ADE
Put in E
Determine the Missing Step of the Mechanism
Mechanism 5
A + B → C
C + D → B + AD
AD + E → ADE Overall: A + D + E → ADE
Put in ADE
Determine the Missing Step of the Mechanism
Mechanism 6
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
→ C
C + D → CD
CD + E → ABE + D Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
→ C
C + D → CD
CD + E → ABE + D Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 6
A + B → C
C + D → CD
CD + E → ABE + D Overall: A + B + E → ABE
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
Determine the Missing Step of the Mechanism
Mechanism 7
CFCl3 → CFCl2 + Cl
Cl + O3 → ClO + O2
ClO + O → Cl + O2
Overall: O + CFCl3 + O3 → CFCl2 + 2O2 + Cl
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
Provincial Exam 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.
PE Diagram for a Reaction Mechanism
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
PE Diagram for a Reaction Mechanism
• The Ea for the overall forward reaction is the difference in energy between the reactants and the top of the highest peak.
PE Diagram for a Reaction Mechanism
Example: Given the following Potential Energy Diagram for a reaction mechanism:
PE
Reaction Proceeds
PE Diagram for a Reaction Mechanism
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 ________
PE Diagram for a Reaction Mechanism
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.
Provincial Exam Questions
HOMEWORK
• Hebden Textbook
• Page 30 Questions 54-55
• Page34 Exercises 56-61
WE ARE DONE UNIT 1!