Reaction Mechanism Examples
1.
Given the reaction:
Name: ________________
HCOOH  CO + H2O
a) This reaction, without a catalyst, is very slow at room temperature. Suggest why.
Reaction has a large activation energy and particles do not
possess sufficient KE at room temperature to overcome Ea and
have a successful collision.
b) This reaction is thought to take place by means of the following mechanism when
the catalyst H+ is added:
Step 1: HCOOH + H+  HCOOH2+
(fast)
Step 2: HCOOH2+  H2O + HCO+
Step 3:
HCO+  CO + H+
(slow)
(fast)
c) Identify the two intermediates HCOOH2+ , HCO+
d) Identify the catalyst in this mechanism H+
e) Another catalyst is discovered which increases the rate of only Step 1.
How will this affect the rate of the overall reaction?
NO EFFECT.
Explain your answer.
RATE OF THE OVERALL REACTION IS DETERMINED BY THE
SLOWEST STEP, FOR THIS REACTION, THIS IS STEP 2.
f) Which step has the greatest activation energy? STEP 2
g) How many "bumps" will the potential energy diagram for the catalyzed reaction?
3
h) Which step is called the rate determining step in this mechanism? STEP 2
i) In order to have successful collisions, the colliding particles must have both the
proper amount of energy and the proper GEOMETRY
On the set of axes below, draw the shape of the curve you might expect for the
reaction in this question. The overall reaction is exothermic! Make sure you get the
"bumps" the correct relative sizes.
Potential
Energy
Progress of Reaction
2.
Given the following mechanism, answer the questions below:
Step 1:
O3 + NO  NO2 + O2 (slow)
Step 2:
NO2 + O  NO + O2 (fast)
a) Give the equation for the overall reaction.
O3 + O  2 O2
b) What could the catalyst be in this mechanism? NO
c) What is an intermediate in this mechanism? NO2
CHEMISTRY 12
REACTION MECHANISM WORKSHEET
The following are proposed reaction mechanisms. For each, either find the overall reaction
or the missing step. In addition, list all reaction intermediates and catalysts for each
reaction.
1.

A 
C 
Step 1 : A + B
I1
Step 2 : I1 +
I2
Step 3 : I2 +
Overall Reaction :
3.
J
Step 2 : J + K  L +
Step 3 : 2 L  M + N
2. Step 1 : H + I
D
Overall Reaction :
 SeO2
O2  SeO4
4. Overall reaction : OCl- + I-  OI- + Cl-
Step 1 : Se + O2
Step 2 : SeO2 +
Step 1 : OCl- + H2O  HOCl + OHStep 2 : I- + HOCl  HOI + ClStep 3 :
Overall Reaction :
5.
Step 1 : HCOOH + H+  HCOOH2+
Step 2 : HCOOH2+  HCO+ + H2O
Step 3 : HCO+  H+ + CO
Overall Reaction :
6.
Overall reaction : H2 + Cl2
Step 1 :
7.
 HCl
H + Cl2  HCl
Cl + Cl  Cl2
 2HCl
Step 2 : Cl + H2
+ H
Step 3 :
+ Cl
Step 4 :
 NO3
NO  2NO2
Step 1 : NO + O2
Step 2 : NO3 +
Overall reaction :
8.
Overall reaction : 2Ce4+ + Tl+
Step 1 : Ce4+ + Mn2+
Step 2 :
Step 3 : Mn4+ + Tl+
H
 Ce3+
 Tl3+

2Ce3+ + Tl3+
+ Mn3+
+ Mn2+
9.
 H3O2+
Br- + H3O2+  HOBr
HOBr + H+  H2OBr+
Br- + H2OBr+  H2O
Step 1 : H+ + H2O2
Step 2 :
Step 3 :
Step 4 :
Overall reaction :
10.
+ Br2
Overall reaction : NH4+ + HNO2
Step 1 : HNO2 + H+
Step 2 :

Step 3 : NO+ + NH3
 NH3NO+
Step 4 : NH3NO+
11.
+ H 2O
 N2
+ 2H2O + H+
H2O + NO+
 H 2O
+ H+ + N2
Overall reaction : I- + OClStep 1 :
 Cl-
 Cl- +
OH-  H2O
Step 2 : HOCl + I-
HOI
Step 3 : HOI +
+ OI-
+ OI-
ANSWERS :
Missing Step/Reaction
1
2A + B + C
2
I+K+L
3
4
5
6
7
8
9
10
11

D
M+N
Se + 2O2  SeO4
OH- + HOI  H2O + OIHCOOH  H2O + CO
Cl2  2Cl
2NO + O2  2NO2
Ce4+ + Mn3+  Ce3+ + Mn4+
2H+ + 2Br- + H2O2  2H2O +
NH4+  NH3 + H+
H2O + OCl-  HOCl + OH-
Reaction Intermediates
I1 , I2
Catalysts
J , L
H
SeO2
HOCl , OH- , HOI
H2O
HCOOH2+ , HCO+
H+
Cl , H
Cl2
NO3
Mn3+ , Mn4+
Br2
Mn2+
H3O2+ , HOBr , H2OBr+
NO+ , NH3 , NH3NO+
H+
HOCl , HOI , OH-
H2O
Reaction Mechanism Examples
1.
Given the reaction:
Name: ________________
HCOOH

CO + H2O
a) This reaction, without a catalyst, is very slow at room temperature. Suggest why.
_____________________________________________________________________________________________
c) This reaction is thought to take place by means of the following mechanism when
the catalyst H+ is added:
Step 1: HCOOH + H+  HCOOH2+
Step 2: HCOOH2+
Step 3:
HCO+


(fast)
H2O + HCO+
CO + H+
c) Identify the two intermediates
d) Identify the catalyst in this mechanism
(slow)
(fast)
_______________________________________
________________________________
f) Another catalyst is discovered which increases the rate of only Step 1.
How will this affect the rate of the overall reaction?
______________________________________________________________________________________________
Explain your answer.______________________________________________________________________
f) Which step has the greatest activation energy? ________________________________________
h) How many "bumps" will the potential energy diagram for the catalyzed reaction?
_________
h) Which step is called the rate determining step in this mechanism? ___________
ii) In order to have successful collisions, the colliding particles must have both the
proper amount of energy and the proper ________________________________
On the set of axes below, draw the shape of the curve you might expect for the
reaction in this question. The overall reaction is exothermic! Make sure you get the
"bumps" the correct relative sizes.
Potential
Energy
Progress of Reaction
2.
Given the following mechanism, answer the questions below:
Step 1:
O3 + NO  NO2 + O2 (slow)
Step 2:
NO2 + O  NO + O2 (fast)
a) Give the equation for the overall reaction.
b) What could the catalyst be in this mechanism?
c) What is an intermediate in this mechanism?
_________________________
_______________________________