REACTION MECHANISMS
1.
A reaction is proposed to occur by the following mechanism
Ce4+ + Mn2+ → Ce3+ + Mn3+
slow
Ce4+ + Mn3+ → Ce3+ + Mn4+
fast
+
4+
3+
2+
Tl + Mn → Tl + Mn
fast
a. Write the overall reaction
b. Identify each component in the mechanism as a reactant, product, intermediate or catalyst
c. Identify the rate determining step and write the rate law for the reaction
2. Consider the following mechanism
C4H9Br → C4H9+ + Br(slow)
C4H9+ + H2O → C4H9OH2+
C4H9OH2+ + H2O → C4H9OH + H3O+
a. Write the overall balanced chemical reaction
b. What are the intermediates of this mechanism?
c. Write the rate law for the reaction
3. For the reaction
NO2 (g) + CO(g) → NO (g) + CO2 (g)
The experimentally determined rate law is known to be second order with respect to NO2. One suggested
mechanism for the reaction is
NO2 + NO2 → NO3 + NO
NO3 + CO → NO2 + CO2
a. Write the rate law for each elementary step
b. Is this a reasonable mechanism? (Does is meet all 3 criteria for a mechanism?)
c. Which step must be the rate determining step?
d. Identify the intermediates in the mechanism.
4. Below is a proposed mechanism
A+B↔C
(fast)
C +A → B + E
(slow)
E+F→G+D
(fast)
a. Write the overall balanced equation
b. Identify all intermediates and catalysts
c. Write the rate law for the reaction
5. Consider the following mechanism
BrCl + H2 ↔ HBr + HCl
HCl + BrCl → HBr + Cl2
(slow)
a. Write the overall balanced equation
b. Identify all intermediates and catalysts
c. Write the rate law for the reaction
6. The reaction below
2 NO + Cl2 → 2 NOCl
is found by experiment to be second order in NO and first order in Cl2
a. Write the rate law for this reaction
b. A proposed mechanism for this reaction is
i. NO + Cl2 → NOCl2
ii. NOCl2 + NO → 2 NOCl
What is the intermediate in this mechanism?
c. Identify the rate determining step for this mechanism.
REACTION MECHANISMS
7. For the reaction
a. A + B+ C → D + E
b. The experimentally determined rate law is
c. Rate = k [A][B][C]
d. For each mechanism below, determine if it is a valid possible mechanism. If the mechanism is not valid,
state why.
Valid or not valid?
Mechanism I
A + B + C→ X + E (slow)
X→D
(fast)
Mechanism II
A+B↔X
(fast)
X+C→Y+E
(slow)
Y→D+E
(fast)
Mechanism III
A+B↔X
(fast)
X+C→Y+E
(slow)
Y→D
(fast)
Mechanism IV
A+B→X+E
(slow)
X+C→D
(slow)
8. The figures below show the potential energy diagram for a chemical reaction that can occur by 2 different paths.
a. Write the balanced equation for the reaction
b. Write a mechanism for each pathway
c. On each figure, identify the activation energy and the enthalpy of reaction
d. Identify the role of species as reactant, intermediate, product, catalyst or transition state.
AB
Energy
Energy
AE
A+B
A+B+E
BF
F+B+C
C+D
Reaction progress
C+D+E
Reaction progress
9. The reaction below was studied by the method of initial rates.
2 NO + H2 → N2O + H2O
Experiments show that the rate of the reaction doubles when the concentration of hydrogen gas is doubled.
Additionally, the rate increases by a factor of four when the concentration of nitric oxide is doubled.
a. Based on the information above, write a rate law for the reaction.
b. Evaluate each of the four mechanisms below to determine which mechanism(s) are consistent with the
data above.
Mechanism A
NO + H2 → N + H2O (slow)
NO + N → N2O
Mechanism B
NO + NO ↔ N2O2
N2O2 + H2 → N2O + H2O (slow)
Mechanism C
H2 ↔ 2 H
H + 2 NO → N2O + OH (slow)
OH + H →H2O
Mechanism D
NO + NO →NO2 + N (slow)
NO2 + H2 → NO + H2O
N + NO → N2O
c. For each mechanism above, sketch a reaction profile (see #8). You may assume the overall reaction is
endothermic and that the rate determining step has the highest activation energy