Download reaction mechanism problems

AP CHEMISTRY: REACTION MECHANISM PROBLEMS
1. The reaction between chloroform and chlorine gas proceeds in a series of three elementary steps.
Cl2(g) W 2Cl(g)
CHCl3(g) + Cl(g) 6 CCl3(g) + HCl(g)
CCl3(g) + Cl(g) 6 CCl4(g)
CHCl3(g) + Cl2(g) 6 CCl4(g) + HCl(g)
Step 1: Fast, reversible
Step 2: Slow
Step 3: Fast
Overall reaction:
(a) Which of the steps in the rate determining step?
(b) Write the rate expression for the rate determining step.
(c) What is the molecularity of each step?
2. A second mechanism for the Raschig process which produces hydrazine is as follows:
Step 1:
Step 2:
Step 3:
NH3(aq) + OCl-(aq) 6 NH2Cl(aq) + OH-(aq)
NH2Cl(aq) + OH-(aq) 6 NHCl-(aq) + H2O(R)
NHCl-(aq) + NH3(aq) 6 N2H4(aq) + Cl-(aq)
2NH3(aq) + OCl-(aq) 6 N2H4(aq) + H2O(R) + Cl-(aq)
fast
slow
fast
(a) Which of these steps is the rate determining step? What is its molecularity?
(b) What is the rate expression for the rate determining elementary step?
3. At temperatures less than 500 K the reaction between carbon monoxide and nitrogen dioxide
CO(g) + NO2(g) 6 CO2(g) + NO(g)
follows the rate expression “Rate = k[NO2]2.” Which of the three mechanisms suggested below
agrees with the experimentally observed rate expression?
Mechanism I:
NO2 + CO 6 CO2 + NO
slow
Mechanism II:
NO2 + NO2 6 NO3 + NO
fast
NO3 + CO 6 NO2 + CO2
slow
Mechanism III:
NO2 6 NO + O
fast
CO + O 6 CO2
4. Propose two possible mechanisms consistent with the observed rate law expression, Rate = k[NO2Cl],
for the reaction below.
2NO2Cl 6 2NO2 + Cl2
5. Nitric oxide reacts with hydrogen to produce nitrogen and water vapor according to the following
equation:
2NO(g) + 2H2(g) 6 N2(g) + 2H2O(g)
This reaction is thought to proceed by the following two-step mechanism:
2NO + H2 6 N2O + H2O
N2O + H2 6 N2 + H2O
slow
fast
According to this mechanism: (a) What is the rate law expression for this reaction? (b) What is the
overall reaction order?
6. The rate equation for the reaction:
Cl2(aq) + H2S(aq) 6 S(s) + 2HCl(aq)
was found to be rate = k[Cl2][H2S]. Which of the following mechanisms are consistent with this rate
law?
(a) Cl2 6 2ClC
ClC + H2S 6 HCl + HSC
ClC + HSC 6 HCl + S
Cl2 + H2S 6 S + 2HCl
(slow)
(fast)
(fast)
(b) Cl2 6 2ClC
ClC + H2S 6 HCl + HSC
ClC + HSC 6 HCl + S
Cl2 + H2S 6 S + 2HCl
(fast)
(fast)
(slow)
(c) Cl2 + H2S6 ClC + HSC + HCl
ClC + HSC 6 HCl + S
Cl2 + H2S 6 S + 2HCl
(slow)
(fast)
7. The ozone, O3, in the stratosphere can be decomposed by the reaction with nitrogen (II) oxide, NO,
(commonly called nitric oxide), from high-flying aircraft.
O3(g) + NO(g) 6 NO2(g) + O2(g)
The rate expression is rate = k[NO]. Which of the following mechanisms are consistent with the
observed rate law?
(slow)
(a)
NO + O3 6 NO3 + O
(fast)
NO3 + O 6 NO2 + O2
O3 + NO 6 NO2 + O2
(b)
NO + O3 6 NO3 + O2
(slow, one step)
(c)
O3 6 O2 + O
NO + O 6 NO2
O3 + NO 6 NO2 + O2
(slow)
(fast)
(d)
NO 6 N + O
O + O3 6 2O2
N + O2 6 NO2
O3 + NO 6 NO2 + O2
(slow)
(fast)
(fast)
(e)
NO 6 N + O
O + O3 6 2O2
N + O2 6 NO2
O3 + NO 6 NO2 + O2
(fast)
(slow)
(fast)
8. Hydroxide ions are involved in the mechanism of the following reaction but are not consumed in the
net reaction.
OCl-(aq)+ I-(aq) 6 OI-(aq) + Cl-(aq)
(a) From the data given, determine the order of the reaction with respect to OCl-, I-(aq), and OH-.
(b) Write the rate equation and determine the value of the rate constant, k.
Trial
[OCl-]
[I-]
[OH-]
rate of formation
of OImolCL-1Cs-1
1
0.0040
0.0020
1.00
4.8 x 10-4
2
0.0020
0.0040
1.00
4.8 x 10-4
3
0.0020
0.0020
1.00
2.4 x 10-4
4
0.0020
0.0020
0.50
4.8 x 10-4
5
0.0020
0.0020
0.25
9.6 x 10-4
(c) Show that the following mechanism is consistent with the rate law established above.
Step 1:
Step 2.
Step 3.
OCl-(aq)+ H2O(R) W HOCl(aq) + OH-(aq)
HOCl(aq)+ I-(aq) 6 HOI(aq) + Cl-(aq)
HOI(aq)+ OH-(aq) 6 H2O(R) + OI-(aq)
fast
slow
fast
(d) Given the mechanism above, how can Le Chatelier’s principle explain the negative effect of the
hydroxide ion concentration on the rate of the reaction?
9. Substances A and B react in elementary steps according to the following equations:
Step 1:
Step 2:
A+B6C+D
D+A6C+E
k = 0.25 M/min
k = 0.0002 M/min
(a) Determine the equation for the overall reaction.
(b) Give the expected rate law for the reaction from the information above.
10. For the reaction between hydrogen and iodine:
H2(g) + I2(g) 6 2HI(g)
The rate expression is: rate = k[H2][I2]. Show that this mechanism is consistent with the rate law.
Step 1:
Step 2:
I2(g) W 2I(g)
H2(g) + I(g) + I(g) 6 2HI(g)
fast
slow
Step #1:
Cl2 W 2ClC
Step #2: CHCl3 + ClC 6 CCl3C + HCl
Step #3:
CCl3C + ClC 6 CCl4
CHCl3 + Cl2 6 CCl4 + HCl
fast
slow
fast
Therefore, the slow step, step #2, is the rate
determining step and gives the rate law:
Step #1: NH3 + OCl- 6 NH2Cl + OH- fast
Step #2: NH2Cl + OH- 6 NHCl- + H2O slow
Step #3: NHCl- + NH3 6 N2H4 + Cl- fast
2NH3 + OCl-6 N2H4 + Cl- +H2O
The rate determining step is the second, slow
step giving the rate law:
Rate = k2[NH2Cl][OH-]
Neither NH2Cl or OH- are part of the overall
reaction and must therefore be removed.
Since NH3 and OCl- produce NH2Cl and OH- in
step #1, they can be substituted in the rate law
giving the relationship:
Rate = k1k2[NH3][OCl-]
or
Rate = k[NH3][OCl-]
OCl- + H2O W HOCl + OHfast
HOCl + I- 6 HOI + Clslow
HOI + OH- 6 H2O + OIfast
OCl- + I- 6 Cl- + OIStep #2 is the rate determining step since it is the
slowest step giving the rate law:
Step 1:
Step 2:
Step 3:
Rate = k2[HOCl][I-]
HOCl is an intermediate species in the reaction
and must be substituted for with reactants.
Give two possible mechanisms for the reaction
2NO2Cl 6 2NO2 + Cl2
which are consistant with the rate law,
rate = k[NO2Cl]
Step 1;
Step 2:
NO2Cl 6 NO2 + Cl
NO2Cl + Cl 6 NO2 + Cl2
2NO2Cl 6 2NO2 + Cl2
slow
fast
Step 1:
Step 2:
Step 3:
NO2Cl 6 NO + OCl
NO2Cl + NO 6 NO2 + NOCl
NOCl + OCl 6 NO2 + Cl2
2NO2Cl 6 2NO2 + Cl2
slow
fast
fast
Trial
[OCl-]
[I-]
Rate of
Formation of
OI[OH-] molCL-1Cs-1
1
0.0040 0.0020
1.00
4.8 x 10-4
2
0.0020 0.0040
1.00
4.8 x 10-4
3
0.0020 0.0020
1.00
2.4 x 10-4
4
0.0020 0.0020
0.50
4.8 x 10-4
5
0.0020 0.0020
0.25
9.6 x 10-4