CHM 123
HW 2 – Chemical Kinetics
Name
Due Thursday 1/5/15
1. (4pts) The half-life of a radioisotope is found to be 4.55 minutes. If the decay follows first order kinetics,
what percentage of isotope will remain after 2.00 hours?
2. Acetaldehyde decomposes to methane and carbon monoxide as follows…
CH3CHO (g)  CH4 (g) + CO (g) at 527°C
Its rate law expression is … Rate = k[CH3CHO]2
The specific rate constant at 527°C is… k = 2.0 x 10-2 M-1hr-1
a) (2pts) What is the order with respect to acetaldehyde?
b) (3pts) What is the half-life if 0.10 mole of CH3CHO is injected into an empty 1.0 L vessel at 527°C?
c) (4pts) What percentage of CH3CHO is remained after 200.0 hrs?
3. (5pts) In the presence of excess thiocyanate ion, SCN-, the following reaction is first order in chromium
(III) ion, Cr3+; the rate constant is 2.0 x 10-6 s-1
Cr3+(aq) + SCN-(aq)  Cr(SCN) 2+
If 85.0% reaction is required to obtain a noticeable color from the formation of the Cr(SCN)2+ ion, how many
hours are required?
4. (5pts) Cows digesting grass emit large quantities of global-warming methane gas into the atmosphere. As an
atmospheric chemist, you are interested in knowing what role that methane plays in the cycling of chlorine
in the atmosphere via the reaction:
Cl(g) + CH4(g) → CH3Cl(g) + H(g)
The rate constant at 25.0 °C is 6.02×107 M-1s-1. At 65,000 ft in the stratosphere, where the temperature is -60.0 °C,
the rate constant is 1.08×107 M-1s-1.What is the activation energy for this reaction?
Dang1
5. (5pts) In automobile exhausts, the following reaction can occur between nitrogen dioxide and carbon
monoxide.
NO2(g) + CO(g) → NO(g) + CO2(g)
To design better emission control systems, chemists need to know as much as possible about the kinetics of this
reaction. The experimental rate law is rate = k[NO2]2. Explain which of the two mechanisms below better represents
this reaction.
Mechanism A
Mechanism B
k1(slow)
k1(slow)
NO2  NO + O
k2(fast)
2NO2 
CO + O  CO2
CO + NO3  NO2 + CO2
NO3+ NO
k2(fast)
6. (12pts) The mechanism of a reaction is shown below.
HOOH + I¯  HOI + OH¯
(slow)
HOI + I¯  I2 + OH¯
(fast)
2OH¯ + 2H3O+  4 H2O
(fast)
a) What is the overall reaction?
b) Which compounds are intermediates?
c) Predict the rate law based on this mechanism.
d) What is the overall order of the reaction?
e) What is meant by the rate determining step in a reaction mechanism?
f) Is there any catalyst present?
Dang2