Page 148 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:37 9in x 6in b985-appA Elementary Physical Chemistry

Appendix A: Homework Problem Sets 133

5. The concentration of a reactant in a second-order reaction dropped
4
from 440mmol L −1 to 112mmol L −1 in 2.44 × 10 s. Calculate the rate
constant of this reaction.
6. The decomposition of ammonia is a zero-order reaction in NH 3 .The
partial pressure of NH 3 drops from 20 to 10 kPa in 750 s.
(a) What is the rate constant for the reaction?
(b) How long will it take for all ammonia to disappear?
7. The half-life of a substance is 250 s. The reaction is first order. How
long will it take for the substance to reduce to 10% of its initial value?
8. 14 C decays with a half-life of 5750 years. The reaction is of first order. If
the percentage of 14 C in a skeleton contains 90% of 14 Cthat is present
in living matter, how old is the skeleton?
2
9. The reaction 2A → P is of second order, with r = k[A] .If k A =
s
0.75 L mol −1 −1 , how long would it take for the concentration of A to
change from 0.40 mol L −1 to 0.02 mol L −1 ?
10. The concentration of B in the reaction A → 2B was measured at 20min
intervals and gave the following results:

[B]/mol L −1 0 0.178 0.306 0.400 0.460 0.624
t/ min 0 20 40 60 80 ∞

What is the order of the reaction and what is the value of the rate
constant, k?
11. For a given reaction, the rate constant is
k =2.0 × 10 −4 Lmol −1 −1 at T = 298 K, and

s

s
k =4.5 × 10 −3 Lmol −1 −1 at T = 345 K.
Calculate the Arrhenius parameter A.
12. The activation energy of a given decomposition reaction is E a =
−1
125 kJ mol . Estimate the temperature at which the reaction rate
would be 15% greater than the rate at 300 K.
13. Food decomposes (rots) 50 times faster at 30 Cthanat0 C. Estimate
◦
◦
the activation energy of the decomposition.
14. The activation energy of the first-order decomposition of N 2 Ointo N 2
and O is E a = 250 kJ/mol. If the half-life of the reactant is t 1/2 =
6
7.5 × 10 s at 673 K, what will it be at 773 K?
−
15. The mechanism for the reaction (catalyzed by Br )
2H 2 O 2 (aq) → 2H 2 O(l) + O 2 (g)