CHM 122 General Chemistry II Lecture (Section 01) Spring 2010 Kinetics Worksheet Three (1) The rate constant for the decomposition of nitrogen dioxide, 2N02 2N0 +02 is 3.40 Mr min 1. If the initial concentration of NO2 is 2.00 M, how long will it take for the concentration to be reduced to (2) The radioactive isotope 32P decays by first-order kinetics and has a half-life of 14.3 days. How long will (3) The dimerization of C2F4 follows second-order kinetics and has a rate constant of 0.0488 M min1 at 1.50 M? it take for 95.0% of the sample to decay? 425.0 . If we start with 0.120 mol of C2F4 in a 2.00 L flask, (a) what will be the concentration of C2F4 after 1.00 hr? (b) How many grams of C4Fs will be produced in 1.00 hr? (4) Strontium-90 is a radioactive isotope of Sr and has a half-life of 28.0 years. How long will it take for all the strontium-90 presently on earth to be reduced to 1.00% of the present amount? Radioactive decay follows first-order kinetics. (5) The decomposition of NO2 follows second-order kinetics and had a rate constant equal to 1.4 × 10-5 Mr\'s\". (a) If 4.0 mol of NO2 is initially present in a 2.0 L vessel, what concentration of NO2 remains after 2.0 hr? (b) Referring to the conditions in part (a), what is the half-life of the reaction? (6) Radioactive decay follows first-order kinetics. How many grams of a 15.0 g sample of Co-60 disintegrate after 30.0 yrs? The half-life of Co-60 is 5.27 yrs.
1.
2NO2 ------------- 2 NO + O2
K= 3.40 M-1S-1
Initial concentration= [A]0 = 2.00M
after t time concentration = [A]t= 1.50M
for second order reacton
1/[A]t = Kt + 1/[A]0
1/[A]t - 1/[A]0 = Kt
1/1.50 - 1/2.00 = 3.40 xt
t= 0.049 min.
2.
half life= 14.3 days
for first order reaction
half life = t1/2 = 0.693/K = 0.693/14.3 = 0.0485 day-1
Initial amount = 100%
after t time = 100 - 95 = 5%
for first order reaction
K = 2.303/t xlog[A]0/[A}t
t = 2.303/K xlog[A]0/[A]t
t= 2.303/0.0485 xlog(100/5)
t= 61.78 days
3.
rate constant = K=0.0488 M-1 min-1
number of moles of C2F4= 0.120 mole
volume = 2.0 L
Concentration = number of moles/volme = 0.120/2.0 = 0.06M
Initial concentration = [A]0 = 0.06M
t= 1 hour= 60 min
for second order reaction
1/[A]t = Kt + 1/[A]0
1[A]t = 0.0488 x 60 + 1/0.06
1/[A]t = 19.598 M
[A]t = 0.05M
Concentration after 1 hr= 0.05M
molar mass of C2F4 = 100 gram/mole
concemtration = mass/molar mass x 1000/volume
mass = concentration x molar massx volume/1000
mass -= 0.05 x 100 x2000/1000 = 10 grams
mass of C2F4 =10 grams.
Homework Sourse
