HOMEWORK #2 (updated 1/14/07)

 

 

Kehew Ch.2, #9, #10

 

1.  Strontium 90 is a radioactive element which has a half-life of 29 years. How long would a given amount of 90Sr need to be stored to obtain a 99.9% reduction in quantity?

 

2.  Tritium levels in the atmosphere reached their peak of about 1000 T.U. in 1963.  Assuming ground water recharge occurring at that time was in equilibrium with atmospheric tritium and that the titrated water moved along the flow path without dispersion, how long would it take for the tritium level to decay to less than 5 T.U.?

 

Drever Ch.11, #4:

 

3. The rates of dissolution of quartz, K-feldspar, albite, and anorthite at 25°C and pH 5 are 10-13.39, 10-12.50, 10-12.26, 10-8.55 mol-cm-2-s-1, respectively (Lasaga et al., 1994). 

a) Assuming a density of 2.65 g/cm, how long would it take a planar surface of each mineral to retreat 100 micron? 

b) How far would each mineral retreat in your experiment during a 5 year PhD program?

c) How do these times relate to the preservation/loss of these minerals during weathering and transport?

 

 

Snoeyink and Jenkins Ch. 2, #3:

 

4. If the initial rate of a second-order reaction at 20°C is 5x10-7 moles/liter sec, and the initial concentration of the two reacting substances are each 0.2 M, what is the k in liters/mole sec?  If the activation energy is 20 kcal/mole, what is k at 30°C?

 

 

Using data from Zhang and Millero, 1993:

 

5. Oxidation of sulfide gas by oxygen in seawater occurs in multiple steps, but the consumption of sulfide can be expressed by the first step:

 

H2S + 2O2 --> SO32- + H2O

 

Laboratory experiments that measured the concentration of H2S over time gave these results (using [O2]o = 212mm) for three different initial concentrations of H2S ([H2S]o):

 

 

Concentration of H2S as ln[H2S] (mm)

Time (hours)

[H2S]o =10mm

[H2S]o = 17mm

[H2S]o = 30mm

0

2.25

2.8

3.3

6

2.1

2.7

3.2

12

1.9

2.5

3.0

18

1.75

2.25

2.85

24

1.55

2.1

2.6

30

1.4

1.95

2.5

 

  1. Give an expression for the loss of H2S over time.  What is the reaction order?
  2. Plot ln[H2S] vs. time (in minutes).
  3. Give k for the reaction in min-1.

 

 

Grad Only Problem (S&J, Ch.2, #6):

 

6. The forward rate law for the reaction

 

H+ + OH- ↔ H2O

 

is given as

 

d[H+]/dt  =  d[OH-]/dt  =  -k[H+][OH-]

 

The rate constant, k, at 20°C was found to be 1.3x1011 liters/mole sec.  Assuming NaOH is rapidly added and mixed with HCl in aqueous solution such that the initial concentrations of H+ and OH- are 10-4 M, how much time is required for one half of the acid and base to react?  (Assume for this calculation that the reaction is irreversible.)