1. Explain how layer charge influences the following layer silicate
properties:
(a) Interlayer bonds
(b) Crystal size
(c) SwÅelling
(d) Surface area
(e) c spacing
(f) Exchangeability of adsorbed cations
2. Write the half-unit-cell formula for a montmorillonite mineral with a
layer charge of 0.3, with 90% of the substitution in the octahedral layer
( Mg2+ for Al3+ ) and 10% of the substitution in the tetrahedral layer
( Al3+ for Si4+ ). The saturating cation is Na+. Draw a diagram of this
mineral, indicating the position of Mg2+, Al3+, Si4+ and Na+ ?
3. What is meant by the zero point of charge? What are the functional
groups responsible for pH-dependent charge in soil?
4. An acidic pesticide of pK 5 is being applied to soils of a given region.
Would leaching be greatest for soils of
(a) pH > 5 ?
(b) pH 6 to 8 ?
(c) pH < 5 ?
Why?
5. A soil of pH 5.5 retains 60 mmol(+) kg -1 of exchangeable bases and
has a CEC at pH 7 of 80 mmol(+) kg-1 :
(a) What is its percent base solution?
(b) What is its approximate CEC at pH 5.5?
6. An irrigation water contains 750 mg L-1 soluble salts. If used at an
average leaching fraction of 0.15, what would be the average EC of the
drainage water leaving the bottom of the crop root zone?
7. A 30-cm depth of surface soil contains 28% exchangeable sodium and
has a CEC of 150 mmol(+) kg-1. How many tonnes per hectare of
sulfur will be required to lower its ESP to 5% ?
8. A vermiculite surface soil has the ability to fix 25 mmol kg -1 of K+ or
NH4+. What rate of (NH4)2SO4 or KCl fertilizer (in kg ha-1) would be
require to saturate this fixation capacity for a 30-cm depth of soil?
9. For a CEC procedure that uses Na+ as the index cation, H2O/ethanol as
the wash solvent, and Mg2+ as the displacing cation, discuss the effect
of each of the following on CEC measurements:
(a) Hydrolysis due to excess washing
(b) Presence of large amounts of lime or gypsum in the soil
(c) Incomplete index-cation saturation
(d) Precipitation of an insoluble Na+ salt in the ethanol
(e) Incomplete removal of the index cation by Mg2+
10. To calculate b(maximum amount of adsorbate) and K(affinity constant)
of a pesticide with Langmuir equation from the data : Each 25 mL 20, 40,
60, 80, 100 mg/L respectively add to each 0.5 gm soil, and final solution
cocentration respectively is 0.4, 2, 20, 35, 50 mg/L.
11. What kinds of function groups or segments contribute to the
hydrophobicity of soil organic matters? What are the hydrophilic
function groups in soil organic matter? Which function groups are the
hard Lewis bases and which ones are the soft Lewis bases?
12. For Ca2+, Mg2+, K+, P5+, Si4+, or other selected chemical elements, trace
all the possible pathways that the ion might follow from its state in
igneous rock through soils to its most stable state at the earth’s
surface?
13. How are Fe(III) and Al(III) reactions both different and similar in soils?
Which transition metals are essential to (a) Plants? (b) Animals?
14. If K+ is release both from “fixed” forms and by weathering during the
growing season, why is exchangeable K+, as determined by a single
extraction, well correlated with the soil’s K-supplying power?
15. What are Gapon equation and Vanselow equation? What’s difference
of their explaining cation exchange and exchange coefficient on the
surface of soil colloids?
16. A 10 gm Na+-saturated montmorillonite (CEC = 100 cmol c/kg) is mixed with
1.0 liter of water containing 10 ppm Na+ and 10 ppm K+ as the only cations. if
the exchange equation K = [ExK+][Na+]/[ExNa+][K+] = 2.0 can be accepted as
valid for this system, calculate the equilibrium concentrations of Na + and K+
in solution?
17. Narrate the developed process of DDL theories from Helmholtz to
Gouy-Chapman to Stern, and explain their differences and suitability on the
adsorption of ions.
18. A sample of pure goethite, FeOOH, with a PZC of 7.5, is suspended in 10 -3 M
NaCl. The pH is adjusted to 5.0 with acid. What would be the effect of
increasing the NaCl concentration to 10 -2 M on (a) solution pH (b) CEC and
AEC (c) net surface charge?
19. Explain the mechanisms of buffer reactions of soils with acids, and describe
various forms of acid in the soils?
20. A mineral soil with pH 5 retains 7 cmolc/kg of exchangeable bases and 1
cmolc/kg of exchangeable Al3+ . The CEC is 10 cmolc/kg at pH 7. Try to
answer the questions: (a) What is the percent base saturation at pH 5.0? (b)
What fraction of the total acidity at pH 5 is nonexchangeable? (c) What is the
approximate lime requirement ( metric tons/ ha) of surface soil (15 cm depth
and B.D. = 1.33 gm/cm3)?
21. What is Jackson’s “Frayed-edge” theory during the weathering of mica
minerals? Describe the proposed K-release processes and explain the
difference of released mechanism between native K and Fixed K?
22. What are the kinetic considerations (limitations) in applying Lindsay’s
phosphate-phase diagram? Make mathematical calculations of (H 2PO4-) for
dica-phosphate (CaHPO4) at pH 7, pK = 6.66 and (Ca2+) = 0.005.
23. Describe reduction reaction sequence and Eh in waterlogged soils, explain the
difference of electrode current for Fe3+-Fe2+ redox pair between
[Fe3+]=[Fe2+]=10-3 M and [Fe3+]=[Fe2+]=10-4 M at pH 2 in water with
measurement of the platinum electrode potential.
24. Explain clearly chemisorption and precipitation of inorganic ions, and
describe a bonding mechanism that would explain Cu 2+, Cd2+ and Pb2+
showing enhanced adsorption on Al hydroxide after the surface has adsorbed
phosphate?
25. The logarithm of the association constants, logK 1M, for selected trace metals
with acetate and ethylenediamine at 25℃ are give below:
Metal
Acetate
Ethylenediamine
2+
Ni
1.43
7.32
2+
Cu
2.22
10.48
Zn2+
1.57
5.66
2+
Cd
1.93
5.41
(a) Why do all of these metals, regardless of electronic properties
and size, complex much more strongly with ethylenediamine
than with acetate?
(b) Why does Cu2+ complex most strongly with both ligands,
particularly in the case of ethylenediamine?
(c) Which metal shows the largest relative preference for
ethylenediamine over acetate? Why?
26. Soils of arid climates commonly contain smectite minerals with Na + as a
predominant exchange cation, yet Ca2+ can be present in the form of CaCO3.
Explain this separation of Ca2+ and Na+ into separate solid phases. Explain
the pH change if Na+-saturated smectite is added to a suspension of CaCO3,
the pH is observed to rise from about 8 to 10 or higher.
27. Substitution of electron withdrawing groups such as Cl or NO 2 on the
aromatic ring of phenols has the effect of lowering the pK a. Explain how the
sorption characteristics of pentachlorophenol(PCP) would differ from those
of phenol (a) at low soil pH (b) at high soil pH. (PCP is much less soluble in
water than phenol).
28. Suppose that 1.0 gm of a clay is equilibrated with 100 mL of 10 -4 M
methylene blue. After equilibration, the solution concentration is only 0.6×
10-4 M. The experiment is then repeated using 2.0 gm of clay in 100 mL of
10-4 M methylene blue, and this time the final concentration is 0.4×10-4 M. If
the Langmuir equation describes methylene blue adsorption, calculate the
specific surface area of the clay in square meters per gram. Assume that each
adsorbate molecule occupies 65 Å2 on the surface.
29. Why should hydrated alkali and alkali earth cations deprotonate at extremely
high pH values in soil environment? Does the deprotonation of these cations
occur in nature?
30. What are the weathering mechanisms as the rocks exposed to water, O 2, and
CO2?
31. The dissociation of H4SiO4 follows the equation described below,
H4SiO4 + H2O = H3SiO4- + H3O+
The pKa of H4SiO4 is around 9.71. What is the activity ration of H 3SiO4- to
H4SiO4 at pH of 5.71?
32. Differentiate complex ion and ion pair in a soil solution system.
33. What is the ionic strength of a solution? Calculate the ionic strength of a
reaction solution controlled by 0.01 M CaCl2
34. Prove that 1 mmho cm-1 = dS m-1. Convert the conductance limit of 750μmho
cm-1 for the standard of irrigation water quality to the quantity expressed by
SI unit.
35. What are ionic potential and its usage of a cation? What are some chemical
behavior of K+ and Mg2+ in soil and the related environment not accounted
for their ionic potential?
36. What is the fate of Na+, K+, Ca2+ and Mg2+ in the soil profile in the regions of
limited rainfall?
37. Draw a soil profile to represent the soil development under moderately
weathered condition. Explain the reasons for the formation of the sequence
for various acidic and basic zones through the soil profile.
38. What is the ionic strength of a solution containing 0.015 M Ca 2+, 0.05 M Na+,
0.030M Cl-, 0.02M SO42-, and 0.01 M HCO3-?
39. Describe the stepwise process of changes in water when an ion enters an
aqueous solution. What are the two major characteristics of an ion that
govern the ion’s interaction with water?
40. Why do highly weathered soils, such as Oxisols or Ferrasols, contain high
amounts of iron, aluminum, titanium, and manganese oxides?