CHEMICAL UNITS AND DEFINITIONS

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CHEMICAL UNITS AND DEFINITIONS
Soil Chemistry A discipline that considers the
chemical composition, changes, reactions, and
final equilibrium within the soil matrix that
emphasizes the soil solution and the solid/liquid
interface --
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• -- as affected by
• Soil minerals (parent materials)
• Inorganics (water, salts, acids, bases)
• Organics (non-humus, humus,
• contaminants)
• Physical Chemistry (surface activity,
• adsorption, ion exchange, thermodynamics,
• kinetics)
• Chemical Properties (solubility, stability,
• equilibrium)
• Pollutants (trace elements, pesticides,
• nuclides)

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Most systems may be conceptualized as consisting
of one or more reactants that undergo some degree
of interaction resulting in the formation of one
or more products.
REACTANTS PRODUCTS
INTERACTIONS
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Problem Solving Approach

• Draw the system being considered
• Itemize the given information
• Identify the question
• Itemize the additional information needed
• Look up, derive, and apply the additional
• information
• ANSWER THE QUESTION

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GRAM MOLECULAR (ATOMIC) WEIGHT (gmw)
A substance composed of one kind of an atom is
known as an element.
A substance composed of two or more elements is
known as a compound.
A mole stands for the amount of element or
compound containing Avogadros Number of atoms or
molecules (6.023x10-23)
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WHAT IS

• PPM ??
• Molarity (Molar Solution M)
• Molality (Molal Solution m)
• Gram Equivalents (moles of charge)
• Normality (N)

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How many grams of hydrated calcium chloride
(CaCl2 . 2H2O) is required to make exactly 1 L of
a 1 M solution?
Since the number of moles of products and
reactants are often different, a more convenient
expression is the number of equivalents of
chemical charge (molec) that interact which are
always equal. How many equivalents of CaCl2 are
required for the 1 M solution? How many
equivalents of Ca? What is the solution normality
(N)?
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Selected Properties of Water

• Good Solvent
• Zero Net Charge
• Weakly Ionized
• Strong Dipole
• Density 1g/cm3

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• Water dissociates and surrounds ions and mineral
  soil forming a solvation or hydration sphere.
• The degree of hydration is characterized by the
  heat produced when a known quantity of ions are
  added to a know volume of water and is called the
  heat of hydration. The heat of hydration tends
  to decrease with ions of larger ionic radius, and
  increase with greater ionic charge. This is of
  significance to the force of ionic attraction to
  surfaces and ion replaceability (see later
  discussion).

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Chemical Activity and Ionic Strength

• The tendency to react is related to the
  potential of each component to participate in the
  reaction, and is influenced by both the degree of
  hydration and solution concentration.
• This tendency to react is referred to as the
  chemical potential or chemical activity, and
  is the driving force for chemical reactions.

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The chemical activity of any ion is related to
the corresponding molar concentration by way of
an activity coefficient.
ai ?i Mi
Mi
0
1
?i
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The activity product of an electrolyte is
expressed as the product of the respective
activities raised to the appropriate power.
AB A B-
a a- (? M) (?- M-) ? ?- A B- ( )
ionic or chemical activity ionic
concentration
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NaCl aNa aCl (?Na MNa) (?Cl MCl) ?Na ?Cl
Na Cl- CaCl2 aCa aCl2 (?Ca MCa) (?Cl
MCl)2 ?Ca ?Cl2 Ca2 Cl-2
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There is no known thermodynamically exact method
that allows the determination of individual ion
activities. However, the quantification of soil
chemical processes such as ion exchange requires
that individual ion activities at leas be
estimated. Since Mi is easily determined, the
activity coefficient is the main variable.
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It has been empirically observed, that the mean
activity coefficient of a strong electrolyte is
the same in dilute solutions of the same ionic
strength (I) regardless of ionic composition,
but can vary at higher concentrations. It is
therefore necessary to determine the variation of
ionic activity as some function of ionic strength.
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Debye-Huckel (I 0.001-0.001 M)
I 1/2 ? Mi zi2 where zi is the ionic charge,
and I is the ionic strength. As I increases, the
effective concentration or ai decreases such
that log10 ?i -A zi2 (I)1/2 Where A 0.511,
and for any solution of a given I, one may
estimate the individual ionic activity
coefficient and solve for the respective ionic
chemical activity.
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Extended Debye-Huckel (I 0.01-0.10 M)
log10 ?i -A (zi)2 I 1/2 (1 ß oai
(I)1/2)
Where ß has a value of 0.33, A 0.5091, and oai
is approximated by the size of the hydrated ion
(Table 1.1 McBride Table 2.6 Bohn).
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Davis Equation (I gt 0.1 M)
log10 ?i -A (zi)2 I1/2 1 (I)1/2
- 0.3 I
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Guggenheim (I gt0.1 M)
Log10 ?i -A (zi)2 I1/2 1 I1/2
bI cI2
Non-Electrolytes
log10 ?i km I Where km is the salting
coefficent 0.01-0.20