Sn

1
Physical properties of water and their importance
Property Consequence
Excellent solvent Transport of nutrients and waste products, prerequisite of biogeochemical processes
High dielectric constant Solubility of ionic compounds
High surface tension Physiological control factor droplets and surfaces
Transparent for visible and (partially) for UV radiation Allows photosynthesis in aqueous media
Highest density in liquid state at 4 C Floating ice, stratification, isolation of water biota from freezing
High heat of vaporization Controls the transfer of vapor between atmosphere and water
High heat of melting Stabilization of temperature regime at freezing/melting
High heat capacity Stabilization of temperature
2
Hydrogen bonds
3
Anomalous water properties boiling point
Boiling points of structurally similar compound
from the 4.-7. period
4
Anomalous water properties density
Consequence density of ice is lower than
density of liquid water
5
Solubilty of liquids and solids
Water as a solvent Water is the most common polar
solvent. Some solutes remain in aqueous solution
in molecular form, other electrolytes
dissociate to ions. Ionic crystals are usually
well soluble (i.e. solubility at least 0.1-1
mol/l). Solubility of salts generally increases
with temperature, in contrary to gas solubility.
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Some rules for solubility of solids with ionic
structure

• Most sodium, potassium and ammonium salts are
  well soluble. Exception is KClO4, which is often
  used for precipitation of potassium ion from
  aqueous solutions.
• Nitrates are usually well soluble.
• Carbonates and phosphates are usually insoluble
  or sparingly soluble, exceptions are sodium,
  potassium and ammonium salts. Potassium-magnesium
  phosphate is used for precipitation of magnesium
  ion from aqueous solutions.
• Halides are usually well soluble, exceptions are
  silver, lead and mercury (I) halides. PbCl2 is
  sparingly soluble, silver and mercury (I)
  chlorides are essentially insoluble.
• Sulfates are usually well soluble, exceptions
  are calcium, barium strontium, lead and mercury
  (I) sulfates. Silver sulfate is sparingly
  soluble.
• Sulfides are usually insoluble in water.

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Solubility of nonelectrolytes
Solubility in the form of molar concentration in
aqueous solution can be estimated also from
Henrys law constant and vapor pressure.
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Dissolution as a chemical reaction
Dissolution can be described in terms of chemical
reaction, e.g. for gas in water
Thermodynamic relations derived for chemical
reactions can be applied to this process, e.g.
the equilibrium constant
K is the equilibrium constant of the reaction ai
is the equilibrium activity of i compound, ?i is
the stoichiometric coefficient of i compound
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Activity and standard states
Activity is defined as the ratio of actual
fugacity of a compound to its fugacity in a
standard state. Standard states are chosen
differently for compounds in different phases.
E.g. for gases the standard state is ideal gas at
standard pressure p 101325 Pa. Corresponding
activity is
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Standard states II
Standard state for (aqueous) solutions is
solution at unit concentration
Standard state for pure solid or liquid compounds
is chosen as pure solid or liquid, leading to
unit activity at all conditions. The same
standard state is used for solvents in solutions.
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Equilibrium in dissolution reactions
Equilibrium constant for dissolution of A gas in
water is
Henrys law constant is apparently a certain form
of equilibrium constant. Solubility of solid
ionic compound that is (partially) dissolved in
water is described by the ion product
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Dissolution of minerals - examples

• Calculate molar solubility of AgCl in water
  dissolution reaction is AgCl(s) --gt Ag Cl-.
• From strochiometric ballance Ag Cl-. Ks
  1.76 x 10-10 AgCl- Ag2, Ag 1.33
  x 10-5 and molar solubility of AgCl is 1.33 x
  10-5 mol/l.

• Concentration of Ca2(aq) equal to 3.32 x 10-4
  mol/l was obtained from analysis of water in
  contact with fluorite (CaF2). Calculate the ion
  product of CaF2 .
• Equilibrium reaction is CaF2(s) lt--gt Ca2(aq)
  2F-(aq) and Ks Ca2F-2. 1 mol of CaF2 leads
  to 1 mol of Ca2 and 2 moles of F- upon
  dissolution, F- 2Ca2
• Ks Ca2(2Ca2)2 Ks (3.32 x 10-4)(6.64 x
  10-4)2 1.46 x 10-10.

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Dissolution of reactive gases CO2 in water
Dissolution reaction is (1)
for which we apply Henrys law (H 0.034
mol/(lbar) 29.41105 Pal/mol, atmospheric
content of CO2 is about 0.038)
Dissolved carbon dioxide is subject to hydrolysis
leading to carbonic acid, reaction (2)
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CO2 in water II
Carbonic acid dissociates to hydrogen carbonate,
reaction (3), and further to carbonate, reaction
(4)
Water autoprotolysis also has to be considered,
reaction (5)
All values of equilibrium constants relate to
25C.
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CO2 in water III
Dissolution of CO2 in water is described by the
system of reactions (1)-(5). Reactions (4) and
(5) may be neglected for an open system (in
equilibrium with the atmosphere), allowing a
simplified solution
pH of water in equilibrium with the atmosphere
(open water not in contact with buffering
minerals such as calcite, atmospheric water) is
about 5.6. In reality pH of rain droplets is
slightly higher (about 6) due to non-equilibrium
conditions.
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CO2 in water pressure dependence

• The amount of dissolved CO2 in water depends only
  on partial pressure of CO2 (and temperature).
  Examples
• in deep waters, where hydrostatic pressure adds
  up to atmospheric pressure
• carbonated beverages

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CO2 in water pH dependence
In buffered waters where pH is fixed, only the
concentrations of other species are calculated.
Their relative abundance is shown in the graph
vs. pH
Total amount of dissolved CO2 increases with pH.
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Limestone solubility
In contact with limestone, reaction (6) is added
to the system of reactions (1)-(5)
The reason for variation of ion product is the
unknown mineralogical character of limestone.
Solution of reaction system (1)-(6) is a function
of CO2 partial pressure and pH. Minimum
solubility of limestone (expressed as
concentration of Ca2 ions) in open water is
about 0.3 mmol/l Ca2.
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p(CO2) pH c(Ca2) mol/l
Limestone solubility II Dependence on partial
pressure of CO2 and pH.