Geology Reference
In-Depth Information
Box 4.2 The effect of temperature on equilibrium constants
Like the equilibria considered in Chapter 2, the positions
of aqueous equilibria vary with temperature (and to a
much smaller extent with pressure). An equilibrium con-
stant is therefore only constant for a particular set of phys-
ical conditions. The temperature variation is important,
considering the wide range of temperatures at which aque-
ous solutions are encountered in the geological world
(from 0 °C to hundreds of degrees).
Raising the temperature of a system in equilibrium will
alter the equilibrium constant in a direction that favours
the higher enthalpy side of the equilibrium (Le Chatelier's
principle - Chapter 2). Consider for example the solubility
of a salt like BaSO 4 :
-
BaSO
+ +
Ba
2
SO
2
(4.2.1)
4
4
Solid
Saturated solution
This reaction has a positive Δ H (the reaction is endo-
thermic). As enthalpy is higher on the right-hand side, Le
Chatelier's principle (Chapter  2) indicates that a rise in
temperature will increase the equilibrium constant
(Equation 4.17), shifting the equilibrium to the right, in
accordance with the everyday experience that salts are
more soluble in hot water than cold.
During the 1980s a programme of exploration in the
Pacific Ocean using the manned submersible Alvin located
active, chimney-shaped hydrothermal vents, which dis-
charge jets of water at temperatures up to 350-400 °C. 2
Such vents have since been identified on mid-ocean ridges
in all of the major oceans. The hot, mildly acid, oxygen-free
fluids discharged from them are saturated with metal
sulfides and other salts, which are immediately precipi-
tated upon coming into contact with cold, neutral, oxygen-
bearing seawater (see Figure  4.2.1). The fine sulfide
precipitates form dense black clouds in the water, so
Figure 4.2.1 A black smoker issuing from a chimney in
the Logatchev Hydrothermal Field of the Mid-Atlantic
Ridge. (Source: Reproduced with permission of GEOMAR
Helmholtz Centre for Ocean Research, Kiel, Germany.)
these vents have become known as 'black smokers'
(Edmond and von Damm, 1983).
Gases, on the other hand, become less soluble at higher
temperatures. The expulsion of dissolved CO 2 as water is
heated causes the deposition of the familiar 'fur' inside
kettles and heating pipes, particularly in hard-water areas.
'Hard' waters contain calcium bicarbonate which, with the
expulsion of dissolved CO 2 as the water is heated, forms
the less soluble calcium carbonate:
2
Ca
HCO aCOHOC
hardwater
cold
+
+ →++
2
O
expelled
gas
3
3
2
2
(4.2.2)
solid
fur
water
hot
(
)
(
)
''
Some solids also have inverse solubility/temperature
relationships, for example, the anhydrite (CaSO 4 ) from
which black smoker chimneys are initially built. Anhydrite
is deposited from seawater as it is heated by the hot vent.
2
The highest vent discharge temperature so far discovered
is 464 °C (Koschinsky et al ., 2008).
CaF 2 left over, is said to be saturated with this
component.
When dissolved in aqueous solution, calcium
fluoride - being an ionic compound (Chapter  7) - is
completely dissociated into calcium and fluoride ions
(Ca 2+ and F respectively). The calcium atom loses two
electrons and becomes a doubly charged positive ion
(called a cation ). Each fluorine atom (of which there
 
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