Section 13'4: Solubility as an Equilibrium Process

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Section 13.4 Solubility as an Equilibrium Process
Solute is constantly
dissolving and recrystallizing. Equilibrium
Rate of dissolution Rate of
recrystallization ? No ? concentration Dissolutio
n Rate of dissolution gt Rate of
recrystallization ? Conc. increases Recrystalliza
tion Rate of dissolution lt Rate of
recrystallization ? Conc. decreases
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Section 13.4 Solubility as an Equilibrium Process
Qualitative Description of How Much Solute is in
Solution
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Relevance to Ocean Acidification What are
happening in oceans w.r.t. CO32- ?
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Section 13.4 Temperature Effects on Solubility
Solid Solutes
Most solids have a ?Hsoln because ?Hlattice gt
?Hhydr
?Hsoln ?Hlattice ?Hhydr
(?Hhydr ?Hsolvent ?Hmix)
How does this explain why adding more heat
results in more dissolution?
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Section 13.4 Temperature Effects on Solubility
Gas Solutes
The opposite is true for gases Energy must
be added to separate solids (heat helps), but
gases are already separated (?Hsolute 0)
?Hhydr step is always exothermic (lt 0)
That means ?Hsoln (?Hsolute ?Hhydr) must be
negative.
Why is this? (Think about intermolecular forces.)
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Section 13.4 Temperature Effects on Solubility
Gas Solutes
Thermal pollution O2 less soluble and fish cannot
breathe
What are implications for ocean acidification?
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Section 13.4 Pressure Effects on Solubility
Solid Solutes
Because solids (and liquids) are almost
incompressible, pressure has little effect on
their solubility, but there is still some effect.
Figure 13.21
Saturation state ? Ca2CO32-/Ksp
If product of ion concentrations are
greater than the product of the ion
concentrations at equilibrium ? gt 1 ?
recrystallization If product of ion
concentrations are greater than the product
of the ion concentrations at equilibrium
? lt 1 ? dissolution
Ksp Ca2CO32-
Saturation horizon ? 1 (at equilibrium)
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Depth of calcite and aragonite saturation horizons
Saturation horizon ? 1 (at equilibrium)
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Section 13.4 Pressure Effects on Solubility
Gas Solutes
Henrys Law The quantitative relationship between
gas pressure and solubility Sgas kH x
Pgas where Sgas ? solubility of a gas (mol/L)
Pgas ? pressure of gas (atm)
kH ? Henrys law constant (mol/L atm)
The partial pressure of CO2 in the atmosphere is
0.000314 atm. What is the concentration of CO2
in the ocean? Henrys law constant for CO2 Is
2.3 x 102 mol/L atm.
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Why is the amount of CO2 coming out of the ocean
less than that going in?
Adapted from NASA Earth Observatory
http//earthobservatory.nasa.gov/Library/CarbonCyc
le
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The Bends
http//www.elmhurst.edu/chm/vchembook/174temppres
.html
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Relevance to Ocean Acidification Salinity
Effects in Solubility
Figures from http//www.advancedaquarist.com/200
2/3/chemistry
What does this mean for the impact of salinity of
CaCO3 solubility?
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Section 13.5 Quantitative Ways of Expressing
Concentration
Molarity (M) moles of solute / volume of
solution Molality (m) moles of solute / kg of
solvent
Find the molarity and molality of a solution
where 20 g of CaCO3 were dissolved in 2 L of
water at 4 ºC (density of water 1 kg/m3).
Mass percent ( w/w) mass solute / (mass
solute mass solvent) x 100 Volume percent
(v/v) volume solute / volume solution x
100 w/v mass solute / solution volume x
100 Find the w/w and the w/v for the CaCO3
solution described above. What is the v/v
for a solution where 70 mL of alcohol was added
to 2 L of water?
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Suggested Problems
Solubility Henrys Law 13.44, 13.46, 13.49,
13.50 Ways of expressing concentration 13.56,
13.58, 13.60, 13.62, 13.66, 13.68, 13.70