Dissociation of H2O: H2O H OH- Kw = aH aOH- aH2O

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Dissociation of H2O
H2O ? H OH-
Under dilute conditions ai i And aH2O
1 Hence Kw H OH-
Kw aH aOH- aH2O

At 25oC Kw 10-14
Kw is temperature dependent
At 25oC Kw 10-14 Neutral water H OH-
At 25oC pH of neutral water 7
At 0o C Kw 10-14.727 and neutral water has a
pH of 7.363
At 60o C Kw 10-13 and neutral water has a pH
of 6.5
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pH of natural precipitaion
National Atmospheric Deposition Program Data from
Denali National Park, Alaska
Measurements of the pH of precipitation at Denali
National Park over the last 23 years average
approximately 5.3
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pH at other remote localities Siberia
6.0 Australian Desert 5.8 Amazon River Basin
near coast 4.7 Amazon River Basin 2230
kilometers inland 5.3 Iceland 5.5
pH of natural precipitation between 5 and 6 this
is due largely to the dissolution of atmospheric
gases (especially CO2 ) into the precipitation,
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River ( Lake) Water
Normal range between 6.5 and 8.5 with maximum
frequency around 7.5
Why if natural rain water is acidic is river
water typically neutral to slightly basic?
Most weathering reactions consume H MgSiO3 2
H H2O ? Mg H4SiO4
5
pH of ocean water between 7.8 and 8.4 controlled
by equilibrium involving CO2
Photosynthesis uses up CO2 in photic zone (near
surface) creating higher pHs. Respiration and
decay release CO2 in deeper waters leading to a
decrease in pH.
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Formation of carbonic acid begins with
dissolution of CO2 gas in water CO2 (gas)
H2O ? H2CO3 (aq)
This dissolution follows Henrys
law H2CO3(aq) KCO2 PCO2
Where PCO2 is the partial pressure of CO2 in the
gas and KCO2 is the Henrys law constant.
kCO2 is a function of temperature (Table 2.1).
At 25oC it is 3.92 10-2 10-1.47
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What will the pH of precipitation be in 2100 if
PCO2 doubles?
Today pCO2 10-3.5
Equations
1. H2CO3 10-1.47 2 10-3.5
Assuming that the solution is dilute so that ai
i

Additional equation comes from charge
balance Number of negative charges number of
positive charges
Note that we must multiply the concentration of
the ion by the absolute value of its charge
3. H OH- HCO3- 2 CO3-2 Where
concentrations are in moles/liter
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Simplifying assumption in an acidic solution
OH- and CO3-2 are very small so charge
balance (Equation 3) becomes 3. H HCO3-
Plugging equations 1 and 3 into 2 we get
H 10-5.51
pH 5.51
In contrast pH of precipitation in equilibrium
CO2 in atmosphere today 5.66
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Next we will consider the effect of carbonate
minerals (minerals that contain CO3-2) on pH of
water
2nd most abundant carbonate mineral is
Dolomite CaMg(CO3)2
Dolomite dissociates through the reaction CaMg
(CO3)2 (solid) ? Ca2(aq) Mg2 (aq) 2 CO3-2
(aq) Ksp 10-17.02
What is the pH of an aqueous solution in
equilibrium with dolomite and the atmosphere.
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Our equations (note T 25o)
2. H2CO3(aq) KCO2 PCO2 10-1.47PCO2
5. 2 Ca2 2 Mg2 H OH-
HCO3- 2 CO3-2
Our simplifying assumptions
i. CaMg(CO3)2 is a pure solid so that its
activity 1
ii. Ai i (at least for first pass)
iii. All Ca2 and Mg2 comes from the
dissociation of dolomite so that Ca2 Mg2
iv. Ca2 ,Mg2 and HCO3- are the dominant ions so
that HCO3-1 2 Ca2 2 Mg2 4Ca2
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It is now all over but for the algebra
A. Combining equation 1 with simplifying
assumptions i, ii and iii we get 1. Ca22
CO3-22 Ksp or Ca2 CO3-2 vKsp
C. Substitute the expression for H2CO3 in
equation 2 into equation 3. Then use simplifying
assumption ii and the new equation 3 to solve for
HCO3-. Substitute the results into 4 to get
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Or
Plugging the numbers in we get a pH of 8.35
Waters will initially be acidic but once they
come into contact with dolomite they will become
basic.
As your book shows calcite has a very similar
effects.
Interaction with carbonate minerals on of the
best ways of countering the effects of acid
precipitation.
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If water is in contact with the atmosphere pH
calculation known as open system calculations if
not they are closed system calculations.
For closed systems we often do not know PCO2 must
have additional information.
One assumption often used is the constant total
carbon (CT) assumption see book page 47.
Open systems
Closed systems