Geoscience Reference
In-Depth Information
Natural
rainwater
(5-5.6)
Sea
water
(7.8-8.3)
Acid
rain, fog
(2-5.6)
pH
-0
1
2
3
4
5
6
7
8
9
10
11
12 3 4
Distilled
water
(7.0)
Ammonium
hydroxide
NH 4 OH(aq)
(11.1)
Apples
(3.1)
Lye
NaOH(aq)
(13.0)
Slaked lime
Ca(OH) 2 (aq)
(12.4)
Battery
acid
(1.0)
Lemon
juice
(2.2)
Vinegar
CH 3 COOH(aq)
(2.8)
Baking
soda
NaHCO 3 (aq)
(8.2)
Milk
(6.6)
More basic or alkaline
More acidic
Figure 10.3. Diagram of pH scale and pH levels of selected solutions.
At a pH of distilled water (7), only the first dissociation
(to the bicarbonate ion) occurs. The added H + (proton)
decreases the pH of the solution, increasing its acidity.
In the background air, the mixing ratio of CO 2 (g) in
2011 was about 393 ppmv. A fraction of this CO 2 (g)
always dissolves in rainwater. Thus, rainwater, even in
the cleanest environment on Earth, is naturally acidic
due to the presence of background carbonic acid in it.
The pH of rainwater affected by only carbonic acid is
about 5.6 ,indicating that its hydrogen ion molarity is
twenty-five times that of distilled water.
Example 10.1
What is the pH of water at equilibrium when (a)
[OH ]
10 −7 mol L −1 and (b) [OH ]
10 −11 mol
=
=
L −1 ?
Solution
Because [H + ][OH ]
10 −14 mol 2 L −2 ,tosatisfy
=
Equation 10.4, [H + ]
10 −7 mol L −1 and [H + ]
=
10 −3 mol L −1 in the two respective cases. Substi-
tuting these values into Equation 10.3 gives pHs
of (a) 7 and (b) 3, corresponding to neutral and
acidic conditions, respectively.
=
10.2.2. Sulfuric Acid
When sulfuric acid gas dissolves in or condenses onto
raindrops, the resulting aqueous-phase sulfuric acid
[H 2 SO 4 (aq)], a strong acid, dissociates by
10.2.1. Carbonic Acid
Water can be acidified in one of several ways. When gas-
phase carbon dioxide dissolves in water, it reacts rapidly
with a water molecule to form aqueous carbonic acid
[H 2 CO 3 (aq)], a weak acid, which partly dissociates by
the reversible reactions
CO 2 (aq)
Dissolved
H +
Hydrogen
ion
HSO 4
Bisulfate
ion
H 2 SO 4 (g)
Sulfuric
acid gas
H 2 SO 4 (aq)
Dissolved
sulfuric acid
+
2H +
Hydrogen
ion
SO 4 2
Sulfate
ion
carbon dioxide +
H 2 O(aq)
Liquid
water
H 2 CO 3 (aq)
Dissolved
carbonic acid
+
(10.6)
H +
Hydrogen
ion
HCO 3
Bicarbonate
ion
+
At pH levels greater than
2, complete dissociation
to the sulfate ion is favored, adding two protons to
solution. The increase in [H + ] decreases the pH further,
increasing the acidity of rainwater. Sulfur dioxide can
also dissolve in rainwater and produce the sulfate and
+
2H +
Hydrogen
ion
CO 3 2
Carbonate
ion
+
(10.5)
 
 
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