I. The Problem
Topic 6
Rain is becoming more acidic. This results in
damage to lakes, forests, architecture and
human health for people who breathe the
acidic vapor.
Acid Rain
What should we do?
II. Acids and Bases
A. Acids: release H+ (a hydrogen ion, also
called a proton) in water.
II. Acids and Bases
B. Bases: produce OH- (hydroxide ion) in water.
B + H2O
HA + H2O
H+ + A-
*formula of BH+ depends on original base
II. Acids and Bases
C.
BH+ + OH-
Ex. HBr(aq) + Ba(OH)2(aq)
Neutralization reactions: When acids and bases react
with each other, we call this a neutralization reaction.
HCl(aq) + NaOH(aq)
NaCl(aq) + H2O(l)
In neutralization reactions, H3O+ from an acid combines
with hydroxide ion from a base to form molecules of water.
The other product is a salt (an ionic compound).
6.3
1
III. Concentration
A. Important to know how many protons are
given up by each acid.
III. Concentration
B. Molarity
= M = moles solute
L solution
HCl → H+ + ClHNO3 → H+ + NO3H2SO4 → 2H+ + SO42-
How much H3O+ is in a volume of water?
III. Concentration
C. [H+]
Example 1. 2 moles of HCl are dissolved in
1L water. Find M of H+.
III. Concentration
C. [H+]
Example 2. Find [H+] if 10 g H2SO4 is
dissolved in 2L water.
10g H2SO4 x mol H2SO4 x 2 mol H+ = 0.2 mol H+
98g
1 mol H2SO4
III. Concentration
III. Concentration
D. [OH-]
Use the same approach for bases but focus on
[OH-].
E. Acidity
NaOH → Na+ + OH-
Acidic: more H+
Basic: more OHNeutral: equal amounts of H+ and OH-
Ba(OH)2 →
+
Ca(OH)2 → Ca2+ + 2OHNH3 + H2O → NH4+ + OHBa2+
2OH-
All water contains some H+ and some OH-.
Whether it’s acidic, basic, or neutral depends on relative
amounts.
2
III. Concentration
E. Acidity
III. Concentration
F. pH
To express [H+] without using exponents, use the pH
scale.
In H2O,
[H+] x [OH-] = 1.0 x 10-14 …….always
pH = -log [H+]
[H+]
10-7,
If
>
acidic
If [H+] < 10-7, basic
If [H+] = 10-7, neutral
M
[H+]
pH
III. Concentration
IV. pH of Rain
pH of pure water = 7.0 (neutral)
pH of normal rain = 5.3 (acidic)
* The lower the pH the more acidic the solution.
pH > 7 basic
pH < 7 acidic
Demo:
pH = 7 neutral
IV. pH of Rain
CO2 + H2O → H2CO3
Why?
H+ + HCO3-
From this rx in atmosphere [H+] = 2.5 x 10-6,
pH 5.6
V. SOx and NOx
• Produced in developed areas (industry or
traffic)
• acid anhydrides
• in water they react to form strong acids
Normal rain has a pH of 5.3 due to small
amounts of other naturally occurring acids.
3
V. SOx and NOx
V. SOx and NOx
A. Review of SOx
B. NOx Review
•
•
•
•
• from car exhaust ( fog in CA has pH 1.5,
pH vinegar is 2.5)
Mainly from coal used in power plants
Desired rx : C + O2
CO2 + E
Side rx :
Later :
4 NO2(g) + 2 H2O(l) + O2(g)
4HNO3(aq)
nitric acid
Effects of acid rain: damage to marble
VI. Effects of Acid Rain
• Dissolves limestone and marble
1944
At present
These statues are made of marble, a form of limestone composed
mainly of calcium carbonate, CaCO3.
Limestone and marble slowly dissolve in the presence of H+ ions:
CaCO3(s) + 2 H+(aq)
Ca2+(aq) + CO2(g) + H2O(l)
6.10
Effects of acid rain: damage to lakes and streams
VI. Effects of Acid Rain
• Damage to forests
6.13
4
VII. What to Do?
A. Reduce SOx emissions
–
–
–
–
use less power (burn less coal)
switch to coal with less S
remove S before burning ( density)
“scrub” smokestack emissions
VII. What to Do?
B. Reduce NOx Emissions
• more walking
• more efficient cars
• new catalytic converter
VII. What to Do?
C. Neutralize acid rain in lakes
Add a base (limestone)
5