Transcript Document 7396777

Chem. 250 – 11/18 Lecture
Announcements I
A. Exam 2 Results
Average = 73
B. New Homework
Set (Text Ch. 4:
25; Ch. 7: 3, 5, 6,
8, 10, 24, 25, 26,
35, 44 +
Furlough
Questions)
Score
Range
90-92
N
80s
4
70s
5
60s
2
<60
3
2
Announcements - II
C. Topic Covering
1.
2.
3.
4.
5.
6.
Cloud Chemistry
Precipitation Chemistry
Hydrologic Cycle
Water Properties
Water Composition
Some of the above topics may be covered in
the next lecture
Announcements - III
D. Rough Drafts due today (one copy to me
+ keep track of who receives other
copies)
E. Next Wednesday – Furlough Day
- I will give a couple of additional homework
problems for you to do as practice toward
understanding of concepts (not collected or
graded)
Furlough Problems:
1. A cloud is nucleated on an aerosol containing
8.0 μg m-3 NH4HSO4 with 75% efficiency and
reaches a LWC of 0.5 g m-3. The SO2 mixing
ratio is present at 2.0 ppb. If P = 0.8 atm and
T = 15ºC, calculate the pH of the cloud water.
Calculate the pH independently for aerosol
only acidity and SO2 only acidity and use only
the source which adds the most acidity. Then
determine the following: [H2SO3 (aq)], [HSO3-],
[SO42-]
2. Derive the equation on slide 34.
Cloud Chemistry
• Rationale for Studying
- Cloud reactions can be important (e.g.
formation of H2SO4)
- Precipitation composition depends on
cloud composition
- Provide introduction to aqueous
chemistry
Cloud Chemistry
- Incorporation of Pollutants
Cloud Chemistry
- Incorporation of Pollutants
• Main mechanisms
- Nucleation of cloud droplets on aerosol
particles
- Scavenging of gases
- Reactions within the droplet
Cloud Chemistry
Nucleation of Cloud Droplets (some review?)
• Cloud droplets can not form in the absence of
aerosol particles unless RH ~ 300%.
• Cloud droplets nucleate on aerosol particles at
RH of ~100.1 to ~101%.
• Cloud droplets should nucleate when RH =
100% except that the vapor pressure over a
curved surface is less than that over a flat
surface (due to water surface tension)
• Smaller particles (d < 50 nm) have more curved
surfaces and are harder to nucleate
Cloud Chemistry
- Nucleation of Cloud Droplets
• Nucleation more readily occurs with:
- Larger particles
- Particles with more water soluble
compounds (due to growth according to
Raoult’s law)
- Compounds that reduce surface tension
- Smaller aerosol number concentrations
(less competition for water so higher RH
values)
Cloud Chemistry
- Nucleation of Cloud Droplets
• The concentration of constituents incorporated from
nucleation depends on the efficiency of nucleation and
on the liquid water content (or LWC).
• LWC = g liquid H2O/m3 of air
• The higher the LWC, the lower the concentration
(dilution effect)
• Cloud nucleation leads to heterogeneous cloud droplet
composition – Ignored here for calculations
Cloud Chemistry
Nucleation Example Problems
• Why is a RH over 100% required for cloud
droplet nucleation?
• Why is nucleation efficiency higher in less
polluted regions?
• An ammonium bisulfate aerosol that has a
concentration of 5.0 μg m-3 is nucleated with
50% efficiency (by mass) in a cloud that has a
LWC of 0.40 g m-3. What is the molar
concentration? What is the cloud pH?
Cloud Chemistry
- Scavenging of Gases
• Also Important for covering water chemistry
(e.g. uptake of CO2 by oceans)
• For “unreactive” gases, the transfer of gases to
cloud droplets depends on: the Henry’s law
constant (always)
• In special cases, transfer can depend on LWC (if
high), or can be limited by diffusion
• Henry’s Law:
where KH = constant (at

X
given T) and X = molecule
KH 
of interest
PX
-
Cloud Chemistry
Scavenging of Gases: “unreactive” gases
• When LWC and KH are relatively low, we can assume
that PX is constant
Then [X] = KH∙PX
• When KH is high (>1000 M/atm), conservation of mass
must be considered (PX decreases as molecules are
transferred from gas to liquid)
• We will only consider 2 cases (low KH case and 100%
gas to water case)
• Example Problem (low KH case): What is the
concentration of CH3OH in cloud water if the gas phase
mixing ratio is 10 ppbv and a LWC of 0.2 g/m3? The
Henry’s law constant is 290 M/atm (at given temp.).
Assume an atmospheric pressure of 0.9 atm and 20°C.
-
Cloud Chemistry
Scavenging of Gases “unreactive” gases
• For compounds with high
Henry’s law constants, a
significant fraction of
compound will dissolve in
solution
• fA = 10-6KHRT(LWC)
where fA = aqueous
fraction (not used in
assigned problems)
• When fA ~ 1, can use
same method as for cloud
nucleation
From Seinfeld and Pandis (1998)
-
Cloud Chemistry
Scavenging of Gases: “reactive” gases
• Many of the gases considered are acidic and
react further
• Example: Dissolution of SO2 gas
Reaction:
Equation:
SO2(g) + H2O(l) ↔ H2SO3(aq)
H2SO3(aq) ↔ H+ + HSO3HSO3- ↔ H+ + SO32-
KH = [H2SO3]/PSO2
Ka1 = [H+][HSO3-]/[H2SO3(aq)]
Ka2 = [H+][SO32-]/[HSO3-]
Note: concentration of dissolved SO2 = [S(IV)]
= [H2SO3] + [HSO3-] + [SO32-] = [H2SO3](1 + Ka1/[H+] + Ka1Ka2/[H+]2)
“Effective” Henry’s law constant
= KH* = KH(1 + Ka1/[H+] + Ka1Ka2/[H+]2) = function of pH
Cloud Chemistry
- Scavenging of Gases: “reactive” gases
• For SO2 problems in homework, assume:
– Little SO2 is depleted from gas phase (usually valid)
(This means PSO2 and [H2SO3] are constant)
– pH is just determined from SO2 (usually not valid)
– The third reaction can be ignored (dissociation of
HSO3- doesn’t affect pH)
• Dissolution of HNO3
– Because both KH and Ka are large, we can not
assume little HNO3 is depleted from gas phase
– Better assumption is 100% transfer to aqueous phase
-
Cloud Chemistry
Scavenging of Gases: “reactive” gases
• Example problem:
Determine the pH and aqueous NO3- concentration
(in M) if air containing 1 ppbv enters a cloud
with a pressure of 0.90 atm, a T = 293K, and a
LWC of 0.50 g/m3. Assume 100% scavenging.
-
Cloud Chemistry
Combining two scavenging methods
example including ammonium bisulfate, sulfur dioxide and carbon dioxide
Equilibrium pH where
sum of anion charge =
sum of cation charge
Calculation method is fairly
complex (uses systematic
method)
Cloud Chemistry
-
Reactions in Clouds
• Cloud reactions are important for water
soluble species because of higher
concentrations in clouds
• Only sulfur chemistry covered here
Cloud Chemistry
- Reactions in Clouds
• Reaction of S(IV) and H2O2
- HSO3- + H2O2 → HSO4- + H2O (acid catalyzed)
- Rate = k[HSO3-][H+][H2O2]
- Rate = k’[H2O2]PSO2
- Effectively pH independent (despite what text
says)
Cloud Chemistry
- Reactions in Clouds
• Reaction of S(IV) and Ozone
- Two main reactions:
HSO3- + O3 → HSO4- + O2 moderately fast
SO32- + O3 → SO42- + O2 fast
reaction is faster at high pH because more
S(IV) is present in reactive forms
Cloud Chemistry
-
Reactions in Clouds
1 E -0 2
S(IV) loss rate (M/s)
1 E -0 4
1 E -0 6
1 E -0 8
1 E -1 0
1 E -1 2
1 E -1 4
2
3
4
5
6
pH
O3
H CH OH 2 O 2
7
Precipitation Chemistry
• Precipitation Formation
– Cloud droplets are collected by collisions with
rain droplets or snow crystals and transfer
their contents
– Snow crystals also can form mainly through
diffusion from water vapor and are very clean
• Below Cloud Scavenging
– Incorporation of gases or particles
Cloud/Precipitation Chemistry
Some Questions
1. Which reactant for sulfur dioxide oxidation is
likely to be most important if a cloud is
nucleated on a soil dust aerosol? on an acidic
sulfate aerosol?
2. Two snow events occur down-wind of a
pollution source. In one case, the snow is
mostly crystals formed from diffusional growth.
In the other the snow grew by removing cloud
droplets. How will the snow composition be
different?
Water Chemistry
Hydrological Systems
• Most of water on Earth is in the ocean
• Much of the freshwater is inaccessible for use
• Groundwater is becoming an increasingly important
resource
from Girard
Water Chemistry
Hydrological Systems
• The hydrologic cycle is the cycle
by which water is distributed
around the Earth
• Evaporation removes most of the
non-volatile constituents of water
• For this reason, atmospheric
source of many compounds are
not large (although they are
important atmospheric sinks)
• As with clouds, regions of heavier
precipitation tend to have greater
“dilution” of pollutants
• Water flowing through sediments
can add or remove constituents
from Girard
Water Chemistry
Hydrological Systems
Data From Chem. 31
Sacramento Valley
N
Groundwater??
122.2
122.0
121.8
121.6
121.4
121.2
Longitude (deg. W)
sediments
River Water??
Tap Water West East
Transect
granite
121.0
90
80
70
60
50
40
30
20
10
0
120.8
Mg (ppm)
Mg - Long. Plot
Water Chemistry
Properties of Water
• See Text for boiling
point/melting point and heat
capacity properties
• Temperature – Density
Relationship: density maximum
occurs at 4°C and ice density is
much lower than water density
• Note: if density increases with
depth, water is stable.
from Girard
Kotz et al., “Chemistry and
Chemical Reactivity” (6th Ed.)
Water Chemistry
Properties of Water
• In the oceans, production of dense water
that can sink occurs when warm water
evaporates producing cool water with high
salinity
• This only occurs in two areas (near
Iceland and near Antarctica)
• The volume of deep water formed equals
the volume of upwelling water
Water Chemistry
Water Composition
• Salt Water
- main ions are sodium (1.06%) and chloride (1.9%)
with lower amounts of magnesium and sulfate
- main compound affecting pH is HCO3- ion (a weak
base)
• Fresh Water
- main ions are HCO3-, Mg2+, Ca2+, Na+, and Cl- main source of major ions is dissolution of carbonates
e.g. CaCO3(s) + CO2(g) + H2O(l) ↔ Ca2+ + 2HCO3-
Water Chemistry
Water Composition
• Dissolved solids
– Mass of material left after evaporating water
– Expressed in ppm
– Surrogate measure is electrical conductivity
Water Chemistry
Chemical Reactions
• Acid-Base Equilibria
Dissociation of water (always important)
H2O ↔ H+ + OH-
Carbon dioxide reactions:
1) Acid-Base Reactions
CO2 (g) ↔ CO2 (aq)
M/atm
CO2 (aq) + H2O ↔ H+ + HCO3HCO3- ↔ H+ + CO32-
KH = 0.0338
Ka1 = 4.45 x 10-7
Ka2 = 4.7 x 10-11
Water Chemistry
Chemical Reactions
• Acid-Base Properties – continued
Note: If water is in contact with atmosphere, PCO2 =
fixed value, so [CO2] = independent of pH
Other equations useful for solving water chemistry
equations:
Mass balance: T = [CO2] + [HCO3-] + [CO32-]
where T = total carbonate concentration
Charge balance equation:
Σ(zi*[cation]i) = Σ(zj*[anion]j) zi = charge of ion i
Water Chemistry
Chemical Reactions
• Form of carbonate as a function of pH
The fraction of carbonate species α present in a single form (e.g.
HCO3-) can be calculated as follows:
 (HCO ) 

3
[HCO3 ]
T

K a 1 H  
H   K H   K
 2
a1

K a2
a1
The right part to the equation
can be derived from
equilibrium equations
When pH < pKa1, CO2 is the dominant species, when pKa1 < pH
< pKa2, HCO3- is the dominant species, and when pH > pKa2,
CO32- is the dominant species
Water Chemistry
Chemical Reactions
Composition of Carbonates
1.2
Alpha Values
1.0
0.8
CO2
0.6
HCO3CO32-
0.4
0.2
0.0
0
2
4
6
8
10
12
pH
80% of US surface water
14
Water Chemistry
Chemical Reactions
• Second source of carbonate: dissolution or
weathering of carbonate rock/soil
CaCO3(s) ↔ Ca2+ + CO32Ksp = 4.6 x 10-9
This reactions normally must be considered with other
reactions (because in most waters, the pH is such that
[HCO3-] >> [CO32-])
Problems normally can be solved using 1) the systematic
method or 2) simplifying assumptions
Water Chemistry
Chemical Reactions
Example of simplifying assumption:
Solubility of CaCO3 in pure water (no CO2 present)
Water with carbonate soils is usually in regime where α(HCO3-) >
α(CO2) > α(CO32-), so a more representative reaction would result in
HCO3By combining CaCO3(s) ↔ Ca2+ + CO32with H+ + CO32- ↔ HCO3- and H2O ↔ H+ + OHThe following is obtained: CaCO3(s) + H2O ↔ Ca2+ + HCO3- + OHwhere: K = KspKw/Ka2 = 9.7 x 10-13 = [Ca2+ ][HCO3-][OH-]
Assumption that [Ca2+] = [HCO3-] = [OH-] leads to
[Ca2+] = solubility = 9.9 x 10-5 M pH = 10.00
Vs. solubility = 6.8 x 10-5 M and pH = 7.00 considering
solubility reaction only
Water Chemistry
Chemical Reactions
Simplifying assumption when both CaCO3 and CO2
are present
Combine simplified equation for CaCO3 solubility
with first 2 CO2 reactions:
CaCO3(s) + H2O ↔ Ca2+ + HCO3- + OHand CO2 (g) + H2O ↔ H+ + HCO3- (and H+ + OH- ↔ H2O)
Net reaction: CaCO3(s) + CO2 (g) + H2O ↔ Ca2+ + 2HCO3-
Notes: 1) increased CO2 leads to increased
solubility
2) 2[Ca2+] = [HCO3-] expected
Water Chemistry
Chemical Reactions
From Harris,
Quantitative
Chemical Analysis,
6th Ed., 2003
Additional CO2 sources
Low Carbonate Soils
Water Chemistry
Chemical Reactions
Buffering capacity and alkalinity
Through their reactions carbonate soils buffer water from
the addition of acids.
Alkalinity is a measure of the buffering capacity of water.
Alkalinity = mmol of acid that can be added to a 1 L water
sample before the pH → 4.5.
Alkalinity = [OH-] + 2[CO32-] + [HCO3-] (approximate)
Alkalinity = [OH-] + 2[CO32-] + [HCO3-] – [H+] (better, but
still approximate equation)
Water Chemistry
Some Problems
1. What are first and second largest reservoirs of
water on Earth?
2. What two ions are the most prevalent in seawater?
3. What three ions are the most prevalent in
fresh water?
4. How do the three major ions in fresh water
generally get into fresh water?
5. How do the concentrations of major ions in
rain water compare with fresh water?
Water Chemistry
Some Problems - II
1. At 5°C, the water hydrolysis equilibrium
constant is 2.0 x 10-15. What is the pH of pure
water (no CO2, no other sources of trace
species)?
2. Determine the solubility of CaCO3 in water in
equilibrium with 380 ppm CO2. What is the pH
of the water? What is its alkalinity?
3. Coral is largely CaCO3. As PCO2 goes up, what
will happen to the solubility of coral in the
ocean? What should happen to the pH of the
ocean? What will happen to [Ca2+]?
Water Chemistry
One last problem
1. A water sample has a measured alkalinity of
0.4 mM and a pH of 6.7. Determine the
concentration of [OH-], [HCO3-], [CO32-], and
[CO2].