chapter 1 鈥 concepts in aquatic chemistry
TRANSCRIPT
1
Tonight’s topicsSolubility rules
Table 1.2 in the text
Expressions for concentration
pp 9-20
Ionic strengthpp 21-22
Activity and activity coefficients
pp 22-40
Page numbers refer to the handout revised Chapter 1
2
Our goal: Describe equilibrium composition of solutions
Some solutions not at equilibriumEquilibrium assessment still valuable
ApplicationsWater and wastewater treatmentNatural and disturbed systems
Techniques apply to analysis and design of treatment systems
3
Common environmental systems are dilute systems
1 kg water 1 L water
1 mg/L 1 mg/kg = 1 part per million by mass
1 g/L 1 g/kg = 1 part per billion by mass
4
Copper (Cu) in waterWhere does the Cu come from?What form(s) is (are) present?
Think about a material balance TOTCu = ?
5
Possible forms of copper in a drinking water systemCu pipe (Cu0)Cuprous (Cu(I))
Cu+ and complexesCupric (Cu(II))
Cu2+ and complexesPrecipitated copper
CuCO3.Cu(OH)2 = malachite
Adsorbed copper S.O-Cu (S represents a solid surface)
6
Why does speciation matter?
Speciation affects:MobilityToxicityReactivity
7
8
Structure and charge distribution of a water molecule
9
Hydrogen atomsThere is excess positive charge here
Oxygen atom
Unshared electron pairs create negative charge
in these regions
The H2O molecule approximates a tetrahedron
Four-sided moleculeOxygen in the centerHydrogen at two cornersPolar molecule
Oxygen is electrophilic Electron affinity toward oxygen
Hydrogen region is more positive
10
Water is an unusual fluid
Unusual fluid properties result from:Molecular shapeHydrogen bonds
Unusual properties include:DensityHeat capacityHeat conductivitySolvent properties
11
Properties of water videoProperties of water
12
Solutes in waterWhat happens when a substance (A)
dissolves in water?A-A bonds breakH2O-H2O bonds break
A-H2O bonds form
Hydrophilic Dissolution in water favored
Hydrophobic Dissolution in water not favored
13
Ions in solution
Cations = positive charge (Na+, Ca2+, Fe3+,…)
Anions = negative charge (Cl-, SO42-,
PO43-,…)
Charged solute surrounded by and stabilized by water molecules
What are examples of uncharged dissolved substances?
14
Electroneutrality
(anion charge) = (cation charge)
Charge balanceImportant fundamental equation Used to solve equilibrium problems
For example…
15
General solubility rulesSee Table 1.2, page 8 of our text
What is the difference among soluble, slightly soluble, and insoluble?
Are the following compounds soluble in water? NaNO3
CaCO3
PbS
16
Ways of expressing concentrationsMass/volume and mass/mass unitsGas phase concentrations
Ideal gas lawConcentrations represented by a single
elementHardness and alkalinityEquivalentsComposite parameters
17
What is a mole?Abbreviation = mol
Avogadro’s number6.0231023
That many items make up 1.0 mol
18
Solubility calculationsHow many grams of each of the
following compounds should be added to 1.0-L of water to prepare a 0.01 M solution?
Assume these substances dissolve completely
NaCl
Ca(OH)2
FeS
19
10-2 M solution of NaCl requires:
10-2 (mol/L)
(22.99 g Na/mol + 35.45 g Cl/mol)
= 0.584 g/L
For Ca(OH)2 = 0.761 g/ L
For FeS = 0.879 g/L
20
Gas phase concentrationsWe apply gas phase concentrations in
Chapter 7
Ideal gas lawPi = (ni / Vtotal) R T = Ci R T
Vi = (ni / Ptotal) R T
R = 8.314 J/mol.K = 0.0821 L.atm/mol.K
21
What is the concentration of oxygen in the atmosphere?
C = P / RT
= 0.2095 / (0.0821 × 298)
{atm / (L.atm/mol.K) × K}
= 8.56 × 10-3 mol/L
32 g/mol = 0.27 g O2 / L air
22
Mass or moles per volumeTypical units are:
mg/L, g/L, mol/L, mmol/L
Atomic weights listed inside the front cover of our text
23
A lake contains 10-4 M NO3- (nitrate)
Express the concentration as mg NO3- / L
NO3- = 14 g N/mol + (316) g O/mol
= 62 g NO3-/mol
10-4 mol NO3- / L 62 g NO3
- / mol NO3-
= 6.2 10-3 g/L = 6.2 mg NO3
- / L
24
Species expressed as a single elementFor example, nitrogen (N)
An important nutrient for growing corn
An important nutrient in eutrophication
Bacteria convert N compoundsDifferent species promote different
reactions NO3
- (nitrate)
NO2- (nitrite)
NH4+ (ammonia)
org-N (organic nitrogen)
25
Concentrations from groundwater analyses:
NO3- = 12 mg/L
NO2- = 7 mg/L
NH+4 = 9 mg/L
What is the total N?
12 (mg NO3-/L) (1/62) (mmol NO3
-/mg NO3-) 1
(mmol N/mmol NO3-) 14 (mg N/mmol N)
= 2.7 mg NO3- as N/L (or mg NO3
--N/L)
26
7 (mg NO2-/L) (1/46) (mmol NO2
-/mg NO2-) 1
(mmol N/mmol NO2-) 14 (mg N/mmol N)
= 2.1 mg NO2- as N/L (mg NO2
--N/L)
9 (mg NH4+/L) (1/18 mmol NH4
+/mg NH4+) 1
(mmol N/mmol NH4+) 14 (mg N/mmol N)
= 7 mg NH4+ as N/L (mg NH4
+-N/L)
Total N = 11.8 mg N/L
27
Species expressed as a different compoundHardness = multivalent metal ions
Ca2+, Mg2+, Fe2+, Mn2+
Alkalinity = acid neutralizing capacity
CO32-, HCO3
-, OH-
Often reported as CaCO3 mg/L as CaCO3 (common expression)
mg as CaCO3 / L (logical expression)
28
If water contains 35 mg Ca2+/L and 15 mg Mg2+/L, what is the hardness expressed as mg CaCO3 /L?
29
24
3
135 1 8.73 10
40.08 10
mg Ca mol Ca mol hardness mol hardness
L mg Ca mol Ca L
24
3
115 1 6.17 10
24.305 10
mg Mg mol Mg mol hardness mol hardness
L mg Mg mol Mg L
Total hardness = 1.49 x 10-3 mol/L
3 53 3 3
3
1.49 10 1 10 149mol CaCO mg CaCO mg as CaCOmol hardness
L mol hardness mol CaCO L
What is an equivalent concentration? It depends on the reaction
1 eq = 1 mole charge
1 eq = 1 mole H+
1 eq = 1 mole e-
30
What is the normality (eq/L) of a 10-4 M
solution of Fe(NO3)3?
It depends on the reaction.
For precipitation of Fe(OH)3(s) the reaction is:
Fe3+ + 3 H2O = Fe(OH)3(s) + 3 H+
31
4 4 410 3 3 10 3 10mol Fe eq eq
NL mol Fe L
For reduction of Fe3+ to Fe2+
Fe3+ + e- = Fe2+
32
4 4 410 1 10 10mol Fe eq eq
NL mol Fe L
Pause for introductions…Interview a neighbor, report back to
class:NameBirthplaceAcademic backgroundWork experienceFamous person you want to have dinner
with
33
Chemical reactivity or activityOverall tendency to participate in
reactionsNotation used in our text
Concentration: [i] = mole of i per L ci = mole of i per L
Activity: {i} = activity of i Dimensionless
{i} i ci
i activity coefficient
“represented by”
34
How do solution properties affect reactivity?
Reactivity = f(solution properties)Dissolved solids can be:
Neutral (examples?)Positively charged (cation)Negatively charged (anion) Charges can be
Monovalent ( 1) Divalent ( 2) Trivalent ( 3) Greater (z)
35
Ionic strengthA measure of ionic charge
36
21
2 i iI c z ci = mole of i per L
zi = valence (charge) associated with species i
Solution includes 10-3 M NaNO3 and 10-4 M CaCl2. Assuming everything dissolves, what is the ionic strength?
37
i ci zi ci zi2
Na+ 10-3 1 10-3
NO3- 10-3 -1 10-3
Ca2+ 10-4 2 4 10-4
Cl- 210-4 -1 210-4
I = 1.3 × 10-3
See Figure 1.6 on page 32 of our textAt this ionic strength:
1 0.96 monovalent
2 0.88 divalent
3 0.75 trivalent
Ionic strength effects are greater for higher charge ions…
Minimal effect on neutral solutes
38
Activity depends onConcentrationEnvironmental components
Other constituents’ concentrationsTemperaturePressure (usually 1 bar)
39
Chemical activity measured relative to a standard state
Standard state includes:ConcentrationReference environmental conditions
Also known as standard environmental conditions
40
Standard state common definitions
StateStandard Conc.
T (C)P (bar)
Other
Solid Pure solid 25 1 -
Liquid
Pure liquid 25 1 -
Gas Pure gas 25 1
Ideal gas behavior
Solute
1.0 M 25 1
Infinite dilution
41
Activity coefficients
42
realsystem reactivity per moleof i
standardstate reactivity per moleof ii
Real
StandardState
1.01.0
ii i i
i
i ii iC
i CCi
i
Dimensions from these terms are not visible
= f(ionic strength)For example, the Davies equation:
A = 0.51 for water at T = 25CApplicable at I < 0.5 M
43
0.52
0.5log 0.2
1i
IA z I
I
That symbol is 1.0, not I
Compare activity coefficients of Na+ and
Ca2+ at I = 10-1 and I = 10-4.
44
I
Na+ Ca2+
Log Log
10-4 5.04 10-3 0.988
2.02 10-2 0.955
10-1 0.112 0.772 0.448 0.356
Chemical equilibriumEquilibrium occurs when:
No driving force to change composition
Consider generic reaction:
a A + b B c C + d D
45
The forward reactionRate of conversion of A and B to C and D
can be described by:= kf {A}a {B}b
kf is the rate constant for the forward reaction
46
Reverse reactionRate of conversion of C and D to A and B
can be described by:= kr {C}c {D}d
kr is the rate constant for the reverse reaction
47
Net reaction rate is: rnet = kf {A}a {B}b – kr {C}c {D}d
Net rate is zero at equilibrium
kf {A}a {B}b = kr {C}c {D}d
Rearranging terms yields:
48
c d
feqa b
req
k C DK
k A B
In general:
Q = activity quotientAt equilibrium Q = Keq Dynamic equilibrium
Forward and reverse reactions continueRates are equal
49
c d
a b
C DQ
A B
What is pH? pH = - log{H+} - log[H+]
pH = 7 -log[H+] = 7Log[H+] = -7
[H+] = 10-7 mol/L
What is the range of pH values?
50
Temperature effects Keq depends on the change in enthalpy
(Hr) associated with the reaction
51
21,,
11lnln
12 TTR
HKK r
TeqTeq
How does water temperature affect pH?
Equilibrium expression:{H+} {OH-} = Kw = 10-14.0 (T = 25 C)
Charge balance in pure water:[H+] = [OH-]
What is the pH at T = 1 C?
52
Hr = Hproducts – Hreactants
Reaction is: H2O = H+ + OH-
Hr = Hproducts – Hreactants
= 0 – 230.0 – (-285.83) = 55.83 kJ/mol.K
2
1
3,
, 1 2
1 1 55.83 10 1 1ln
8.314 298 274eq T r
eq T
K H
K R T T
2
2 1
1
,, ,
,
ln 1.97 exp( 1.97)eq Teq T eq T
eq T
KK K
K
53
KT = 274 = 1.39x10-15
54
1581.39 10
{ } { } 3.73 10{ }
H HH
pH = - log{H+} = 7.4
Combining chemical reactions
H2CO3 HCO3- + H+ Ka1 = 10-6.35
HCO3- CO3
2- + H+ Ka2 = 10-10.33
H2CO3 CO32- + 2 H+ Ka = 10-16.68
55
Key conceptsExpressions for concentrations
Mass/volumeMol/volumeEquivalentsAs another compound
Activity and ionic strength effects
Equilibrium
Temperature effects
Combining chemical reactions
56
Team surveyIf you are here…
Turn results in before you leave
If you are not here…Send results by Monday
57