chapter 1 鈥 concepts in aquatic chemistry
TRANSCRIPT
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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
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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
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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
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Copper (Cu) in waterWhere does the Cu come from?What form(s) is (are) present?
Think about a material balance TOTCu = ?
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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)
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Why does speciation matter?
Speciation affects:MobilityToxicityReactivity
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Structure and charge distribution of a water molecule
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Hydrogen atomsThere is excess positive charge here
Oxygen atom
Unshared electron pairs create negative charge
in these regions
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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
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Water is an unusual fluid
Unusual fluid properties result from:Molecular shapeHydrogen bonds
Unusual properties include:DensityHeat capacityHeat conductivitySolvent properties
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Properties of water videoProperties of water
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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
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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?
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Electroneutrality
(anion charge) = (cation charge)
Charge balanceImportant fundamental equation Used to solve equilibrium problems
For example…
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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
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Ways of expressing concentrationsMass/volume and mass/mass unitsGas phase concentrations
Ideal gas lawConcentrations represented by a single
elementHardness and alkalinityEquivalentsComposite parameters
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What is a mole?Abbreviation = mol
Avogadro’s number6.0231023
That many items make up 1.0 mol
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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
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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
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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
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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
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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
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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
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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)
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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)
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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
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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)
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If water contains 35 mg Ca2+/L and 15 mg Mg2+/L, what is the hardness expressed as mg CaCO3 /L?
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3
135 1 8.73 10
40.08 10
mg Ca mol Ca mol hardness mol hardness
L mg Ca mol Ca L
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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
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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-
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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+
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4 4 410 3 3 10 3 10mol Fe eq eq
NL mol Fe L
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For reduction of Fe3+ to Fe2+
Fe3+ + e- = Fe2+
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4 4 410 1 10 10mol Fe eq eq
NL mol Fe L
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Pause for introductions…Interview a neighbor, report back to
class:NameBirthplaceAcademic backgroundWork experienceFamous person you want to have dinner
with
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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”
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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)
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Ionic strengthA measure of ionic charge
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2 i iI c z ci = mole of i per L
zi = valence (charge) associated with species i
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Solution includes 10-3 M NaNO3 and 10-4 M CaCl2. Assuming everything dissolves, what is the ionic strength?
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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
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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
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Activity depends onConcentrationEnvironmental components
Other constituents’ concentrationsTemperaturePressure (usually 1 bar)
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Chemical activity measured relative to a standard state
Standard state includes:ConcentrationReference environmental conditions
Also known as standard environmental conditions
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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
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Activity coefficients
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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
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= f(ionic strength)For example, the Davies equation:
A = 0.51 for water at T = 25CApplicable at I < 0.5 M
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0.52
0.5log 0.2
1i
IA z I
I
That symbol is 1.0, not I
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Compare activity coefficients of Na+ and
Ca2+ at I = 10-1 and I = 10-4.
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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
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Chemical equilibriumEquilibrium occurs when:
No driving force to change composition
Consider generic reaction:
a A + b B c C + d D
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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
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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
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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:
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c d
feqa b
req
k C DK
k A B
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In general:
Q = activity quotientAt equilibrium Q = Keq Dynamic equilibrium
Forward and reverse reactions continueRates are equal
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c d
a b
C DQ
A B
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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?
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Temperature effects Keq depends on the change in enthalpy
(Hr) associated with the reaction
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21,,
11lnln
12 TTR
HKK r
TeqTeq
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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?
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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
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KT = 274 = 1.39x10-15
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1581.39 10
{ } { } 3.73 10{ }
H HH
pH = - log{H+} = 7.4
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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
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Key conceptsExpressions for concentrations
Mass/volumeMol/volumeEquivalentsAs another compound
Activity and ionic strength effects
Equilibrium
Temperature effects
Combining chemical reactions
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Team surveyIf you are here…
Turn results in before you leave
If you are not here…Send results by Monday
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