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8/11/2019 Lecture 3 - Activity Water Chemistry
http://slidepdf.com/reader/full/lecture-3-activity-water-chemistry 1/21
EES 3204C Fall 2014
Jong-Yeop Kim, Ph.D., P.E.Associate Professor
Department of Environmental and Civil Engineering
Florida Gulf Coast University
8/11/2019 Lecture 3 - Activity Water Chemistry
http://slidepdf.com/reader/full/lecture-3-activity-water-chemistry 2/21
Announcements
No class on 9/1 (Labor day observed)
Office hours
M &W: 10:30–11:30 am, T & R 9:00–10:30 am or by
appointment
Contact: email – [email protected], phone – 239.590.1363
Office location: Holmes Hall, Room 311 (inside 317)
Reading assignment
Chapter 1.5 (p.g 19-35)
Assignments
HW 1 due Aug 25, at the beginning of class
Prep quiz#2 due Aug 25 by 7:30 am. Read pg 12-34 before
8/11/2019 Lecture 3 - Activity Water Chemistry
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Today’s Learning Objectives Recall equivalent concepts for expressing concentrations
of various chemical compounds.
Identify important types of chemical reactions in the
natural environment.
Define chemical reactivity (activity) and the standard
state. Explain the physical/chemical significance of activity
coefficient.
Compute the ionic strength (I) of a solution containingvarious dissolved ions.
Predict the activity coefficient of each dissolved ion by
using various equations and the calculated I for the
solution.
8/11/2019 Lecture 3 - Activity Water Chemistry
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8/11/2019 Lecture 3 - Activity Water Chemistry
http://slidepdf.com/reader/full/lecture-3-activity-water-chemistry 5/21
Important types of chemical rxns in natural waters
4. Redox (Oxidation and reduction)
e.g. Reduction of O2(g) with e-
O2 + 4e- + 4H+ H2O
5. Sorption (Adsorption/Desorption):
e.g. adsorption Pb2+ onto a sediment surface
S + Pb2+ S - Pb
6. Combination of two or more types of reactions
In the natural environment, a number of difference types of
chemical reactions are often taken place either in series(one after the other) or simultaneously…
8/11/2019 Lecture 3 - Activity Water Chemistry
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8/11/2019 Lecture 3 - Activity Water Chemistry
http://slidepdf.com/reader/full/lecture-3-activity-water-chemistry 7/21
Chemical Activity
Standard state (Section 1.5.2)
Reference state: standard temperature and pressure
(STP)T = 25°C = 298.15 K
P = 1 bar = 0.987 atm
Infinite dilution (solutes), Ideal gas behavior (gas)
Standard concentration
dissolved solutes: [i] = 1.0 mole per liter = 1.0 M;
bulk liquids, solids: xi = 1.0 (mole fraction);
pure substance
gases: Pi = 1 bar (~ 1 atm); pure substance;
8/11/2019 Lecture 3 - Activity Water Chemistry
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Chemical Activity Activity is a measure of relative reactivity (dimensionless)
activity of compound i is ai or {i}
defined for a standard state (“STP”)
at standard state, the activity is always {i} = 1
in system different from standard state
{i} > 1 means more reactive than standard state
{i} < 1 means less reactive than standard state
activity related to concentration (Eqn. 1.8b)
statestandardini""of reactivity
systemrealini""of reactivity}{ i
][}{thus,1/][
}{[i]/[i]
{i}std.
iiii
ii I : the activity coefficient[i]: the concentration
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity Ionic strength (p. 19)
composite effect of all ionic charge in solution
expressed as I (or ) in mole/L (or M)
where ci is the molar concentration of i and zi is the charge
on i (individual species) (Eqn. 1.6)
Example problem 1 (also see Example 1.5)
What is the ionic strength (I) of a 10-3 M NaCl and
0.2 x 10-3 N K 2SO4 solution?
21
2 i i
i
I c z
I = 1.3x10-3 M
8/11/2019 Lecture 3 - Activity Water Chemistry
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Example 1: Ionic strength
Calculate the ionic strength (I) for a 1 L solution in
which 0.01 mole of FeCl3 and 0.02 mole of H2SO4 are
dissolved and completely dissociated into ions:
8/11/2019 Lecture 3 - Activity Water Chemistry
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Worksheet 1: Ionic strength
The following quantities of salts were added to a
volume of water to make 1 liter of solution:
1170 mg/L NaCl
0.03 N CaCl2
1042 mg/L BaCl2
What is the ionic strength of the solution?
8/11/2019 Lecture 3 - Activity Water Chemistry
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Solution chemistry and activity coefficient
from infinite dilution to ~1 M ionic strength
ions are electrostatically attracted to each other associated ions decrease ability of ion to react ({i})
increasing ionic strength ( I ) more associated ions = less reactive ions
increase ionic charge ( z)
more electrostatic attraction = less reactive ions
effect accounted for by i
Na+
Cl-
Cl-
Ca2+
Cl-
Cl- Cl-
Chemical Activity
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Activity Activity coefficient ( i) characterizes the difference between
chemical environment in real system and that in the reference state
Estimating the activity coefficient i Three methods (See Table 1.4a)
Debye-Hückel limiting law: applicable at I < 0.005 M
Ions can be treated as point charges
Extended Debye-Hückel: applicable at I < 0.1 M
Ionic size are taken into account. But only same ionic size
Davies: applicable at I < 0.5 M Davies usually best for fresh water
1/ 22
1/ 2log 0.21i
I Az I I
D e b y e
H ü c k e l
ii i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity Typo (for old version textbook)
Table 1.4a (pg 30)
Extended Debye-Hückel, parameter B
B ≈ 0.33 in water at 25oC
1/ 2
1/ 2
50.3
50.3
B T
should be
B T
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1/ 2
21/ 2
6 3/ 2
6 3/ 2
log 0.21
1.82 10 ( )
1.82 10 [(78.4)(298.2 )]
0.51
i I Az I I
A T
A K
A
Activity Estimating the activity coefficient i
parameter A in Debye-Hückel and Davies
= 78.4 T = 298 K (25oC)
a number t o remember
ii i
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Worksheet 1 (cont.) - (activity coeff.)
With the ionic strength (I) that you calculated in the
worksheet 1, please predict by using the following
equations:
b) Debye-Huckel limiting law (1, 2)
c) Davis (1, 2)
d) Compare the results from b) and c) and discuss any
difference or similarity between them.
i i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity Difference between calculations: Na+
spreadsheet
ionic strength (M)10
-6
10-5
10-4
10-3
10-2
10-1
100
a c
t i v i t y
c o e f f i c i e n t
0.0
0.2
0.4
0.6
0.8
1.0
Debye-Hückel
ExtendedDebye-Hückel
Davies
ii i
i i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity (by Davies equation)
Effect of z : Na+, Ca2+, and Al3+
ionic strength (M)
10-6 10-5 10-4 10-3 10-2 10-1 100
a c t i v i t y
c o e f
f i c i e n t
0.0
0.2
0.4
0.6
0.8
1.0
Al3+
Ca2+
Na+
ii i
Why increases?
i i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity
Solution chemistry and activity coefficient
above ~1 M ionic strength
water molecules attracted to ions
activity of water decreases
“less water” increases abilityof ion to react ({i})
ii i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Example problem -2: Application of
What is the concentration of Pb2+ in water in equilibriumwith Pb(OH)2(s) in 0.1 M NaCl at pH 11 and 25C?
Pb(OH)2(s) = Pb2+ + 2 OH- K sp = 10-10.9
Pb2+
2 2
2
2 2
{ }{ }
{ }{ }
sp
sp
Pb OH K
Pb OH s
K Pb OH
i i
i i
8/11/2019 Lecture 3 - Activity Water Chemistry
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Activity What is the concentration of Pb2+ in water in equilibrium
with Pb(OH)2(s) in 0.1 M NaCl at pH 11 and 25C?
Pb(OH)2(s) = Pb2+
+ 2 OH-
K sp = 10-10.9
activity coefficients by Davies equation
I = 0.1 M
2
2
2 2 2 2 2
22 2
{ }{ } [ ] [ ]
[ ][ ]
sp Pb OH
sp
Pb OH
K Pb OH Pb OH
K Pb
OH
[Pb2+
] = 3.54 x10-5
M
ii i