Environmental Engineering Reference
In-Depth Information
E
XAMPLE
4.3
K
aw
FROM A
B
OND
C
ONTRIBUTION
S
CHEME
Given the following bond contributions to log
K
aw
, calculate
K
aw
for (a) benzene, (b)
hexachlorobenzene, and (c) chloroform.
log
K
aw
Bond
H-C
ar
−
0.154
C
ar
-C
ar
0.264
H-C
al
−
0.119
Cl-C
ar
−
0.024
Cl-C
al
0.333
The symbols above are self-explanatory. The subscript ar denotes aromatic while al
refers to aliphatic.
(a) In benzene there are six C
ar
-C
ar
bonds and six H-C
ar
bonds. Thus
−
log
K
aw
=
6
(
−
0.154
)
+
6
(
0.264
)
=
0.66. Hence
K
aw
=
0.219. The experimental value is
0.231.
(b) In hexachlorobenzene there are six C
ar
-C
ar
bonds and six Cl-C
ar
bonds.
Hence
−
log
K
aw
=
6
(
0.264
)
+
6
(
−
0.024
)
=
1.46.
K
aw
=
0.038.Theexperimen-
tal value is 0.002.
(c) In chloroform we have one H-C
al
and three Cl-C
al
bonds. Hence
log
K
aw
=
−
0.119
+
3
(
0.333
)
=
0.87.
K
aw
=
0.134. The experimental value is 0.175.
The bond contribution scheme suffers from a major disadvantage in that interactions
between bonds are usually not considered. This will be a serious drawback when polar
bonds are involved.
−
E
XAMPLE
4.4
K
aw
FROM A
G
ROUP
C
ONTRIBUTION
S
CHEME
Estimate the Henry's constant for benzene and hexachlorobenzene using the following
group contributions:
Group
−
log
K
aw
C
ar
-H(C
ar
)
2
0.11
C
ar
-(C)-(C
ar
)
2
0.70
C
ar
-Cl(C
ar
)
2
0.18
For benzene we have six C
ar
-H(C
ar
)
2
groups. Hence
−
log
K
aw
=
6
(
0.11
)
=
0.66.
K
aw
=
0.218. The experimental value is 0.231. The prediction is only marginally better
than the bond contribution scheme.
For hexachlorobenzene we have six C
ar
-Cl(C
ar
)
2
. Hence
−
log
K
aw
=
6
(
0.18
)
=
1.08.
K
aw
=
0.083. The experimental value is 0.002. The agreement is poor and is no
better than that for the bond contribution scheme.
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