5.112 Principles Of Chemical Science Worksheet With Answers - Problem Set #4 Solutions - 2011
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Fall 2011: 5.112 PRINCIPLES OF CHEMICAL SCIENCE
Problem Set #4 solutions
This problem set was graded out of 99 points
Question 1 (out of 7 points)
(i) For each pair, determine which compound has bonds with greater ionic character. (3 points)
(a) NH
or AsH
: The N-H bond is more ionic than the As-H bond since the difference in
3
3
electronegativity of N and H (3.0 vs. 2.2) is greater than the difference between As and H
(2.2 vs. 2.2).
(b) SiO
or SO
: S and O have similar electronegativities (3.0 and 3.4) whereas for Si and O the
2
2
difference is greater (1.9 vs. 3.4) so the Si-O bonds have greater ionic character.
(c) SF
or IF
: S and I have very similar electronegativities (2.6 and 2.7), thus the bonds are
6
5
very similar. S-F bonds may have a slightly higher ionic character, but it would be
difficult to determine experimentally.
(ii) In which molecule, O=C=O or O=C=CH
, do you expect the C–O bond to be more ionic?
2
(2 points)
The C–O bond in O=C=O will have greater ionic character because O is more electronegative
than the carbon in the CH
fragment, so in absolute values, the C in CO
has twice as much
2
2
charge removed as the C attached to one O and the CH
fragment.
2
(iii) In which molecule do you expect the bonds to be more ionic, CCl
or SiCl
? (2 points)
4
4
The difference in electronegativity between C and Cl is less than the difference between Si and Cl,
so the bonds in SiCl
will be have greater ionic character.
4
Question 2 (out of 3 points)
The gaseous potassium chloride molecule has a dipole moment of 10.3 D, which indicates that
it is a very polar molecule. The separation between the nuclei in this molecule is 2.67 Å.
Calculate the dipole moment of a KCl molecule if there were opposite charges of one
-19
fundamental unit (1.60 X 10
C) at the nuclei?
We can calculate the dipole moment of the molecule if it was completely ionic from the
following formula:
(
)
(
)
1.602 x 10 -19 C
2.67 x 10 -10 m
QR
µ =
=
= 12.8 D
3.336 x 10 -30 C m/D
3.336 x 10 -30 C m/D
The dipole moment of KCl is 12.8 D if it were 100% ionic in character.
Question 3 (out of 6 points)
Consider two diatomic molecules with 100% ionic character. One has a bond length of 1.75
Angstroms, the other a bond length of 3.25 Angstroms.
(a) Calculate the dipole moment for each molecule. (3 points)
The equation for dipole moment is: µ = QR , where Q = the charge and R is the distance
separating the two ions, if we want to have the dipole moment to be in debye, we need to have
-1
the conversion fact of 0.2082 A D
.
We can now go ahead and calculate the dipole moments for the two molecules:
0
⎛
⎞
0
( )
For the 1.75 A
bond length:
e
1.75 A
⎝ ⎜
⎠ ⎟
QR
µ =
=
= 8.41 D
0
0
−1
−1
0.2082 C A
D
0.2082 e A
D
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