5.112 Principles Of Chemical Science Worksheet With Answers - Problem Set #4 Solutions - 2011 Page 15
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16Problem set #4 solutions for 5.112, Fall 2011
Page 15 of 16
The potential energy minimum for the helium dimer is at an intermolecular distance of 0.287
nm, while the minimum for argon is 0.380 nm.
c)
Why does argon have a greater separation distance at the minimum than helium? (1
point)
Argon has a larger atomic radium (lower in the group on the periodic table), therefore (in
hard sphere model) unable to come “closer to each other.
d)
(Bonus) Was Lennard-Jones one person or two? Answer ONE!
Question 18 (out of 4 points)
Name the types of attractive forces that will contribute to the interactions among atoms,
molecules, or ions in the following substances. Indicate the one(s) you expect to predominate.
(a) KF
Potassium and fluorine different considerably in electronegativity. The bonding in
the compound potassium fluoride is expected to be ionic. These ion-ion attractions
predominate in this compound; dispersion forces are also present.
(b) Cl
Dispersion forces are the only intermolecular forces possible between molecules of
2
Cl
.
2
(c) HBr
Dipole-Dipole attractions predominate in the interactions between molecules in
hydrogen bromide. The positive (H) end of one molecule is attracted by the negative
(Br) end of another, but repelled by its positive (H) end. Dispersion forces are also
present.
(d) Kr
Dispersion forces are the only forces operating among the atoms in a sample of
Krypton. Single Kr atoms have completely symmetrical (spherically symmetrical)
distributions of charge. Two neighboring Kr atoms induce temporary dipoles in
each other that cause them to attract each other.
Question 19 (out of 2 points)
Predict whether a bromide ion will be most strongly attracted to a sodium ion, a molecule of
hydrogen chloride, or an atom of krypton.
A bromide ion should be most strongly attracted to a sodium ion. The attraction between ions of
-1
unlike charge such as these is stronger than the ion-dipole attraction between Br
and HCl and
-1
the ion-induced dipole attraction between Br
and Kr.
Question 20 (out of 3 points)
Under room conditions, fluorine and chlorine are gases, bromine is a liquid, and iodine is a
solid. Explain the origin of this trend in the physical state of the halogens.
The boiling point and melting point of a substance depend on the strength of the attractive forces
operation amount the particles that comprise the liquid or solid substance. These forces tend to
increase with increasing molar mass in a group of related substances. The halogens are certainly
closely related chemically. Therefore their boiling and melting points tend to rise with increasing
molar mass.
Question 21 (out of 4 points)
List the following substances in order of increasing normal boiling points T
, and explain your
b
reasoning: NO, NH
, Ne, RbCl.
3
Substances with the strongest intermolecular forces require the highest temperature to make
them boil. Liquid RbCl has strong Coulomb (electrostatic) forces holding its ions together. It has
the highest boiling point. Liquid NH
has dipole-dipole attractions, as does liquid NO. In liquid
3
NH
, these are particularly strong. They are hydrogen bonds. In liquid NO, the dipole-dipole
3
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