Homework Solutions

January 28

13.11
a.    Electronegativity is how strongly an atom pulls electrons to itself; any electrons.  Including its own.  Electron affinity is the energy change of a system associated with the addition of an (extra) electron to an (uncharged) atom.  Related concepts, but different.
b.    In a covalent bond, electrons are shared equally between two atoms.  In a polar covalent bond, electrons are shared, but not equally.
c.    In a polar covalent bond, electrons are shared, but they spend more time/have greater density on one of the atoms.  In an ionic bond, there is no sharing of electrons at all.

16.17  Excellent answers in the back of your textbook.  Note that LD is the same thing as induced dipole-induced dipole.

16.21    A recurring theme with respect to this question is that stronger intermolecular attractions mean that a higher temperature is required in order to separate molecules from the condensed, liquid phase to the gaseous phase.  So, the trick is to determine which molecules will have stronger intermolecular forces. 
a.    Although both molecules are non-polar (do not have a dipole moment), it is easier to induce a dipole in the more linear n-pentane.  Hence the intermolecular attractions are stronger in n-pentane, therefore its boiling point is higher.
b.    Dimethyl ether is slightly polar, ethanol is more polar, so it attracts itself more.  In addition, ethanol is capable of forming hydrogen bonds (which we did not talk about).  Hydrogen bonds are stronger intermolecular forces than dipole-dipole attractions.
c.    HF is more polar than HCl.
d.    LiCl is an ionic compound.  Ion-ion attractions are very much stronger than dipole-dipole.
e.    n-pentane is heavier so it takes more energy to vaporize it. 
f.    Dimethyl ether is polar, n-propane is not.

16.28  Liquids and solids both have defined volume at a given temperature; liquids and solids are difficult to compress.  Liquids and gases both flow; liquids and gases both assume the shape of their container.

16.34    CSe₂ > CS₂ > CO₂

16.35

a.    In crystalline solids, the atoms are arranged in some regular, repeating pattern.  There is a unit cell.  In amorphous solids there is not regular repeating patter.

b.    In ionic solids, the ions are in some regular repeating pattern.  Usually the larger ion is arranged occupy lattice sites in one of the unit cells and the counter ions fill in the holes.  The ratio of ions in the unit cell must match the ratio of ions in the formula for the ionic compound.  It is not possible, however, to remove exactly one formula of an ionic compound.  By that I mean that it is not possible to remove exactly one 'NaCl' from solid NaCl.  The term molecular solid means whole molecules are in each lattice position.  It is possible, whatever the structure, to remove exactly one molecule from the solid.

c.    In a network solid atoms occupy specific lattice sites.  In a molecular solid molecules occupy lattice sites.

d.    In network solids, there exist strong, directional bonds.  In metals the bonds are equally strong in all directions.