Class Notes (1,100,000)
US (470,000)
UofL (900)
CHEM (50)
Lecture 4

CHEM 202 Lecture Notes - Lecture 4: Crystal Structure, Intermolecular Force, Ionic Bonding


Department
Chemistry
Course Code
CHEM 202
Professor
Neal Stolowich
Lecture
4

This preview shows pages 1-2. to view the full 7 pages of the document.
Lecture 4
Review Chap 37 - three intermolecular forces: H-bonding, dipole-dipole, and dispersion
Liquids and solids – Chap 38
Properties of the Liquid State (chap 38.1) – recall from our earlier phase comparisons, that the liquid
phase is a hybrid of the two other phases: particles are in loose contact and intermolecular forces can
exert an influence much as in the solid phase, however increased entropy gives the particles more
options like a gas, and hence are less oriented then the solid phase. As we’ll see, the types of IF can
now have a significant influence on a liquid’s properties.
Main properties of the liquid phase:
A. Viscosity is the resistance to flow. We commonly equate viscosity with “thickness” and slow to
pour. The stronger the intermolecular forces in a liquid resist the ability of the particles to flow,
and higher viscosity results. Also, more surface contact (dispersion!) results in higher viscosity.
Glycerol is a thousand times more viscous then water as it has 3 O-H groups (H-bonding) and
longer chain length (dispersion). Oils are even more viscous despite no H-bonds because of
dispersive forces dominating. Viscosity decreases with increasing temperature as kinetic energy
will overcome IFs.
B. Surface tension is more of a property of the liquid – gas interface then the bulk liquid, allowing
the surface of a liquid to behave as a “skin”. Surface tension results unbalanced attraction
between particles at the surface, relative to the balanced attraction in bulk liquid. In general, the
stronger the intermolecular forces, the greater surface tension, however the way some liquids
can orient themselves on the surface can increase surface tension as well (such as liquid metals)
(Fig S12.19)
Surface tension results in a liquid’s ability to bead, or form spheres- which is the most efficient
way to minimize its surface area.
C. Capillarity or Capillary Action– competition exists between intermolecular forces within the
liquid versus with the molecules composing the wall. Accordingly, no direct trends base on the
property of the liquid (although surface tension is important), but depends on the nature of the
solid wall and whether cohesive or adhesive attractions dominate. Cohesion is the attraction
between particles in a single medium (the liquid), while adhesion is the attraction between two
media (liquid and solid wall).
Water has high capillary action, resulting from both its high surface tension, but also adhesion
to the wall of the glass (mainly silicon dioxide), resulting in the typical “smile” of the surface.
Water molecules are pulled up the wall on the glass to maximize the adhesive forces. On the
other hand, poor adhesion (mercury vs glass wall, water vs. waxed glass wall) results in a
“frown”.
(Fig S12.20)
find more resources at oneclass.com
find more resources at oneclass.com

Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

Structure and property of the Solid State (38.2) – Better understood on the basis of IFs, since the
particles are not moving around. Orientation is indeed important! Solids locked into an orderly
arrangement are called crystalline solids while those arranged with little or no order are called
amorphous solids. We will focus on the former.
A. Classification of solids by spatial arrangement (38.3) – the spatial arrangement of a crystalline
solid is described by a lattice.
The Crystal Lattice and the Unit Cell - Definitions:
Lattice – the overall pattern of the arrangement of chemical units within the crystal.
unit cell – simplest repeating pattern of the lattice
Both of these are subject
To orientation, ie-
Types of unit cells- many types possible, but we’ll consider just three (p. 395)
Types of unit cells- (summary ; Silberberg summary figure 12.27)
A.i.1.a.i. Simple cubic unit cell (sc) – 8 spheres on
each corner of the cube (none internally). Since only 1/8 of each sphere lies within a given
unit cell, 1/8 x 8 = 1 sphere/unit cell.
A.i.1.a.ii. Body-centered unit cell (bcc) – A simple
unit cell containing one additional sphere in the center of the cube, to give 2 spheres / unit
cell.
A.i.1.a.iii. Face-centered unit cell – 1/8 from sc plus ½
sphere on each of 6 faces (sides) = 4 spheres/unit
B. Classification of solids by chemical type (38.4) –
Molecular compounds – components of the lattice are individual molecules of the substance
(ie, H2O, CO2 , O2, and even monoatomic atoms such as He). Many are fcc lattices where the
entire molecule represents each sphere on the lattice, resulting in 4 molecules per unit cell.
(show example for solid CH4, fig S12.30) HCN has a bcc lattice with only two HCN molecules
per unit cell
find more resources at oneclass.com
find more resources at oneclass.com
You're Reading a Preview

Unlock to view full version