Chem 109 - General Chemistry - Spring 2013

Lecture Notes 33: 24 April

Liquids & Solids, cont.

Heating & Cooling Curves

A consideration of vapor pressure etc leads to the behaviors of substances with increasing (or decreasing) temperature (see Fig 10.44, p 476 of Zumdahl 8th ed):

plot of heating curve for water

Solutions —General Information

Solutions: a solution occurs when one chemical is completely dissolved or dispersed in another. We most commonly think of solutions as being liquid, but solid solutions also occur, such as the various metal alloys like steel, brass and bronze.

In a solution the substance present in highest concentration is considered to be the solvent, while components in lesser amounts are considered to be solutes. If you dissolve a sugar cube in water you get a sugar solution, where water is the solvent, and sugar is the solute.

FYI

Example:

What is the solvent in 80 proof rum: 80 proof = 40% alcohol in water, so water is the solvent.
What is the solvent in 151 proof rum: 151 proof = 75.5% alcohol in water, so alcohol is the solvent.

Solubility

All gases are completely soluble in each other.

Liquid solutions

"Like dissolves like."

Solvation - the process of surrounding a solute with solvent molecules).
- Hydration (solvation with water).
Saturation - the maximum amount of solute which can be in solution in equilibria with its pure state.
- Supersaturation - dissolving solute in excess of saturation - unstable to the addition of solute (carbonated beverages, honey, etc.) Video
Temperature effects:
- Gases decrease in solubility with increasing temperature. [example of oxygen solubility]
- Many solids increase in solubility with increasing temperature, but not always true.
Pressure effects:
- Gases increase in solubility with increasing pressure (Henry's Law: P = kC, where k is a constant specific to the solution). [example of carbonated beverages - p 493 in Zumdahl).
  - Note that Henry's Law only holds for gases that are non-reactive with the solvent - e.g. works for oxygen and nitrogen in water, does not work for HCl and water (goes to H⁺ + Cl^-)
- Solubility of solids generally uneffected by pressure, since both liquids and solids essentially incompressible.

Solids and Crystals

Solids:Recall earlier definition - solids have fixed or definite shapes and volumes. By this definition solids are strictly limited to the crystalline solids. (The amorphous (noncrystalline) solids discussed in our text are what we have discussed as supercooled liquids.)

Properties:

Hard and rigid - they have virtually no tendency to flow or diffuse.
Nearly incompressible - need to increase pressure about 1,000,000 times to decrease volume by half.
Very low thermal coefficients of expansion.
Crystal lattice
Melting and freezing points are sharp - all units in the interior of a perfect crystal have the same relationships, and therefore the same bonds. Thus when enough energy is added to break the bonds for one unit, there is enough to break bonds with all, so melting is sudden as all the particles break bonds with each other at same temperature and thus same energy.

heat of fusion/crystallization (saw last time with liquids)

Nearly all solids expand when they melt (after all the particles are moving faster). As a consequence, nearly all solids will sink in their liquid forms (water is of course a notable exception - we'll look at why later).

Structure Determination: So how do we know how atoms are arranged in crystals?

X-ray diffraction is THE tool for crystal structure determination. It gives a full 3-D picture of how atoms are arranged via the interpretation of diffraction patterns. So what kind of information is obtained, and how do we use it to reconstruct a crystal?

Xray scattering (Figure 10.10, Zumdahl p 433) and diffraction patterns.

$Bragg diffraction diagram$

public domain image via Wikipedia Creative Commons

Bragg Equation: we can determine the distances between layers in a repeating structure using x-rays. The relevant distances can be seen in Figure 10.11, Zumdahl p 433, resulting in the Bragg equation:

n = 2d sin

STEM: a newer method with equal resolution is Scanning Tunneling Electron Microscopy. This technique give a highly detailed view of the surface of a crystal (or other object). Distances between surface atoms can be calculated etc.

Crystal (Solid) Structure

Crystal Structure (overheads)

Lattice
- Unit Cell
Crystal lattice
- 3 kinds of cubic lattice: simple cubic, body-centered cubic, and face-centered cubic. (Figure 10.9, p 446 of Zumdahl)
public domain image via Wikipedia Creative Commons
- For identical spheres can get two kinds of close packing: (Figure 10.13, p 451 of Zumdahl)
public domain image via Wikipedia Creative Commons
- cubic close packing, which turns out to be a face-centered cubic lattice (Figure 10.15, p 437 of Zumdahl)
- hexagonal close packing (Figure 10.14, p 437 of Zumdahl)
The images below show the so-called cannon ball stacking in close-packing. Note that the stack is a direct result of the HCP lattice shown in the image above, where just the top ball and the next layer of three balls are darkened. Can you discern the next (triangular) layer in the diagram which cooresponds to the third layer down in the pictures below?

public domain image via Wikipedia Creative Commons

public domain image via Wikipedia Creative Commons

Types of Solids

Metallic solids: these are different in that they have ions at the lattice points in a "sea" of shared electrons (Figures 10.18-.20, p 441-2 of Zumdahl). In metals that are very hard, such as chromium, the ions are also covalently bonded to each other.

Network (or covalent) solids - Carbon as example: carbon has allotropes (different physical forms of the same element) all of which have many atoms linked together in covalent networks, and two of which are covalent solids: (Figure 10.22, p 444 of Zumdahl)

Diamond (covalent solid) - each atom in the bulk solid is covalently bonded to four others in a tetrahedral arrangement. This is the secret to diamonds great hardness: to break a piece off many strong covalent bonds must be broken.

Graphite (covalent solid) - as in diamond the carbon atoms are covalently linked to each other, but this time in layers, where each carbon is linked to three others with strong covalent bonds in a trigonal planar geometry. The layers are then very weakly held to each other, so they can readily slide over each other, making them "slippery." Note that the sheets are very strong, thus graphite fibers are used to reinforce other materials (graphite fishing poles etc.).

Fullerenes (molecular solids) - graphite like sheets are rolled up to make spheres (commonly 60 C's), and tubes.