Liquids and Solids
The Condensed Phases
Liquids
Viscosity
Resistance of a liquid to flow is called viscosity.
It is related to the ease with which molecules can move past each other.
Viscosity increases with stronger intermolecular forces and decreases with higher temperature.
Surface Tension
The result of surface tension is that some liquids "ball up" on a surface while others "splat out". The reason is very similar to capillary action. There are both cohesion forces (intermolecular forces between molecules) and adhesion forces (attractions for the surface). See larger picture below.
Caplillary Action
Capillary action is a combination of surface tension (from cohesive forces) and the attraction a liquid's particles has for the surface of a solid (adhesion). This is why some liquids like water seem to "crawl" through and up narrow glass tubes.
Difference in capillary action for water and mercury
Surface tension differences due balance between adhesion and cohesion
Vapor Pressure
Big idea here!
All related to IMF'S!
We have seen a diagram like this one before when we discussed IMF's. The ease with which molecules can break free from a liquid is directly proportional to the strength of attraction between particles. These curves represent a range of liquids with varying strengths of IMF's.
What happens to vapor pressures when liquids are mixed?
Vapor-liquid equilibria for (a) pure toluene; (b) a mixture of equal amounts of toluene and benzene: and (c) pure benzene. In the solution (b) only half the molecules are benzene molecules, and so the concentration of benzene molecules in the vapor phase is only half as great as above pure benzene. Note also that although the initial amounts of benzene and toluene in the solution were equal, more benzene than toluene escapes to the gas phase because of benzene’s higher vapor pressure.
☛So, which liquid has the stronger intermolecular force, toluene or benzene?
Energy Changes Associated with Changes of State
Calculating Energies for Phase Changes
The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other.
The temperature of the substance does not rise during a phase change.
Enthalpies of phase changes are tied to IMF's
Practice
1. Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) bromine vapor turns to bromine liquid as it is cooled. (b) crystals of iodine disappear from an evaporating dish as they stand in a fume hood. (s) rubbing alcohol in an open container slowly disappears. (d) Molten lava from a volcano turns to solid rock.
2. Compounds like CCl2F2 are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of CCl2F2 is 289J/g. What mass of this substance must evaporate to freeze 200 g of water initially at 15 C? (The heat of fusion of water is 334 J/g; the specific heat of water is 4.18 J/gC.
There are four basic types of crystalline solids
What about solids that are not crystalline?
Practice
1. Silicon is the fundamental component of integrated circuits. Si has the same structure as diamond. Is Si a molecular, metallic, ionic, or covalent-network solid? (12.8)
2. Which type (or types) of crystalline solid is characterized by each of the following: (a) high mobility of electrons throughout the solid; (b) softness, relatively low melting point; (c) high melting point and poor electrical conductivity; (d) network of covalent bonds. (12.10)
3. Indicate the type of crystal (molecular, metallic, ionic, or covalent-network) each of the following would form upon solidification: (a) InAs, (b) MgO, (c) HgS, (d) In, (e) HBr. (12.12)
4. You are given a white substance that sublimes at 3000 C; the solid is a nonconductor of electricity and is insoluble in water. Which type of solid (molecular, metallic, covalent-network, or ionic) might this substance be? (12.14)
Types of crystal lattices
There are seven basic three-dimensional lattices:
Cubic
Tetragonal
Orthorhombic
Rhombohedral
Hexagonal
Monoclinic
Triclinic
Generating a crystal from the lattice and the molecules
Do not be confused by the dots in the lattice structures-they are not the atoms necessarily!
We will take a closer look at the cubic group of lattices
We can learn a lot about the densities of solids by taking a closer look at these structures.
Primitive Cubic
Body-centered Cubic
Face-centered Cubic
In a face‐centered cell, contact is along the face diagonal. Although it is difficult to visualize here, in a face-centered cell each atom has 12 nearest neighbors. The relationship between atomic radius (r) and cell edge (a) is 4r = a √2.
Practice
5. Of the seven three-dimensional primitive lattices, which ones have a unit cell where all three lattice vectors are of the same length? (12.22)
6. Table 12.1 (see below) is helpful for this question: What is the number of atoms that could be contained in the unit cell of an element with a face-centered cubic lattice? (12.24)
7. Sodium metal adopts a body-centered cubic structure with a density of 0.97 g/cm3. Use this information and Avogadro’s number to estimate the atomic radius of sodium. (12.30-(a))
8. Calcium crystallizes with a body centered cubic structure. (a) How many Ca atoms are contained in each unit cell? (b) How many nearest neighbors does each Ca atom possess? (c) Estimate the length of the unit cell edge, a, from the atomic radius of calcium, (1.97 A) (d) Estimate the density of Ca metal(12.32)
9. An element crystallizes in a body-centered cubic lattice. The edge of the unit cell is 2.86 angstroms (one angstrom = 1 x 10-10 m), and the density of the crystal is 7.92 g/cm3. Calculate the atomic weight of the element. (12.34)
Across period 3 there are metals, semimetals, and nonmetals
In elemental samples of nonmetals and metalloids, atoms generally bond to each other covalently.
Metals, however, have a dearth of valence electrons; instead, they form large groups of atoms that share electrons among them.
Nonmetals = covalent bonding
For some semimetals (like Si and C) = network covalent bonding
Metals = metallic bonding
The electronic properties of metals vs semimetals vs nonmetals is governed by the Band Gap
Semiconductors (group 4A) and their TUNABLE properties are of the upmost interest!
Metals share electrons equally
One can think of a metal, therefore, as a group of cations suspended in a sea of electrons.
The electrical and thermal conductivity, ductility, and malleability of metals is explained by this model. Therefore, the melting points for metals should increase with increasing numbers of valence electrons. This trend is observed partially by this sea of electrons model but not entirely as seen below.
Aluminum
Magnesium
Sodium
A molecular orbital approach explains the melting point trend seen for the metals
In the image below, the origin of the band becomes quite clear because as the number of molecular orbitals increases, the bonding and antibonding orbitals get closer together filling in the middle. This results in the band seen on the right hand side. It becomes quite clear that the molecular orbitals become blurred and hence mix with each other, which creates the delocalized cloud of electrons that metals are said to possess.
By describing the molecular orbitals of certain materials as bands, it becomes much easier to understand the properties of metals and semi-metals. Other materials like insulators and semi-conductors will be discussed and concepts such as the valence band, conduction band and the Fermi Level can be defined using the band theory.
Alloys
It is the size of the substituted atom in an alloy that determines the type.
In substitutional alloys, a second element takes the place of a metal atom. Usually this second element about the same size as the original element.
In interstitial alloys, a second element fills a space in the lattice of metal atoms.
Both of these alloys have formulas that look like: Cu0.66Zn0.34 or Ti0.99O0.01.
Intermetallic compounds are just that! These are the types of compounds you are familiar with, like Ag3Sn.
Practice
10. For each of the following alloy compositions indicate whether you would expect it to be a substitutional alloy, an interstitial alloy, or an intermetallic compound. (a) Cu0.66Zn0.34 (b) Ag3Sn (c) Ti0.99O0.01. (12.38)
11. How do you account for the observation that the alkali metals, like sodium and potassium, are soft enough to be cut with a knife? (12.48)
12. For each of the following groups which metal would you expect to have the highest melting point: (a) Au, Re, or Cs; (b) Rb, Mo, In; (c) Ru, Sr, Cd? (12.50)
Ionic solids adopt structures similar to what we have seen for metals with some important differences!
In ionic solids, the lattice comprises alternately charged ions.
Ionic solids, like metals, are not molecular. The "molecule" is a formula unit.
Number of cations per formula unit = anion coordination number
Number of anions per formula unit cation coordination number
Practice
13. The coordination number for the Al3+ ion is typically between four and six. Use the anion coordination number to determine the Al3+ coordination number in the following compounds: (a) AlF3 where the fluoride ions are two coordinate (b) Al2O3 where the oxygen ions are six coordinate (c) AlN where the nitride ions are four coordinate. (12.60)
14. Which of the following properties are typical characteristics of a covalent network solid, a metallic solid, or both: (a) ductility, (b) hardness, (c) high melting point? (12.64)
15. For each of the following pairs of semiconductors, which one will have the larger band gap: (a) InP or InAs, (b) Ge or AlP, (c) AgI or CdTe? (12.66)
16. If you want to dope GaAs to make an n-type semiconductor with an element to replace Ga, which element(s) would you pick? (12.68)
Molecular solids are comprised of discrete molecules
Two forms of carbon (allotropes): one is molecular and one is covalent network.
Practice
17. The molecular formula of n-decane is CH3(CH2)8CH3. Decane is not considered a polymer, whereas polyethylene is. What is the distinction? (12.76)
18. Write a chemical equation for the formation of a polymer via a condensation reaction from the monomers succinic acid (HOOCCH2CH2COOH) and ethylenediamine (H2NCH2CH2NH2). (12.78)
Extra Practice
Notes with examples (no answers)