pole-dipole attractive forces are most important in causing con-In the crystalline solid state, there is little vibrational or densation, and molecules with substantial partial charges, re-translational freedom, and hence diffusion into a crystalline sulting from polar covalent bonds, typically have high condensa-lattice is slow and difficult. As the temperature of a solid is tion temperatures. (Condensation temperature will be the same raised by the input of heat, vibrational and translational motion as the boiling point of a liquid, approached from the opposite di-increases. At a particular temperature - termed the melting rection. )
point - this motion overcomes the attractive forces holding the The liquid state has a minimum of order, and the molecules lattice together and the liquid state is produced. The liquid have considerable freedom of motion. A drop of food coloring state, on cooling, returns to the solid state as crystallization placed in water demonstrates the rapid diffusion that can occur occurs and heat is released by the formation of strong attrac-in the liquid state. The solid state will exhibit no detectable tive forces.
diffusion. If this experiment is tried with a material such as The types of solids, categorized according to the particles iron, the liquid food coloring will merely form a drop on the sur-that make up the crystalline lattice, are listed in Table 2.9.
face of the metal.
The type of crystalline lattice formed by a solid material de-At the liquid surface, molecules can acquire high vibrational pends on the size and shape of the lattice units, as well as on and translational energy from their neighbors, and one will oc-the nature of the attractive forces. Six basic crystalline sys-casionally break loose to enter the vapor state. This phenomenon tems are possible [6]
of vapor above a liquid surface is termed vapor pressure, and will lead to gradual evaporation of a liquid unless the container is covered. In this case, an equilibrium is established between 1. Cubic: three axes of equal length, intersecting at all the molecules entering the vapor state per minute and the mole-right angles
cules recondensing on the liquid surface. The pressure of gas 2. Tetragonaclass="underline" three axes intersecting at right angles; only molecules above a confined liquid is a constant for a given ma-two axes are equal in length
terial at a given temperature, and is known as the equilibrium 3. Hexagonaclass="underline" three axes of equal length in a single plane vapor pressure. It increases exponentially with increasing tem-intersecting at 60 0 angles; a fourth axis of different length perature. When the vapor pressure of a liquid is equal to the is perpendicular to the plane of the other three
36
Chemistry of Pyrotechnics
Basic Chemical Principles
37
TABLE 2.9 Types of Crystalline Solids
TABLE 2.10 Thermal Conductivity Values for Solidsa Units
Thermal conductivity (X 10),
Type of
comprising
Material
cal/see-cm-IC
solid
crystal lattice
Attractive force
Examples
Copper
910
Ionic
Positive and
Electrostatic attrac- KNO 3 , NaCl
Aluminum
500
negative ions
tion
Iron
150
Molecular Neutral mole-
Dipole-dipole attrac- CO 2 ("dry ice"), cules
tions, plus weaker, sugar
Glass
2.3
non-polar forces
Oak wood
0.4
Covalent
Atoms
Covalent bonds
Diamond (carbon)
Paper
0.3
Metallic
Metal atoms
Dispersed electrons Fe, Al, Mg
Charcoal
0.2
attracted to nu-
merous metal atom
nuclei
a Reference 8.
4. Rhombic: three axes of unequal length, intersecting at melting point of the solid, with the solid } liquid transition occur-right angles
ring over a broad range rather than displaying the sharp melting 5. Monoclinic: three axes of unequal length, two of which observed with a purer material.
Melting behavior thereby pro-
intersect at right angles
vides a convenient means of checking the purity of solids.
6. Triclinic : three axes of unequal length, none of which An important factor in the ignition and propagation of burning intersect at right angles
of pyrotechnic compositions is the conduction of heat along a column of the mixture. Hot gases serve as excellent heat carriers, To this point, our model of the solid state has suggested a but frequently the heat must be conducted by the solid state, placement of every lattice object at the proper site to create a ahead of the reaction zone. Heat can be transferred by molecu-
"perfect" three-dimensional crystal. Research into the actual lar motion as well as by free, mobile electrons [6]. The thermal structure of solids has shown that crystals are far from per-conductivity values of some common materials are given in Table 2.10.
fect, containing a variety of types of defects. Even the purest Examining this table, one can readily see how the pres-crystals modern chemistry can create contain large numbers of ence of a small quantity of metal powder in a pyrotechnic compo-impurities and "misplaced" ions, molecules, or atoms in the lat-sition can greatly increase the thermal conductivity of the mixture, and thereby increase the burning rate.
tice. These inherent defects can play an important role in the reactivity of solids by providing a mechanism for the transport Electrical conductivity can also be an important consideration in pyrotechnic theory [7].
of electrons and heat through the lattice. They also can greatly This phenomenon results from the
enhance the ability of another substance to diffuse into the lat-presence of mobile electrons in the solid that migrate when an electrical potential is applied across the material.
tice, thereby again affecting reactivity [7).
Metals are
A commonly-observed phenomenon associated with the pres-the best electrical conductors, while ionic and molecular solids ence of impurities in a crystalline lattice is a depression in the are generally much poorer, serving well as insulators.