11
12
Chemistry of Pyrotechnics
Basic Chemical Principles
13
an ionic compound consisting of sodium and chloride ions, Na+
TABLE 2.3 Electronegativity Values for Some
and Cl- , and one uses the formula NaCl to represent the one-to-Common Elements
one ionic ratio. The attractive forces holding the solid together are called ionic bonds.
Pauling electronegativity
Hence, if one brings together a good electron donor (such as Element
valuea
a sodium atom) and a good electron acceptor (such as a fluorine atom), one might expect a chemical reaction to occur. Electrons Fluorine, F
4.0
are transferred and an ionic compound (sodium fluoride, NaF) Oxygen, 0
3.5
is produced.
Nitrogen, N
3.0
Chlorine, Cl
3.0
Na + F - Na+F- (sodium fluoride)
Bromine, Br
2.8
A three-dimensional solid lattice of sodium and fluoride ions Carbon, C
2.5
is created, where each sodium ion is surrounded by fluoride Sulfur, S
2.5
ions, and each fluoride ion in turn is surrounded by sodium Iodine, I
2.5
ions. Another very important aspect of such a reaction is the Phosphorus, P
2.1
fact that energy is released as the product is formed. This re-Hydrogen, H
2.1
lease of energy associated with product formation is most important in the consideration of the chemistry of pyrotechnics.
aSource: L. Pauling, The Nature of the Chemical In addition to forming ions by electron transfer, atoms may Bond, Cornell University Press, Ithaca, NY, 1960.
share electrons with other atoms as a means of acquiring filled shells (and their associated stability). The simplest illustration of this is the combination of two hydrogen atoms (symbol H, atomic number 1) to form a hydrogen molecule.
H + Cl -> H-Cl (hydrogen chloride)
H + H -} H-H (H
By this combination, both atoms now have "filled shell" elec-2 , a molecule)
tronic configurations and a hydrogen chloride molecule is formed.
The sharing of electrons between two atoms is called a covalent The sharing here is not exactly equal, however, for chlorine is bond. Such bonds owe their stability to the interaction of the a stronger electron attractor than hydrogen. The chlorine end shared electrons with both positive nuclei. The nuclei will be of the molecule is slightly electron rich; the hydrogen end is separated by a certain distance -- termed the bond distance -
electron deficient. This behavior can be noted using the Greek that maximizes the nuclear-electron attractions balanced against letter "delta" as the symbol for "partial," as in the nuclear-nuclear repulsion. A molecule is a neutral species of two or more atoms held together by covalent bonds.
b H-Cl 6
The element carbon (symbol C) is almost always found in nature covalently bonded to other carbon atoms or to a variety of The bond that is formed in hydrogen chloride is termed polar other elements (most commonly H, O , and N). Due to the pres-covalent, and a molecule possessing these partial charges is re-ence of carbon-containing compounds in all living things, the ferred to as "polar." The relative ability of atoms of different chemistry of carbon compounds is known as organic chemistry.
elements to attract electron density is indicated by the property Most high explosives are organic compounds. TNT (trinitrotolu-termed electronegativity. A scale ranking the elements was de-ene), for example, consists of C, H, N, and 0 atoms, with a mo-veloped by Nobel Laureate Linus Pauling. The electronegativity sequence for some of the more common covalent-bond forming ele-lecular formula of C 7H 5N 3O 6. We will encounter other organic compounds in our study of fuels and binders in pyrotechnic mix-ments is given in Table 2.3. Using this sequence, one can assign partial charges to atoms in a variety of molecules; the more tures.
Covalent bonds can form between dissimilar elements, such as electronegative atom in a given bond will bear the partial nega-hydrogen and chlorine.
tive charge, leaving the other atom with a partial positive charge.
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Chemistry of Pyrotechnics
Basic Chemical Principles
15
TABLE 2.4 Boiling Points of Several Small Molecules rule of solubility is "likes dissolve likes" - a polar solvent such as water is most effective at dissolving polar molecules (such as Boiling point (°C at
sugar) and ionic compounds. A non-polar solvent such as gaso-Compound
Formula
1 atmosphere pressure)
line is most effective at dissolving other non-polar species such as motor oil, but it is a poor solvent for ionic species such as so-Methane
CH,,
-164
dium chloride or potassium nitrate.
Carbon dioxide
CO
-
2
78.6
Hydrogen sulfide
H 2S
-60.7
THE MOLE CONCEPT
Water
H ,O
+100
Out of the atomic theory developed by John Dalton and other chemistry pioneers in the 19th century grew a number of important concepts essential to an understanding of all areas of chemistry, including pyrotechnics and explosives. The basic features of the atomic theory are
These partial charges, or dipoles, can lead to intermolecular at-1. The atom is the fundamental building block of matter, and tractions that play an important role in such physical properties consists of a collection of positive, negative, and neutral as melting point and boiling point, and they are quite important subatomic particles.
in determining solubility as well.
The boiling point of water,
Approximately 90 naturally-occurring elements are known 100°C, is quite high when compared to values for other small to exist (additional elements have recently been synthe-molecules (Table 2.4).
sized in the laboratory using nuclear reactions, but these This high boiling point for water can be attributed to strong unstable species are not found in nature).
intermolecular attractions (called "dipole-dipole interactions") of 2. Elements may combine to form more complex species called the type
compounds. The molecule is the fundamental unit of a compound and consists of two or more atoms joined together by chemical bonds.
3. All atoms of the same element are identical in terms of the number of protons and electrons contained in the neutral species. Atoms of the same element may vary in the num-The considerable solubility of polar molecules and many ionic comber of neutrons, and therefore may vary in mass.
pounds in water can be explained by dipole-dipole or ion-dipole 4. The chemical reactivity of an atom depends on the number interactions between the dissolved species and the solvent, water.
of electrons; therefore, the reactivity of all atoms of a given element should be the same, and reproducible, anywhere in the world.
5. Chemical reactions consist of the combination or recombination of atoms, in fixed ratios, to produce new species.
6. A relative scale of atomic weights (as the weighted average The solubility of solid compounds in water, as well as in other of all forms, or isotopes, of a particular element found in solvents, is determined by the competition between attractions in nature) has been developed. The base of this scale is the the solid state between molecules or ions and the solute-solvent assignment of a mass of 12.0000 to the isotope of carbon attractions that occur in solution. A solid that is more attracted containing 6 protons, 6 neutrons, and 6 electrons. An to itself than to solvent molecules will not dissolve. A general atomic weight table can be found in Table 2.2.
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Chemistry of Pyrotechnics
Basic Chemical Principles