and the fulminate ion, CNO - , are ex-
N3-,
tive value for AG for the decomposition (therefore making it a amples of species whose unstable behavior is explainable using spontaneous process).
this approach [ 3] .
_
These considerations make it mandatory that anyone working In structures such as the nitrate ion, N0 3 , and the perchlor-with nitrogen-rich carbon-containing compounds or with nitrate, ate ion, C10 - , a highly electronegative atom has a large, positive 4
perchlorate, and similar oxygen-rich negative ions must use oxidation number (+5 for N in
+7 for Cl in C1O,,) . Such
N03-1
extreme caution in the handling of these materials until their
3 2
Chemistry of Pyrotechnics
Basic Chemical Principles
33
properties have been fully examined in the laboratory. Elevated TABLE 2.8 Melting Points of Some Common Oxidizers temperatures should also be avoided when working with potentially-unstable materials, because of the rate-temperature rela-Melting point,
tionship that is exponential in nature. A non-existent or sluggish process can become an explosion when the temperature of Oxidizer
Formula
°C a
the system is sharply increased.
Potassium nitrate
KNO 3
334
Potassium chlorate
KC10 3
356
STATES OF MATTER
Barium nitrate
Ba(N03)2
592
With few exceptions the high-energy chemist deals with materials Potassium perchlorate
KC1O,,
610
that are in the solid state at normal room temperature. Solids mix very slowly with one another, and hence they tend to be Strontium nitrate
Sr(N0 3 ) 2
570
quite sluggish in their reactivity.
Rapid reactivity is usually
Lead chromate
PbCr0
844
L
associated with the formation, at higher temperatures, of liquids or gases. Species in these states can diffuse into one another Iron oxide
Fe 20 3
1565
more rapidly, leading to accelerated reactivity.
In pyrotechnics, the solid-to-liquid transition appears to be a Reference 1.
of considerable importance in initiating a self-propagating reaction.
The oxidizing agent is frequently the key component in such mixtures, and a ranking of common oxidizers by increasing melting point bears a striking resemblance to the reactivity se-This equation is obeyed quite well by the inert gases (helium, quence for these materials (Table 2.8).
neon, etc.) and by small diatomic molecules such as H 2 and N 2 .
Molecules possessing polar covalent bonds tend to have strong Gases
intermolecular attractions and usually deviate from "ideal" beha-On continued heating, a pure material passes from the solid to vior.
Equation 2.5 remains a fairly good estimate of volume and liquid to vapor state, with continued absorption of heat. The pressure even for these polar molecules, however. Using the ideal gas equation, one can readily estimate the pressure pro-volume occupied by the vapor state is much greater than that duced during ignition of a confined high-energy composition.
of the solid and liquid phases. One mole (18 grams) of water For example, assume that 200 milligrams (0.200 grams) of occupies approximately 18 milliliters (0.018 liters) as a solid or liquid.
One mole of water vapor, however, at 100°C (373 K) black powder is confined in a volume of 0. 1 milliliter. Black occupies approximately 30.6 liters at normal atmospheric pres-powder burns to produce approximately 50% gaseous products sure.
The volume occupied by a gas can be estimated using the and 50% solids.
Approximately 1. 2 moles of permanent gas are i deal gas equation
produced per 100 grams of powder burned (the gases are mainly (equation 2.5).
N 2 , CO 2 , and CO) [5]. Therefore, 0.200 grams should produce V = nRT /P
(2.5)
0.0024 moles of gas, at a temperature near 2000 K. The expected pressure is:
where
P _ (0.0024 mole) (0.0821 liter-atm /deg-mole) (2000deg) V = volume occupied by the gas, in liters
(0.0001 liter)
n = # moles of gas
R = a constant, 0.0821 liter-atm /deg-mole
= 3941 atm!
T = temperature, in K
Needless to say, the casing will rupture and an explosion will P = pressure, in atmospheres
be observed. Burning a similar quantity of black powder in the
g
34
Chemistry of Pyrotechnics
Basic Chemical Principles
35
open, where little pressure accumulation occurs, will produce a external pressure acting on the liquid surface, boiling occurs.
slower, less violent (but still quite vigorous!) reaction and no For solids and liquids to undergo sustained burning, the pres-explosive effect. This dependence of burning behavior on de-ence of a portion of the fuel in the vapor state is required.
gree of confinement is an important characteristic of pyrotechnic mixtures, and distinguishes them from true high explosives.
The Solid State
Liquids
The solid state is characterized by definite shape and volume.
The observed shape will be the one that maximizes favorable Gas molecules are widely separated, travelling at high speeds interactions between the atoms, ions, or molecules making up while colliding with other gas molecules and with the walls of the structure. The preferred shape begins at the atomic or their container. Pressure is produced by these collisions with molecular level and is regularly repeated throughout the solid, the walls and depends upon the number of gas molecules present producing a highly-symmetrical, three-dimensional form called as well as their kinetic energy. Their speed, and therefore their a crystal. The network produced is termed the crystalline lot -
kinetic energy, increases with increasing temperature.
t ice .
As the temperature of a gas system is lowered, the speed of Solids lacking an ordered, crystalline arrangement are termed the molecules decreases. When these lower-speed molecules col-amorphous materials, and resemble rigid liquids in structure and lide with one another, attractive forces between the molecules be-properties. Glass (Si0 2) is the classic example of an amorphous come more significant, and a temperature will be reached where solid. Such materials typically soften on heating, rather than condensation occurs - the vapor state converts to liquid. Di-showing a sharp melting point.