KC10,, + 4 Mg -> KCI + 4 MgO
4. In a neutral molecule, the sum of the oxidation numbers Similarly, the equation for the reaction between potassium ni-will be 0. For an ion, the sum of the oxidation numbers trate and sulfur can be balanced if one knows that the products on all the atoms will equal the net charge on the ion.
are potassium oxide, sulfur dioxide, and nitrogen gas Examples
?KNO 3 +?S-> ?K 2O+?N 2 +? SO2
NH
Again, analysis of the oxidation numbers reveals that potas-3 (ammonia) :
The 3 hydrogen atoms are all +1 by
rule 1. The nitrogen atom will therefore be -3 by sium and oxygen are unchanged, with values of +1 and -2, re-rule 4.
spectively, on both sides of the equation. Nitrogen changes C0 2
from a value of +5 in the nitrate ion (NO - ) to 0 in elemental 3
(the carbonate ion) : The three oxygen atoms 3
are all -2 by rule 1. Since the ion has a net charge form as N 2 . Sulfur changes from 0 in elemental form to a value of -2, the oxidation number of carbon will be of +4 in SO2 . In this reaction, then, sulfur is oxidized and ni-3(-2) + x = -2, x = +4 by rule 4.
trogen is reduced. To balance the equation, 4 nitrogen atoms,
20
Chemistry o f Pyrotechnics
Basic Chemical Principles
21
each gaining 5 electrons, and 5 sulfur atoms, each losing 4 electrons, are required. This results in 20 electrons gained and 20
logically, be the best electron donors, and a combination of a good electrons lost - they're balanced. The balanced equation is electron donor with a good electron acceptor should produce a battery of high voltage. Such a combination will also be a potential therefore:
candidate for a pyrotechnic system. One must bear in mind, however, that most of the values listed in the electrochemistry tables 4KNO 3 +5S- 2K 20+2N 2 +5SO2
are for reactions in solution, rather than for solids, so direct cal-The ratio by weight of potassium nitrate and sulfur correspond-culations can't be made for pyrotechnic systems. Some good ideas ing to a balanced - or stoichiometric - mixture will be 4(101. 1) _
for candidate materials can be obtained, however.
404.4 grams (4 moles) of KNO 3 and 5(32.1) = 160.5 grams (5 moles) A variety of materials of pyrotechnic interest, and their stand-of sulfur. This equals 72% KNO 3 and 28% S by weight. An ability ard reduction potentials at 25°C are listed in Table 2.5. Note the to balance oxidation-reduction equations can be quite useful in large positive values associated with certain oxygen-rich negative working out weight ratios for optimum pyrotechnic performance.
ions, such as the chlorate ion (C10 -3 ), and the large negative values associated with certain active metals such as aluminum (Al).
Electrochemistry
If one takes a spontaneous electron-transfer reaction and sep-THERMODYNAMICS
arates the materials undergoing oxidation and reduction, allow-ing the electron transfer to occur through a good conductor such There are a vast number of possible reactions that the chemist as a copper wire, a battery is created. By proper design, the working in the explosives and pyrotechnics fields can write be-electrical energy associated with reactions of this type can be tween various electron donors (fuels) and electron acceptors (ox-harnessed.
The fields of electrochemistry (e.g. , batteries) idizers). Whether a particular reaction will be of possible use de-and pyrotechnics (e.g., fireworks) are actually very close pends on two major factors:
relatives. The reactions involved in the two areas can look strikingly similar:
1. Whether or not the reaction is spontaneous, or will actually Ag20 + Zn -* 2 Ag + ZnO (a battery reaction) occur if the oxidizer and fuel are mixed together.
2. The rate at which the reaction will proceed, or the time re-Fe20 3 + 2 Al --> 2 Fe + A1 20 3 (a pyrotechnic reaction) quired for complete reaction to occur.
In both fields of research, one is looking for inexpensive, high-energy electron donors and acceptors that will readily yield their energy on demand yet be quite stable in storage.
Spontaneity is determined by a quantity known as the free en-Electrochemists have developed extensive tables listing the ergy change, AG. "A" is the symbol for the upper-case Greek relative tendencies of materials to donate or accept electrons, letter "delta," and stands for "change in."
and these tables can be quite useful to the pyrochemist in his The thermodynamic requirement for a reaction to be sponta-search for new materials. Chemicals are listed in order of de-neous (at constant temperature and pressure) is that the prod-creasing tendency to gain electrons, and are all expressed as ucts are of lower free energy than the reactants, or that AG -
half-reactions in the reduction direction, with the half-reaction the change in free energy associated with the chemical reaction -
be a negative value. Two quantities comprise the free energy of H+ + e } 1/2 H 2
0.000 volts
a system at a given temperature. The first is the enthalpy, or arbitrarily assigned a value of 0.000 volts. All other species are heat content, represented by the symbol H. The second is the measured relative to this reaction, with more readily-reducible entropy, represented by the symbol S, which may be viewed as species having positive voltages (also called standard reduction the randomness or disorder of the system. The free energy of potentials), and less-readily reducible species showing negative a system, G, is equal to H-TS, where T is the temperature of the system on the Kelvin, or absolute, scale. (To convert from values. Species with sizeable negative potentials should then, Celsius to Kelvin temperature, add 273 degrees to the Celsius
2 2
Chemistry of Pyrotechnics
Basic Chemical Principles
23
TABLE 2.5
Standard Reduction Potentials
value.)
The free energy change accompanying a chemical reaction at constant temperature is therefore given by Standard potential
Half-reaction
@25 0 C, in voltsa
AG = G(products) - G(reactants) = AH - TAS
(2.1)
For a chemical reaction to be spontaneous, or energetically fa-3 N
-3.1
2 + 2H+ + 2 e -> 2 HN 3
vorable, it is desirable that 6H, or the enthalpy change, be a Li+ + e - Li
-3.045
negative value, corresponding to the liberation of heat by the reaction.
Any chemical process that liberates heat is termed exo-H
-
0+3e- B +4OH -
-2.5
2B0 3
+ H 2
thermic, while a process that absorbs heat is called endothermic.
Mg+ 2
AH values for many high-energy reactions have been experimen-
+ 2 e -• Mg
-2.375
tally determined as well as theoretically calculated.
The typical
HPO =
-1.71
units for AH, or heat of reaction, are calories/mole or calories/
3
+ 2 H 2O + 3e + P + 5 OH -