+3
gram.
The new International System of units calls for energy Al
+ 3 e + Al (in dil. NaOH soln. )
-1.706
values to be given in joules, where one calorie = 4.184 joules.
TiO
Most thermochemical data are found with the calorie as the unit, 2 +4H++4e+Ti+2H 2 0
-0.86
and it will be used in this book in most instances. Some typical Si0
-0.84
2 +4H++4e+ Si +2H 0
AH values for pyrotechnics are given in Table 2.6.
Note:
1
2
kcal = 1 kilocalorie = 1,000 calories.
S+2e+ S =
-0.508
It is also favorable to have the entropy change, AS, be a posi-Bi
-0.46
tive value, making the -TAS term in equation 2.1 a negative value.
2 0 3 +3H 20+6e-> 2Bi+6OH
A positive value for AS corresponds to an increase in the random-WO 3 + 6 H + + 6 e - W + 3 H 2O
-0.09
ness or disorder of the system when the reaction occurs.
As a
general rule, entropy follows the sequence:
Fe' 3 + 3e- Fe
-0.036
S(solid) < S(liquid) << S(gas)
2H + +2e + H2
0.000
Therefore, a process of the type solids -- gas (common to many N0 - +
+0.01
3
H 20+2e+N0 2 + 2 OH
high-energy systems) is particularly favored by the change in entropy occurring upon reaction. Reactions that evolve heat and H
+0.45
2 SO 3 +4H + +4e-> S+3H 2 0
form gases from solid starting materials should be favored ther-N0 -
3
+ 4 H + + 3 e + NO + 2 H 2 O
+0.96
modynamically and fall in the "spontaneous" category. Chemical processes of this type will be discussed in subsequent chapters.
10 -
+1.195
3
+ 6 H + + 6 e - I - + 3 H 2O
+3
HCr0
+
+1.195
4
+ 7 H + + 3 e - Cr
4 H 2O
Heat of Reaction
C1O,,
+8H++8e+Cl + 4 H 2 O
+1. 37
It is possible to calculate a heat of reaction for a high-energy system by assuming what the reaction products will be and then using Br0
+1.44
3
+ 6 H + + 6 e + Br + 3 H 2O
available thermodynamic tables of heats of formation.
"Heat of
C10
+1.45
formation" is the heat associated with the formation of a chemi-3
+6H + +6e- C1 +3H 20
cal compound from its constituent elements. For example, for Pb0
+1.46
2 +4H + +2e+Pb +2 +2H 20
the reaction
Mn0 4 + 8H++ 5e- Mn +2 + 4 H 2 O
+1.49
2 Al + 3/2 0 2 + A1 20 3
AH is -400.5 kcal/mole of
a
A120 3, and this value is therefore the
Reference 1.
heat of formation (AHf) of aluminum oxide (A1 20 3). The reaction
24
Chemistry of Pyrotechnics
Basic Chemical Principles
25
TABLE 2.6 Typical AH Values for "High-Energy" Reactions The net heat change associated with the overall reaction can then be calculated from
Composition
A H
6H(reaction) = EAH
(2.2)
f (products) - l lH f (reactants)
(% by weight)
(kcal/gram)a
Application
(where E = "the sum of")
KC1O,,
60
2.24
Photoflash
Mg
40
This equation sums up the heats of formation of all of the products from a reaction, and then subtracts from that value the heat NaNO 3
60
2.00
White light
required to dissociate all of the starting materials into their ele-Al
40
ments.
The difference between these two values is the net heat Fe203
75
0.96
Thermite (heat)
change, or heat of reaction. The heats of formation of a number Al
25
of materials of interest to the high-energy chemist may be found in Table 2.7.
All values given are for a reaction occurring at KNO 3
75
0.66
Black powder
25°C (298 K).
C
15
S
10
Example 1
KC1O 3
57
0.61
Red light
Consider the following reaction, balanced using the "oxidation SrCO 3
25
numbers" method
Shellac
18
Reaction
KCIO,, + 4 Mg
- KC1
+ 4 MgO
KC1O 3
35
0.38
Red smoke
Grams
138.6
97.2
74.6
161.2
Lactose
25
Heat of formation
-103.4
4(0)
-104.4
4(-143.8)
Red dye
40
(kcal/mole x # of moles)
a
AH(reaction) = EAHf(products) - EAHf(reactants) Reference 2. All values represent heat released by the reaction.