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The vast majority of explosives are chemical explosives. Explosives usually have less potential energy than fuels, but their high rate of energy release produces a great blast pressure. TNT has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the detonation of a pentane-air mixture, and the 0.34-m/s stoichiometric flame speed of gasoline combustion in air.
The properties of the explosive indicate the class into which it falls. In some cases explosives can be made to fall into either class by the conditions under which they are initiated. In sufficiently large quantities, almost all low explosives can undergo a Deflagration to Detonation Transition (DDT). For convenience, low and high explosives may be differentiated by the shipping and storage classes.
Chemical explosive reaction
A chemical explosive is a compound or mixture which, upon the application of heat or shock, decomposes or rearranges with extreme rapidity, yielding much gas and heat. Many substances not ordinarily classed as explosives may do one, or even two, of these things. For example, at high temperatures (> 2000 °C) a mixture of nitrogen and oxygen can be made to react rapidly and yield the gaseous product nitric oxide; yet the mixture is not an explosive since it does not evolve heat, but rather absorbs heat.
For a chemical to be an explosive, it must exhibit all of the following:
- Rapid expansion (i.e., rapid production of gases or rapid heating of surroundings)
- Evolution of heat
- Rapidity of reaction
- Initiation of reaction
A sensitiser is a powdered or fine particulate material that is sometimes used to create voids that aid in the initiation or propagation of the detonation wave. It may be as high-tech as glass beads or as simple as seeds.
Measurement of chemical explosive reaction
The development of new and improved types of ammunition requires a continuous program of research and development. Adoption of an explosive for a particular use is based upon both proving ground and service tests. Before these tests, however, preliminary estimates of the characteristics of the explosive are made. The principles of thermochemistry are applied for this process.
Thermochemistry is concerned with the changes in internal energy, principally as heat, in chemical reactions. An explosion consists of a series of reactions, highly exothermic, involving decomposition of the ingredients and recombination to form the products of explosion. Energy changes in explosive reactions are calculated either from known chemical laws or by analysis of the products.
For most common reactions, tables based on previous investigations permit rapid calculation of energy changes. Products of an explosive remaining in a closed calorimetric bomb (a constant-volume explosion) after cooling the bomb back to room temperature and pressure are rarely those present at the instant of maximum temperature and pressure. Since only the final products may be analyzed conveniently, indirect or theoretical methods are often used to determine the maximum temperature and pressure values.
Some of the important characteristics of an explosive that can be determined by such theoretical computations are:
- Oxygen balance
- Heat of explosion or reaction
- Volume of products of explosion
- Potential of the explosive
Balancing chemical explosion equations
In order to assist in balancing chemical equations, an order of priorities is presented in table 1. Explosives containing C, H, O, and N and/or a metal will form the products of reaction in the priority sequence shown. Some observation you might want to make as you balance an equation:
- The progression is from top to bottom; you may skip steps that are not applicable, but you never back up.
- At each separate step there are never more than two compositions and two products.
- At the conclusion of the balancing, elemental nitrogen, oxygen, and hydrogen are always found in diatomic form.
|Priority||Composition of explosive decomposition||Products of decomposition||Phase of products|
|A metal and chlorine||Metallic chloride|
|Hydrogen and chlorine||HCl|
|A metal and oxygen||Metallic oxide|
|Carbon and oxygen||CO|
|Hydrogen and oxygen||H2O|
|Carbon monoxide and oxygen||CO2|
- C6H2(NO2)3CH3; → : 7C + 5H + 3N + 6O
Using the order of priorities in table 1, priority 4 gives the first reaction products:
- 7C + 6O → 6CO with one mol of carbon remaining
Next, since all the oxygen has been combined with the carbon to form CO, priority 7 results in:
- 3N → 1.5N2
Finally, priority 9 results in: 5H → 2.5H2
The balanced equation, showing the products of reaction resulting from the detonation of TNT is:
- C6H2(NO2)3CH3 → 6CO + 2.5H2 + 1.5N2 + C
Notice that partial moles are permitted in these calculations. The number of moles of gas formed is 10. The product carbon is a solid.
Example of thermochemical calculations
The PETN reaction will be examined as an example of thermo-chemical calculations.
- PETN: C(CH2ONO2)4
- Molecular weight = 316.15 g/mol
- Heat of formation = 119.4 kcal/mol
(1) Balance the chemical reaction equation. Using table 1, priority 4 gives the first reaction products:
- 5C + 12O → 5CO + 7O
Next, the hydrogen combines with remaining oxygen:
- 8H + 7O → 4H2O + 3O
Then the remaining oxygen will combine with the CO to form CO and CO2.
- 5CO + 3O → 2CO + 3CO2
Finally the remaining nitrogen forms in its natural state (N2).
- 4N → 2N2
The balanced reaction equation is:
- C(CH2ONO2)4 → 2CO + 4H2O + 3CO2 + 2N2
(2) Determine the number of molar volumes of gas per mole. Since the molar volume of one gas is equal to the molar volume of any other gas, and since all the products of the PETN reaction are gaseous, the resulting number of molar volumes of gas (Nm) is:
- Nm = 2 + 4 + 3 + 2 = 11 Vmolar/mol
(3) Determine the potential (capacity for doing work). If the total heat liberated by an explosive under constant volume conditions (Qm) is converted to the equivalent work units, the result is the potential of that explosive.
The heat liberated at constant volume (Qmv) is equivalent to the heat liberated at constant pressure (Qmp) plus that heat converted to work in expanding the surrounding medium. Hence, Qmv = Qmp + work (converted).
- a. Qmp = Qfi (products) − Qfk (reactants)
- where: Qf = heat of formation (see table 1)
- For the PETN reaction:
- Qmp = 2(26.343) + 4(57.81) + 3(94.39) − (119.4) = 447.87 kcal/mol
- (If the compound produced a metallic oxide, that heat of formation would be included in Qmp.)
- b. Work = 0.572Nm = 0.572(11) = 6.292 kcal/mol
- As previously stated, Qmv converted to equivalent work units is taken as the potential of the explosive.
- c. Potential J = Qmv (4.185 × 106 kg)(MW) = 454.16 (4.185 × 106) 316.15 = 6.01 × 106 J kg
- This product may then be used to find the relative strength (RS) of PETN, which is
- d. RS = Pot (PETN) = 6.01 × 106 = 2.21 Pot (TNT) 2.72 × 106