Explosions and Explosives:
An explosion occurs after a chemical reaction when a large amount of energy is created in a short space of time. This energy comes in the form of heat and usually lots of gas. As the reaction occurs so quickly, the gases do not expand immediately but when they do create an enormous ‘blast wave’ which can cause a lot of damage to surroundings. The main steps of an explosion are described below.
Ignition is where an explosive compound is given energy. This could happen via friction, impact, heat, electrical impulse and through many other routes. The energy converts to heat and increases the temperature of the material. Once the heat generated from this extra input of energy is greater than the loss of energy that is naturally lost to the surroundings, the compound ignites. The temperature where this occurs is known as the ignition temperature, which is different for every explosive.
Deflagaration occurs once an explosive compound has been given enough energy to ignite. The process involves burning of the material at a faster rate than during combustion. Sometimes this burning can create sparks, cracking or hissing. The deflagaration is a very exothermic reaction and the heat generated starts decomposition of the compound and allows the reaction to continue. In other words, the reaction propogates itself. If deflagaration occurs in a confined space, the volatile products of the reaction, and the heat generated causes the pressure inside to increase. This is shown by the Ideal Gas Law. Once the pressure has reached a certain value, the material is detonated and an explosion occurs.
Detonation is where an explosive compound decomposes by releasing a shockwave rather than the heat generated by deflagaration. Although nearly all compounds decay via deflagaration, for sensitive explosives, this part of the pathway happens so rapidly that the delay between ignition and detonation is not noticeable. This occurs with primary explosives.
The advantage of detonation is that more stable materials like secondary explosives can be initiated by the shockwave released. If the primary explosive is in close contact with the secondary explosive, the shockwave compressions give it energy and cause it to increase its temperature to above its decomposition temperature and consequently detonate. This will only happen if the speed of the shockwave is greater than the speed of sound.
The detonation of velocity gets faster when the compaction density of the explosive compound increases which is why great care has to be taken when filling shells and bombs that no cracks or gaps are formed. The shockwave is also increased in velocity if the container of the bomb or shell is a certain diameter and shape, particularly that of a cylinder.
The Kinetics of Explosions and the Ideal Gas Law:
When an exothermic reaction occurs, the free energy of the reactants is higher than the free energy of the products. However, energy is often needed to start the reaction. This supply of energy is known as the Activation Energy (Ea). It is the energy needed to break bonds in the reactants and to allow the less stable intermediates, or transition state, of the reaction to form. Only when this barrier has been breached, can the reaction proceed via the downhill pathway, which can be seen on the diagram below.
In an explosive reaction, ignition involves giving the explosive material the activation energy to start deflagaration. Energy can be given in various forms depending on which type of stimulus is used. Friction, impact and heat initiation may provide thermal, potential or kinetic energy to the system while electrical impulses would supply electrical energy. Once the reaction has started, deflagaration can then provide heat energy to the system to allow the rest of the explosive to react.
Ideal Gas Law: pV=nRT
Deflagaration involves generation of gases and heat, which leads to an increase in pressure. How and why does this occur? The Ideal Gas Law can help to explain this.
The Law describes how an ideal gas behaves with reference to the volume, the pressure and the temperature of the surroundings, as well as the amount of gas present. The gases we see around us are not actually ideal gases due to attractions and repulsions between gas molecules but the Law gives a good approximation of how they behave.
During an explosion in a confined space, the volume (V) remains constant. R is known as the gas constant and so also remains constant. This allows the equation above to be modified to give:
Here k is just a number. This means that pressure (p) has a directly proportional relationship with temperature (T) and the number of moles (n) in the mixture, as the number of moles will always increase due to many smaller molecules being made from one big one.
From this graph, it can be seen that if temperature is increased, pressure is increased. As a lot of heat is generated in deflagaration, and the volume remains the same, the pressure in the container becomes greater and greater until the walls can no longer remain intact.
On the molecular scale, the reason is simple. Pressure is a measure of how often molecules hit the walls or sides of an object. As temperature is increased, the molecules are given more energy and they can therefore move faster. The faster they travel, the more often they will hit the walls of a bomb casing, and so the pressure increases. The molecules are also able to hit the walls with a greater force. Once the force and number of hits is great enough, the casing can no longer resist and detonation occurs to cause an explosion.
1. TNT (2,4,6-TriNitroToluene) :
Appearance: Pale yellow solid
Molecular Weight: 227.1
Melting Temperature: 80.8° C, low, good for casting
Thermal Ignition Temperature: 300° C
Stability: Chemically and thermally stable
Solubility: Almost insoluble in water, sparingly soluble in organic liquids
Sensitivity: Low sensitivity to impact and friction
1863 TNT first prepared by a German named Wilbrand.
1870 The 2,4,5 isomer was discovered and a study of the more commonly known 2,4,6-trinitrotoluene was carried out.
1891 Manufacture of TNT started in Germany
1902 TNT replaced picric acid and soon became the standard explosive for both sides during the Great War.
During the development of TNT, explosive mixtures, which included ammonium nitrate and/or aluminium, were also created to meet demands for more powerful and increased amounts of explosives.
Trinitrotoluene is prepared from toluene, which is readily available from coal tar (a faction of oil, which can be separated by fractional distillation). Toluene is mixed with nitric and sulphuric acids so that electrophilic substitution of NO2 groups, commonly known as nitration, can occur. The mono-substituted product is formed first and then with increased reaction temperatures and acid concentrations the di-substituted and then the tri-substituted products are formed. The last step is often carried out with free SO3 in the mixture too. The progression below shows the steps and resulting isomers involved.
In industry, this reaction is carried out as a continuous process where reagents are added, products removed, but the reaction never stops. In order to end up with pure 2,4,6-trinitrotoluene, a pH9 4% sodium sulfite solution is used which attacks the unsymmetrical isomers of TNT resulting from the meta compound in the first substitution. These by-products can then be taken out from the mixture using an alkaline solution. If TNT was not pure and contained a mixture of isomers, it would be more sensitive and have a lower melting temperature.
If TNT is able to undergo full oxidation once it has been detonated, the products formed will be carbon dioxide, water and nitrogen gas. However, the oxygen content of the molecule is very low compared to the about of oxygen needed to form all of these products so extra oxygen is needed, i.e. combustion needs to occur. If this is not possible, as it would be in a confined space, the following reaction can occur.
The unoxidised carbon formed causes the sooty blasts that can often be seen after a bomb containing TNT has exploded. It is only after an explosion, when the reaction products come in contact with oxygen that full combustion can occur. This gives rise to heat released by explosion, and then extra heat released through combustion afterwards.
2. RDX (Research Department EXplosive):
Appearance: White solid
Molecular Weight: 222.1
Melting Temperature: 204° C, high but lowers when mixed with TNT
Thermal Ignition Temperature: 260° C
Stability: High chemical stability, especially compared to TNT
Solubility: Difficult to dissolve in organic liquids
Sensitivity: Easily initiated by impact of friction, often coated with oil or wax
1899 RDX first prepared by a German named Henning for use in medicine.
1920 RDX first recognised as a possible explosive.
1940 A continuous method for the preparation of RDX was developed.
RDX was used with TNT to fill bombs and shells during the Second World War by both sides in order to create more powerful weapons.
Th best way to make RDX is to use the compound pictured above and react it with ammonium nitrate and concentrated nitric acid. The mixture is warmed and when cold water is added, RDX is precipitated. The mechanism for this reaction is very complicated.
RDX, like TNT, does not have enough oxygen in its molecular formula to completely oxidise everything during an explosion. It is only after contact with the atmospheric oxygen that can allow complete combustion to occur. Below is the decomposition reaction of RDX.