A nuclear bomb delivers an incredibly powerful explosion, shown below.
The photo left is of the BADGER nuclear explosion in 1953, part of Operation Upshot-Knothole at the Nevada Test Site in the U.S.
What Is an Explosive?
Wikipedia describes it well: "An explosive device is a device that relies on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy."
Explosives are used for a wide variety of jobs from putting on a New Year's Eve fireworks show to obliterating an entire city. In this article we will explore how chemical explosives work.
Almost all explosions, with the exception of nuclear bombs, are examples of chemical explosions. A nuclear explosion is not a chemical explosion. Instead it relies on the process of fission or fusion, something we will get into in Part 2. This means that all chemical explosives rely on chemical reactions, which are interactions between the electrons of atoms that are part of the explosive compound. Generally heat or a physical shock is required to trigger the explosive chemical reaction. This reaction is an extremely rapid decomposition of the compound, which usually releases a lot of gas and heat. However, not all decomposition reactions are explosive.
An Explosive Reaction is (Usually!) a Decomposition Reaction
A decomposition reaction is a chemical reaction in which a compound separates into either elements or simpler compounds. Under certain extreme environmental conditions such as heat, radiation, humidity or physical agitation, for example, a compound, which is normally chemically stable, will become unstable. An unstable compound may decompose. Explosive compounds vary widely in their chemical stability. They may take anywhere from decades to a mere fraction of a second to decompose.
There are three basic types of decomposition reactions: electrolytic, catalytic and thermal.
An example of electrolytic decomposition is the electrolysis of water into hydrogen gas and oxygen gas. Water is stable under most conditions so it does not spontaneously decompose. An electrical current applied to water, however, will trigger the decomposition reaction:
2 H2O(l) → 2 H2(g) + O2(g)
This is sometimes a tricky concept to understand so I will give it some extra time here. Catalytic decomposition is a case where a catalyst lowers the activation energy of a reaction. In this case the reaction without the catalyst would be either non-spontaneous or it would happen at a very slow rate. A catalyst speeds up the rate of a reaction but it is not consumed or changed in the process. In order for any kind of reaction to take place, the reactant(s) must undergo a rearrangement of chemical bonds. The slowest step in this process is called the transition state. Here, the chemical species are neither reactant nor product. They are in an in-between state and energy is required to form it. This energy is called activation energy. If the reactants do not have as much energy as the activation energy they cannot undergo the reaction. A catalyst works by providing a new route with lower activation energy from reactant to product. It doesn't shift the equilibrium of the reaction but, at any given point in time, it allows a greater proportion of reactants to react, and this is how a catalyst speeds up the reaction rate.
For example, our cells make trace amounts of hydrogen peroxide (H2O2), which functions as part of our immune system and as a signaling chemical. It is also an oxidizer so too much of it in our cells can damage our DNA and proteins. Our cells regulate the level of H2O2 by producing an enzyme called peroxidase to catalyze its decomposition into water and oxygen before it has a chance to react with DNA or proteins.:
2 H2O2 → 2 H2O + O2
Peroxidase lowers the activation energy of the decomposition reaction shown above, which allows this slow spontaneous reaction to happen at a much faster rate.
c) Thermal Decomposition: Endothermal and Exothermal
Many decomposition reactions are thermal, which means that heat is involved. Most decomposition reactions are endothermic, which means that the reaction absorbs heat from the environment. You can verify this for yourself. Although many of these reactions involve toxic chemicals, one basic reaction is very safe and easy to do. It is actually very similar to what goes on chemically when you are baking. Many recipes call for baking soda (a base) to be mixed with an acid, which can be lemon juice, for example. Even buttermilk is slightly acidic. The reaction creates tiny bubbles of carbon dioxide gas that add a light airy texture to your finished product. I have a great tall buttermilk pancake recipe that makes good use of this reaction.
In this case, baking soda is added to a citric acid solution in a cup (the complete method is at the chemistry.about.com link above, with some variations to try).
H3C6H5O7(aq) + 3 NaHCO3(s) → 3 CO2(g) + 3 H2O(l) + Na3C6H5O7(aq)
A thermometer placed in the cup records the falling temperature as the reaction absorbs heat. Three key things occur during this reaction: a loss of mass (which you can measure on a scale), an increase in volume (which can be used to blow up a balloon), and a decrease in temperature (which you can measure with a thermometer). The reaction takes place in the following brief video.
Some thermal decomposition reactions are exothermic, which means they release heat into the environment. Explosive reactions fall into this category. Many exothermic reactions require heat to trigger them but once underway they release heat.
The Bhopal Disaster
A few of these kinds of reactions release a lot of heat very quickly, and this can potentially set up a positive feedback loop, which in turn may create a thermal runaway. In this case, an increase in temperature changes conditions that cause a further increase in temperature. The simple diagram below right gives you an idea of how it works.
In chemical engineering this event is usually accidental and undesirable. A horrific example of an accidental thermal runaway is the 1984 Bhopal disaster at a pesticide plant in India. Wikipedia cites three general causal factors: heat was not able to escape, contaminants accelerated the reaction and gas scrubbers were not functioning. A diagram of the reaction is shown below.
The chemical process is as follows: methylamine (1) reacts with phosgene (2) to form methyl isocyanate (MIC) (3), which is then reacted with 1-naphthol (4) to create carbaryl (5), more commonly known as Sevin, a well-known pesticide. It is legal for home house in Canada but I avoid using it because of its toxic effects.
Several issues at the plant led to a runaway reaction and a toxic gas cloud was released that contained, along with toxic MIC, phosgene, hydrogen cyanide, carbon monoxide and hydrogen chloride. Some of these gases are heavier than air so they stayed close to ground level where they were breathed in. Over 3700 people died and over half a million others were injured by the toxic gases.
In some types of exothermic decomposition reactions, gas escapes so rapidly that a shock wave, or detonation, occurs. High explosives fall into this category. What makes a chemical reaction an explosive one? The reaction must exhibit:
1) Rapid expansion of volume (usually the result of rapid production of gases)
2) Release of heat
3) A rapid reaction rate
4) A reaction trigger, which starts the reaction
There are many different explosive chemicals. Wikipedia has gathered a list of them. Wikipedia also has a list of mixtures and types of explosives. Even sugar, grain and coal dust are explosive under the right conditions. Here we will focus on a few well-known examples of explosive compounds such as nitroglycerin (dynamite), cyclonite (RDX and C-4 explosive), picric acid and trinitrotoluene (TNT). Each of these is an example of a high explosive. Following this, we will explore the differences between high and low explosives.
Nitroglycerin is a contact explosive, which means it requires a physical shock or friction to set it off. It is highly unstable, which means that any jar to it or shaking of it can set off its explosive decomposition, making its manufacture and transport very dangerous. Even while stored carefully, it spontaneously decomposes slowly over time: (rewrite)
4 C3H5N3O9(s) → 6 N2(g) + 12 CO(g) + 10 H2O(g) + 7 O2(g)
Dynamite, shown below, contains nitroglycerin but its sawdust construction makes it much less sensitive to shock. It requires a blasting cap to deliver the shock required to detonate it.
|Image adapted from Pbroks13;Wikipedia|
Still, old dynamite is hazardous because over time liquid nitroglycerin can seep out of the tube and dry into very unstable crystals. Freezing dynamite is not a good safety option. When dynamite is frozen (it freezes at +10°C) it is very stable. However, if it thaws too quickly, it becomes extremely unstable. Most manufacturers recommend storing dynamite under cool conditions for no longer than one year.
Cyclonites or cyclic nitramines is a family of chemically related compounds. RDX, for example, goes by the complex formula hexahydro-1,3,5-trinitro-1,3,5-triazine. Thermal decomposition of these compounds leads to various simple molecules such as HCN, NO, N2O, NO2, CO, CO2, H2O, H2CO, etc. The chemical processes involved in their decomposition are complex and not well understood and reaction products may vary greatly depending on the environmental conditions in which the reaction takes place. Despite their complex reaction chemistry, cyclonites tend to be very powerful and safe to use. You may have heard of the plastic explosive, C-4, for example. An explosive used by the U.S. military, it contains 91% RDX.
C-4 is a plastic moldable claylike explosive which is very stable and more powerful than dynamite. Below right, a crewman of the U.S. Navy inserts blasting caps into blocks of C-4 explosive.
in Germany in 1898 as a medical compound and a propellant, its explosive qualities were later explored and it was used widely in World War II. It currently has a wide variety of military and industrial applications.
Picric acid, a yellow crystalline solid acid phenol (2,4,6-trinitrophenol), is a highly nitrated compound similar to TNT (explored next). It decomposes explosively, producing large volumes of carbon monoxide (CO) gas, steam and nitrogen gas.
One of the most powerful chemical explosions in history took place in Halifax Canada in 1917. The Norwegian SS Imo and the French SS Mont-Blanc collided in the harbour, detonating 2653 tonnes of various explosives, mostly picric acid, which were loaded on the SS Mont-Blanc. This steamship was chartered to carry the explosives from New York to France to supply the French war effort during World War 1. A fire aboard the ship detonated the explosives about 20 minutes after the collision (Picric acid detonates above 300°C). Over 2000 people died and much of Halifax was destroyed.
Below is a photo of Halifax from around 1900, before the explosion. Much of the area visible was destroyed.
Below a photo looks toward the harbor two days after the explosion. On the far side of the harbor you can just make out the steamship IMO, run aground. The SS Mont-Blanc was completely blown apart. Parts of her hull were launched almost 300 metres into the air as the blast ripped through the hull and cargo at more than 1000 m/s. Temperatures of about 5000°C and thousands of bars pressure accompanied the detonation (air pressure at sea level is about 1 bar). Fittings and hull parts flew up to 4 km in all directions.
Researchers estimate that the explosive force was equivalent to 2.9 kilotonnes of TNT. To put this in perspective that is about one tenth the power of the nuclear bomb dropped on Nagasaki, Japan, ending WW 2.
Various kinds of artillery shells were used in World War 1. Four types are shown below left.
Picric acid is more powerful than TNT but it is not as stable, which has lead to its disuse as an explosive. It is still used in fireworks to improve the tone of the colours and to provide the characteristic loud whistling noise. It is stored under water or in solution. If it dries out it becomes extremely hazardous, as it is then unstable and very sensitive to shock and friction, and a bomb squad may be required to dispose of it. It's not something you want to find in an old school chemistry cabinet. Picric acid solution was also once very common in first aid kits. You would soak gauze in it to treat burns. Over the decades these bottles have been known to dry out, leaving behind yellow highly unstable explosive picric acid crystals. If you come across an old metal first aid kit beware!
Finally, TNT (trinitrotoluene), a yellow powdery solid, is very stable compared to other explosive compounds. It is insensitive to shock and friction, impervious to water and has a very high spontaneous detonation temperature. This means it has high activation energy. TNT requires the fairly intense pressure wave from an explosive booster in order to detonate. There are many explosive blends of compounds in current use that contain TNT, many of which are oxygen-rich. The extra oxygen balances TNT's production of carbon so that more carbon monoxide is explosively released. making for an even more powerful explosion. It has many military and peacetime uses in engineering, construction, mining and quarrying.
When TNT (trinitrotoluene) detonates, two similar chemical reactions occur:
2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C + energy
2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C + energy
TNT decomposes into nitrogen gas, carbon monoxide, carbon and either water or hydrogen gas. This reaction yields 2.8 MJ (mega or million joules) of explosive energy per kilogram of reactant. To compare, gunpowder yields 3 MJ per kilogram (kg) and gasoline yields, on average, 47.2 MJ/kg. The energy yield of TNT is used as an explosive energy standard, even for nuclear weapon yields.
Surprising isn't it that gasoline yields significantly more energy than TNT does? We will see in a moment why our vehicle engines don't blow to smitherines.
High Explosives Versus Low Explosives
Detonation and deflagration are two examples of very rapid decomposition reactions. We tend to hear the word detonation but the word deflagration isn't in common use. Both are examples of explosive reactions. A low explosive deflagrates. A high explosive detonates.
Technically, deflagration, characteristic of low explosives, is a combustion reaction that propagates through the process of heat transfer. Hot burning material heats up the layer of cooler material next to it and ignites it. We usually call this process "burning" as in the burning of a log in a fire pit, or gunpowder in a gun chamber, the blue flame of gas/air in a gas stove or even the blast of a rocket engine.
Like explosive decomposition reactions, the combustion reactions involved in deflagration are highly exothermic and happen very fast. Here, however, is a bit of a two-part tricky twist:
First, not all combustion reactions involve the production of heat (though they do produce energy) nor do they all happen rapidly. Combustion is perhaps better defined as an example of a redox (reduction/oxidation) reaction. For example, the cells in our bodies are continuously carrying out cellular respiration, which is a combustion or redox reaction. The simplified version of it looks like this:
C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + energy (ADP into ATP)
Our cells use glucose (from food) and oxygen (from air) to make not exactly heat but high-energy storage compounds called ATP, which deliver the energy that cells need to carry out all their functions. This combustion reaction can be considered a decomposition reaction because glucose, with the addition of oxygen, is broken down into simpler molecules.
Second, not all combustion reactions are decomposition reactions. In rocket (liquid fuel) engines the basic combustion reaction is a synthesis reaction, where water, as water vapour gas, is synthesized from liquid hydrogen and liquid oxygen:
2H2(l) + O2(l) → 2H2O(g) + energy
A schematic diagram of how a liquid-fuel rocket works is shown below.
With this tricky twist being acknowledged, for our purposes we can safely assume that most low explosive reactions are decomposition reactions that rapidly release large amounts of heat and kinetic energy.
Decomposition moves through the compound as a flame front. This definition has no lower speed limit; it may move very slowly. However it does have a top speed limit, which is the speed of sound. What defines a low explosive as low is that this wave front moves slower than the speed of sound. It is subsonic. In reality, combustion propagation speed falls along a spectrum. At the upper end of the limit, a propagation (flame) front can become highly turbulent and it can, often under unpredictable circumstances, become a supersonic front, or shock wave, something characteristic of the blast of a high explosive. An internal combustion engine and fireworks are examples of low explosives (at least for the most part as we will see).
A high explosive detonates rather than deflagrates. In this case, the decomposition reaction moves through the compound as a much faster shock front. Here, the speed of propagation is always supersonic, faster than the speed of sound.
While the combustion reaction characteristic of low explosives consumes oxygen from the air in order to propagate, the faster reaction of a high explosive must consume the oxygen within the explosive compound itself during the redox reaction. Like most low explosive materials, high explosives tend to be organic compounds (organic compounds), composed of oxygen, carbon, hydrogen and nitrogen. If an explosive molecule contains just enough oxygen to convert all of its carbon into carbon dioxide and all of its hydrogen into water it is said to have zero oxygen balance. If it has more oxygen than is needed, it has positive oxygen balance. If doesn't have enough, it has negative oxygen balance. Pure TNT has negative oxygen balance, so it creates carbon-rich soot as a reaction product.
As we learned earlier, the sudden reaction triggered by any chemical explosive is an exothermic one. This means that the reaction releases energy (heat and kinetic energy) rather than absorbing it. The outward blast typical of a chemical bomb (a high explosive) is usually created by the explosive release of gas, which is formed as a product of the reaction. The supersonic moving front of gas and other material creates a shock wave that moves outward ahead of the gas front. In fact, the reaction itself propagates by using the shock wave. This is quite different from the heat transfer method of propagation observed during deflagration in low explosives. Here, compressive heating of the explosive material inside the shock wave drives the reaction forward. It is this shock wave that causes most of the damage of a high explosive.
It may seem that low explosives are the tamer gentler type of explosive. However, it does not necessarily mean that low explosives release less energy than high explosives. TNT, a high explosive that we explored earlier, yields far less energy per kilogram than gasoline, a low explosive. TNT contains 2.8 (MJ) mega joules of energy per kilogram (kg). (A note here: this number is not the same as TNT's heat of combustion, which is 14.5 MJ/kg. This latter figure requires that some carbon in TNT react with the oxygen in the air, a combustion reaction that occurs after the initial detonation reaction). Gasoline, on the other hand, contains on average 47.2 MJ/kg, about 17 times more energy than TNT. The key difference between the two is in their propagation (detonation/deflagration) velocities.
What makes TNT so damaging is its detonation velocity, about 7000 m/s, creating an intense supersonic pressure wave - a very sudden increase (up to 20 bar) followed by a decrease in air pressure, which tends to blow structures apart. (The speed of sound is 340 m/s in air.) Gasoline on the other hand does not detonate but deflagrates instead. Its reaction propagation rate is just 0.34 m/s, creating a smaller but more sustained pressure wave that pushes the pistons down in a car's engine. It does not create a supersonic blast wave that would blow up the whole engine. In fact, most deflagrates create an overpressure of about just 0.5 bar), reflecting their generally low propagation velocities. This table compares the detonation velocities of various chemical high explosives.
Fireworks Are Low (and Sometimes High) Explosives
Devices such as fireworks are technically classified as low explosives.
I've drawn a diagram of a typical fireworks shell, shown below. This one, I believe, would shoot up into the sky, burst in a bright white flash and then burst green and then red stars. The red and green star chemicals are examples of colour-creating compounds, of which there are many. For a more complete list of them, click here.
Some fireworks such as maroons and concussion fireworks, explode with a very loud bang. If you want to know more about various kinds of fireworks try this link at fireworks.co.uk, which has a great glossary of fireworks. The bang is a shock wave. Whether the shock wave is subsonic or not will technically define it as a low or high explosive. Both kinds of fireworks, usually filled with a potent flash powder, are very dangerous and some of these flash powders, depending on which reference source you use, contain high explosive components, and some of the bangs they create verge on detonations. In addition to these, some other fireworks contain trace amounts of picric acid, which is a high explosive. Here, rather than causing as detonation, it is used to create a loud hissing sound and it can also be used as a colour enhancer.
Fireworks are fascinating and their design and usage is both a science and an art. However, there are illegal fireworks available online and at street corner vendors. Beware of these as some have been known to contain high explosives such as dynamite. Instead of shooting upward, a shell filled with high explosive would detonate creating a deadly blast in all directions. To learn how to use store-bought fireworks safely check Consumer Fireworks Safety in Canada. Towns and cities hire certified fireworks operators (pyrotechnicians) to set off and choreograph larger and more dangerous display fireworks. To tame your inner pyromaniac and turn it into a useful skill, check out this training site in Canada or try Pyrotechnics Guild International. To learn more about fireworks - their chemistry and colours - this webpage is a good source of information.
Deflagration and Detonation: Sometimes There is a Fine Line
Sometimes detonation happens by accident in a device that is designed to use deflagration. The exact mechanism for how this works is not well understood but an accidental transition from deflagration to detonation can be caused by partial confinement or obstacles in the way of the flame front. These obstructions can create unpredictable eddy currents that suddenly accelerate the flame front to supersonic speed. For example, in an engine it causes engine knocking. During normal combustion, the fuel/air mixture in the cylinder burns in an orderly controlled way and develops maximum pressure at the right time to push the piston down. Ideally, all the gas/air is burned each cycle. When an engine knocks (it's little detonations you are hearing), the cylinder is not firing correctly and small amounts of unburned gas/air remain. This mixture is outside of the boundary of the next flame front and therefore it is subjected to heat/pressure for a period longer than the gasoline is designed for. Detonation of that gas/air can then occur. A shockwave can rapidly increase pressure in the cylinder beyond its design limits. Repeated knocking can seriously damage or destroy an engine. The following 9-minute video not only explains how the pistons in your engine work but what knocking is and how to deal with it.
In a firearm accidental detonation can cause sudden excessive heating and a potentially lethal structural failure of the weapon. The key difference between a normal firing and a catastrophic detonation has to do with with how the explosion propagates outward. Because the pressure wave caused by the flame front during deflagration does not move faster than a normal (sound) pressure wave, it has time to be "pulled" toward any areas of lower pressure. In a firearm, this means that the bullet is pushed out of the chamber in front of the flame front toward the area of lower pressure further down the barrel. Detonation pressure is faster than the speed of sound so the explosion has no chance of being "pulled" toward any areas of lower pressure. The pressure of the blast front is equal (and much higher) in all directions. This means that the entire chamber of the gun experiences a shockwave, which it is not designed to handle, and that is why the gun can blow apart.
A Fascinating Case of Accidental Detonation
The 1989 U.S.S. Iowa disaster is a fascinating example of accidental detonation in a gun turret ? but even more fascinating for the Navy's handling of the accident. A detonation in the central #2 gun turret killed 47 of the turret crewmen aboard and severely damaged the gun turret itself. The explosion created a fireball as hot as 1600°C traveling at 610 m/s with a pressure wave of 280 bars that spread through all three of the gun rooms, releasing various toxic gases including cyanide from the burning polyurethane foam that lined the turret. The heat from the fire then ignited almost 1000 kg of powder bags in the powder handling area. Nine minutes later another explosion followed, most likely from the buildup of carbon monoxide from the first explosion. Carbon monoxide is highly flammable and readily forms an explosive mixture with air at room temperature. With exposure to heat it will explosively decompose into carbon and carbon dioxide. Below, the #2 turret is cooled with seawater shortly after exploding.
The Navy's initial investigation concluded that the explosion was deliberate. Normally, a mission-specific projectile would be rammed into the bore using a pneumatic ram. Then the powder propellant, which consists of 2-inch long rods of nitrocellulose embedded with diphenylamine (a stabilizer) bundled into bags (usually six are used), is rammed in behind the projectile and the breach is closed before firing.
Their theory was that a crewman, Clayton Hartwig, who died in the explosion, deliberately caused the explosion by inserting either a chemical or electronic detonator in between two of the powder propellant bags as the gun was loaded. Navy officers and investigators leaked to the media than Hartwig and another crewman, Kendall Truitt (who survived), were lovers, that their relationship soured and that Hartwig, suicidal over the breakup, deliberately caused the explosion by using a detonator. Truitt, married to a woman, acknowledged that he was sole beneficiary of a $100,000 life insurance policy on Hartwig. While the Navy focused intensely on the relationship details, the victim's families, the media and U.S. Congress soundly criticized the findings. The U.S. Senate and the U.S. House Armed Services Committees launched their own investigation and to assist them they hired Sandia National Laboratories to review the technical investigation. They found no evidence that any detonator was used and they concluded that the explosion was caused by over-ramming the propellant into the bore. This may have been the result of human error or a mechanical failure of the ram. They concluded that it likely compressed the powder to the point that the bags spontaneously ignited.
Before the technical investigation was completed, the Navy released its own report, finding that the explosion was a deliberate act "most likely" caused by Hartwig by using an electronic timer and that the powder bags were over-rammed under his direction to ensure that the explosion went off. The report was then quickly endorsed by top Navy officials and briefed to the media at the Pentagon.
Meanwhile the Sandia forensic investigation, still incomplete, continued, and its findings showed that the explosion could have been accidentally caused by over-ramming. It recommended a number of improvements to safety procedures. Meanwhile there was speculation about whether the powder itself was the cause of the explosion. Some sources claimed it had been exposed to temperatures as high as 32°C while it was stored for months on uncooled barges in New York River, and this may have made the powder unstable. Sandia requested a series of drop tests to investigate the possibility. A large number of variously designed drop tests done by both Sandia and the Navy found that the powder did detonate a significant number of times, indicating that it was sensitive to even fairly low over-ram pressures.
This story, in which the Navy was not only shown to fumble its own internal investigation but it appeared to try to manipulate the press as well, was the subject of intense media coverage. Several notable books were written on the disaster. Richard Schwoebel, who directed Sandia's investigation, wrote "Explosion Aboard the Iowa," published in 1999. In it, he recounts many of the problems involved with the Navy's investigation. Also in 1999, investigative journalist Charles Thompson published "A Glimpse Into Hell: The Explosion on the USS Iowa and Its Cover-Up" documenting his own investigation, and which is very critical of many of the Iowa's officers involved in the Navy's investigation. It was later made into a made-for-TV movie called "A Glimpse Into Hell."
The Hartwig family sued both the Navy and NBC News for damages and both suits were dismissed. Thirty-eight other Iowa victims sued the Navy and their suit was dismissed. Several Navy officers cited in Thompson's book sued both Thompson and his publisher, W.W. Norton. The suit against Thompson was dismissed and the case against Norton was later settled out of court.
In the next article we will explore the even more devastatingly powerful explosions of nuclear devices, and compare them to chemical explosions.