What if some evil-minded purpose bent on destructive acts reads this page? He (or she, to be as inclusive as possible) might make up some of the explosive mixtures discussed here and use it to hurt people. Perhaps this information should be carefully guarded, with fines and prison sentences for those who dare to reveal the secrets. But it is my opinion that evil-minded people don't need this information to perform acts of destruction. Guns are freely available, the grocery stores are full of poisons (in the automotive and household chemical sections, second aisle past the greeting cards, top shelf), and gunpowder is sold over the counter anywhere they sell reloading supplies. If you are worried about such evil-minded people, rest assured that they will find a way to do evil whether or not the information in these pages is freely available. If you are such an evil-minded person you will probably be bored with these pages unless you happen to be curious about the chemistry of your intended deeds.
Gunpowder was invented by Chinese alchemists of the 9th century as a mixture of sulfur, charcoal, and saltpeter. It was employed in military applications in 10th century China. By the 13th century the secret had spread to Islamic Asia where it was used against Europeans. Ever since, gunpowder has been a staple of the military and political establishments.
Consequently the ingredients of gunpowder, especially sulfur and saltpeter, have had strategic importance. Sulfur is mined from underground deposits. Saltpeter is mined, typically from deposits in caves, where it is leached from guano. In fact, salpeter was mined in Virginia during the Civil War and was central to the Confederate war effort. However the largest deposits of nitrate are in Chile and up until the first decade of the 20th century the mining and shipping of Chile saltpeter (sodium nitrate) were of strategic interest to the world powers.
Saltpeter, or niter, is the common name for potassium nitrate. Biringuccio tels us how to extract it in his Pirotechnia:
As I told you in the chapter on salts, saltpeter is a mixture composed of many substances extracted with fire and water from arid and manurial soils, from that growth which exudes from new walls or from that loosened soil that is found in tombs or uninhabited caves where the rain cannot enter. It is my belief that it is engendered in these soils from an airy moisture that is drunk in and absorbed by the earthy dryness...
Biringuccio goes on to desrcibe a process for extracting saltpeter from this "manurial soil," i.e. soil that has formed from human or animal manure. On the surface of such soil or in certain caves there will be a white crust on the surface of the soil. The important feature of this soil is that it should contain organic material which contains nitrogen: chiefly proteins and their decomposition product, urea. Bacteria in the soil oxidize these nitrogen compounds to a family of nitrate salts: sodium nitrate, potassium nitrate, and calcium nitrate, depending on the other minerals present in the soil. These nitrate compounds are among the most soluble of all compounds. The solubilities of sodium, calcium, and potassium nitrate in boiling water are 952, 376, and 247 g / 100 mL. That is, boiling water will dissolve more than nine times its own weight of sodium nitrate.
The process for making saltpeter is very similar to that for making potash, the chief difference being the starting material. The soluble part of wood ash is mostly potassium carbonate and so when we purify it by recrystalization, the resulting product is purified potassium carbonate. The soluble part of these manurial soils are mostly mixed nitrates. The reason for this is that while the original animal waste may have had a wide variety of soluble materials, as the water wicked up through the soil and evaporated, the less soluble parts fell out of solution as it became more and more concentrated. Only the most soluble parts, in this case the nitrates, made it to the top of the soil and when the last bit of moisture evaporated, they were deposited as a white crust on the surface.
So the beginning of primitive saltpeter production is to collect this white crust
leaving behind as much as possible the underlying soil. Of the three nitrates
present, potassium nitrate is the one we need. To extract it, we use a metathesis
reaction in much the same way that we have previously produced lye:
Ca(NO3)2(aq) + K2CO3(aq) ----->
CaCO3(s) + 2 KNO3(aq)
This removes all the calcium as insoluble calcium carbonate leaving mostly potassium nitrate and a little sodium and potassium carbonate which can be further separated out by repeated recrystalizations in much the same way that we produced potash.
Saltpeter has been mined in Virginia, particularly in the dry caves of the Shenendoah valley. Most of the nitrate mined today comes from Chile and is called "Chile Saltpeter," which is chiefly sodium nitrate. However, most of the nitrate used in explosives and agriculture is today derived from nitrogen in the air (see acids).
We have seen several compounds of sulfur in the form of the sulfide ores: galena, sphalerite, pyrite, and chalcopyrite. But for gunpowder, we need elemental sulfur. Sulfur is one of only a few elements found free in nature, i.e. as the element rather than as compounds of that element. It is a yellow solid with a melting point of about 120 C. It occurs in beds, chiefly in Italy, Texas, Louisiana, Java, Japan, and Mexico. Until the turn of the twentieth century, 95% of the world production was from Sicily, where it was mined in a manner very similar to that of coal. The vast sulfur beds of Texas and Louisiana lay beneath layers of quicksand which were impossible to remove by conventional methods. In the first decade of the twentieth century, Herman Frasch devised a method whereby this sulfur could be drilled similar to oil rather than mined similar to coal. A well is drilled into the sulfur bed and superheated steam is forced down the well shaft. This steam melts the sulfur, which rises up a concentric pipe in the wellshaft. the molten sulfur is cooled above ground and returns to the solid form. By 1919, American exports of sulfur exceeded the entire sulfur production of Italy.
The third ingredient of gunpowder is charcoal. As we have seen, charcoal is produced from wood by heating it in a reducing atmoshphere, i.e. one low in oxygen. Charcoal is almost prue carbon, but the mechanical properties of charcoal, grain size, shape, and structure, depend somewhat on the wood used to make it. Willow is the traditional wood of choice, followed by grapevine, hazelwood, elder, laurel, and pine cones.
Charcoal is not the only fuel we can use, however. Anything that burns can be used as the fuel in a gunpowder-like explosive. We will make gunpowder using either charcoal or sugar.
When a gas expands suddenly enough as to create a shock wave we have an explosion. For example, if you make mead in a glass bottle with the cap sealed, the pressure of carbon dioxide will build to the point that the glass can no longer contain it. The bottle will explode, sending glass fragments and mead everywhere. This may sound like fun but the joke would soon be over if someone were injured by this flying glass.
When people talk about explosives they are usually talking about combustion reactions
that produce a lot of heat and gas in a short time. The explosion comes from the expansion of this gas.
So at the heart of chemical explosives is a fuel which burns. Lets consider
a common fuel which has been important in this course: charcoal. Charcoal burns in
air according to the following reaction:
C(s) + O2(g) -----> CO2(g)
This reaction is indeed energetic. We have seen how hot a charcoal fire can get when
provided with enough air. But this is not an explosion because only the surface
of the charcoal burns and it has to wait for fresh air to be blown in before the
next layer of charcoal can burn. There are two ways we can deal with this problem.
First, we can grind the charcoal up into a fine powder so that as much surface area
as possible is exposed. Second, we can mix it with a solid oxidizing agent so that the burning
charcoal does not have to wait for fresh air. Many oxidizing agents could be used
but one of the best is a compound we have seen before: saltpeter or potassium nitrate.
Potassium nitrate reacts with charcoal according the the following
unbalanced equation:
KNO3(s) + C(s) + S(s) -----> N2(g) + CO2(g) + K2S(s)
You will need the balanced redox equation for this project. If we don't get the amounts in exactly the right proportions, not only will we waste valuable raw materials like saltpeter and sulfur, these excess materials will carry away heat from the reaction and so less heat will be available for heating the product gases, which will then not expand as quickly. The result: less bang for the buck!
You will use the principles of stoichiometry to answer two important questions:
Here the fuel may be charcoal or sugar. The relative amounts will be different for these two fuels. Once you have solved these problems, you will have your formula for gunpowder. Let's suppose we want to make gunpowder from 1 gram of sulfur. How much charcoal and saltpeter would we need?
If we use exactly these proportions and if the reaction actually proceeds according to our balanced reaction, then all of the sulfur, saltpeter, and charcoal will be consumed producing the maximum amounts of heat and gas. In other words, the most bang for the buck! In the case of such a perfect mixture, very little residue will be left behind.
All of the explosives we will discuss require two parts: a fuel and an oxidant. Just about anything that burns can act as a fuel: charcoal, alcohol, fuel oil, sugar, and even metal powders like aluminum and zinc. There are fewer oxidants to choose from. Some of the most popular are saltpeter and ammonium nitrate, both manufactured from nitric acid.
Historically, saltpeter was the oxidant of choice because it was the only oxidant (other than air) which people knew about. Explosives which use saltpeter as the oxidant tend to be relatively slow burning. That means that the combustion takes place in a leasurely few thousanths of a second. While this still sounds pretty fast, it is slow enough that if the container has a hole or loose plug, it will blow the plug out without destroying the container. If the container has a hole, we have a rocket. If the container has a plug, we have a gun. In either case it is important that the gun or rocket not be destroyed as this would undermine its usefulness.
So saltpeter-based explosives have been historically used as gunpowders and rocket propellants. Their function is simply to produce hot gases to propel a projectile. Production of a shock wave is not required. In fact it is to be avoided so that the projectile is not destroyed.
Another popular oxidant is ammonium nitrate, in part because it is such a good oxidant but chiefly because it is such an inexpensive oxidant. It is manufactured entirely from nitrogen and oxygen from the air and hydrogen from water. Consequently there are virtually no materials to buy. The only expenses are the capital invested in the plant and the electricity needed to run it.
Unlike saltpeter-based explosives, those based on ammonium nitrate detonate. That is, they consume their fuel in millionths of a second so that the entire reaction is complete before the gas has a chance to expand. Consequently a shock wave is generated which shatters anything nearby including the container. Such explosives cannot be used as propellents or gunpowders as they will destroy the rocket, gun, or cannon. The principle use of these explosives is in blasting, either for mining or for construction.
Because ammonium nitrate is so cheap and easy to get (anyplace that sells fertilizer), it is attractive for industrial uses. But these same reasons make it attractive for terrorists as well. You may recall that ammonium nitrate was the oxidant used in the explosive that destroyed the World Trade Center in New York and the Federal Building in Oklahoma City. You might think that it would be a good thing to ban ammonium nitrate altogether, but there are so many good and legitimate uses for this material that I believe this would be a mistake.
The gunpowder quiz will consist of three questions on the following topics:
You will build a rocket motor from scratch using the propellant from your quiz.
All of these propellants have saltpeter and sulfur, but may contain either
charcoal or sugar as fuels.
In your quiz you will have determined how much saltpeter
and fuel to use for each gram of sulfur in the propellant.
Begin by adding 1.00 grams of sulfur and the corresponding
amounts of saltpeter and fuel to a plastic bag. Thoroughly mix these ingredients
by kneading them inside the bag. If they are not thoroughly mixed, the propellant
will not burn efficiently.
The original plans for these rocket motors were developed by the Teleflite
Corporation and posted in many places on the internet. Since then, Teleflite
has requested their removal but copies linger. Michael York (Caveman '96)
modified these instructions in Spring 1998 and Caveman assistants Richard
Furr and Mike Latham put together this slideshow of the construction
of rocket motors from scratch. Click on the picture to go there.
You will have passed the gunpowder quiz and built three rockets using homemade propellant. We will then launch them. To pass, at least one of your rockets must get off the ground.