Michael Laing University of Natal, Durban 4001, South Africa Ammonium Nitrate: The Not-So-Docile Fertilizer Anyone who has ever done a little farming or grown vegetables in the backyard garden has bought sacks of limestone-ammonium nitrate mixture (LAN) or possibly even pure granulated ammonium nitrate. The simple compound, N m 3 , is an oxidizing agent. It is explosive and uotentiallv " verv " dangerous if in contact with a suitable reducing agent. Simple thermal decomposition of solid N W 0 3 yields gaseous products:
-
This particular reaction is used commercially to produce nitrous oxide, NzO, a commonly used anesthetic. Note that the NzO molecule can in principle react further to yield very stable nitrogen gas, and free oxygen, which in turn is a potent oxidizer. ~NzO,,, + 2Nzm.,+ Ozm.) (2) To give an idea of the potential danger of NH4N03and the violence of the reaction between it and a reducing agent, wnsider the following reports of explosions and violent fires involving it.
nium flake and silver nitrate are sensitive to 'idion and explode violently on impact or when heated. In addition to the above examples,it is well to remember that ammonium nitrate is by far the most common commercial explosive used worldwide for blasting in mines, quarries, and mad building, in the form of ANFO, a mixture of about 95% ammonium nitrate prills (porous pellets), with 5% fuel oil. Millions of tons of ANFO are pmduced each year. The reason is simple. The ingredients are inexpensive to pmduce and easy and safe to transport and handle. They can be mixed in situ by relatively unskilled labor. The gaseous products, Nz, HzO, and COz are nontoxic. More recently this combination has been developed into "slurry" explosives that wnsist of a gelatinized aqueous solution of m O s containing in suspension the correct proportion of fuel oil and aluminum dust. These explosives either can be contained in plastic "sausages" or simply pumped down the hole drilled for the charge. They are remarkably docile and must be set off by a Pentolite (TNTPETN) "booster" charge of high explosive (6).
NH4N03, and the Ship "Grandcamp", April 16,1947, Texas City ( l ) About 2,500 tons of "fertilizer grade" ammonium nitrate were in the holds of the ship. A small fire began in hold No. 4. The crew wuld not extinguish it, and 37 minutes later the ship was vaporized in a violent explosion. The nearby Monsanto chemical complex was devastated. A second ship, the "High Flyer", also was loading ammonium nitrate, and it too exploded. Over 500 people died, and twothirds of the buildings of Texas City were destroyed. Arson in Seattle, a s Reported in The Economist, 1991 (2) The Seattle f r e department recently has fought several violent fres whose temperatures were so high that they melted steel and crumbled concrete. In all likelihood, the arsonists were using a mixture of aluminum dust and ammonium nitrate, a mixture that can be ignited by addition of water. The temperature of the reaction between aluminum and ammonium nitrate can reach 3000 T. Titanium Metal Explosion at the Arthur D. Little Co., 1990 (3) A worker was killed in a violent explosion when a sludge of titanium metal flake and silver nitrate suddenly exploded. Subsequent tests showed that mixtures of tita-
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Journal of Chemical Education
TNT
Structuresof wmmon commercial and military explosives. Note that all of these explosives are organic compounds, containing nitro, NO2, or nitrate, -ON02,groups. Others, like TACOT, also contain N-N or N=N bonds. The molecule, thus, contains within itself both oxidant and reductant. The products of explosion are typically N2, H20, CO, and in some cases COP
Table 2. Heats of Explosion of Some Explosives (4) (in ~ J W
What Is an Exploslon?
An explosion is an exothermic chemical reaction that takes place in a very short time with the generation of a large volume of gas (4). It is the violent expansion of this gas under high pressure resulting from the high temperature (typically2000 to 3000 'C) that produces a shock wave and causes the damaee. The rate of the reaction has a marked effect on the Lhavior of the explosion. True high explosives like TNT, PETN. and HMX (See the firmre, detoiate with the d&ompokition wave-front travelling through the solid at supersonic velocities of 6900, 8400, and 9100 mls, respectively. These enormous rates are caused by the molecules "falling apart" and forming the gaseous products by an intramolecular free-radical process. Explosions involving nitrates and metals are best thought of as "very fast burning" but are dangerously violent because of the extremely large amount of energy released. Explosions are both enthalpy-driven (exothermic) and entropy-driven (large number of moles of gaseous products per mole of reactant). In all eases, nitrogen is the critically important product. It is a simple diatomic gas and is extremely stable. It is inert. It cannot indulge in any secondary reactions with either metals or nonmetals that would inevitably result in the removal of the all-important gas whose violent expansion is the very life of the explosion. Thus, the combined rise in pressure, increase in entropy and the shock wave is maximized by the formation of nitro~en.There is no other gas that can begenerated chemicallithat has these vitally important properties. All high explosive compounds contain nitrogen. The rate of propagation of the explosion for the ANFO and "slufiy" explosives is slow (compared to the high explosives). The expanding gas gently 'pushes" the rock aside rather than shattering it as a high explosive like TNT would do. This characteristic makes NH4NOs-based explosives ideal for quarrying and blasting. Characteristics of Some Potentially Explosive Reactions
It is illuminating to calculate for the various explosive mixtures the energy released for the ideal balanced reaction between metal and nitrate, and then to compare moles of gas produced, energy per mole of gas produced, and energy per gram of reactants. Calculation of the heat of reaction is a simple application of Hess' Law. The Standard Heats (Enthalpies) of Formation, MPf,for the compounds involved are given in Table 1. (Note that the values of @fof all of these compounds are negative. The compounds themselves are not thermodynamically unstable relative to the elements). Table 1. Standard Heats (Enthalpies) of Formation, A& (in kJlmole)(4,5)
Source NH4N03
AgN03
Tion A l h
ZnO HnW) NH4C104 HCI
Pb(N0s)z
-365 -1 23 -945 -1676
(4) (5) (5) (5)
-348 -242 -296
(5) (5) (4)
-92
(5) (4)
448
Nitroglycerin TNT
PETN RDX (cyclonite) HMX (octogen) Nitrocellulose (gun cotton)
6.77 4.56 6.40 6.32 6.20 4.41
Energies of Reaction
Example 1: Titanium/AgNQ &of
3Ti(soJid) + 2AgNo31solid)+ 3TioZ(aolid)+ %Mid) reaction =3x of ~ i 0 -2 2 x df of A ~ N o ~ = [3x -945 - (2 x -123)l kJ/mole = -2589 kJImole of Nz gas evolved
+ Nzigeaj
(3)
6 3 x -365) - 6 x 2421 kJlmole = -2033 kJImole of A1203produced = -226 kJ1mole of gaseous molecules produced (6H20+ 3Nz) = 1-1676
Notice the difference between these two reactions. Although they are both enormously exothermic, in the WAgNOs case, all of the energy is transferred to only one mole of gas molecules. Hence, the gas expands violently. In the case of AVNH4NOs, the energy released must be distributed among nine moles of gaseous products, with the result that there is no sudden "blast", although the fire may burn violently and resist any efforts to extinguish it. Energy Release per Gram of Reactants In military explosives, it is the energy liberated by 1g of
material that is of interest. These values are well-known, and tabulated (4). Calculation of t h e value for the WAgNOs case is given as an example : Mass of 3 moles of Ti = 144 g Mass of 2 moles &NO3 = 340 g Total mass reacting = 484 g Energy liberated per gram = 25891484 kJ/g = 5.35 kJ/g For comparison, the values of the measured heats of explosion for some well-known high explosives are given above in Table 2. The value of the energy liberated in the WAgNOa reaction lies midway between the heats of explosion of nitroglycerin and TNT! And this total energy is concentrated in the kinetic energy of the one mole of Nz gas. It is quite clear that this combination of TI and &NO3 has the potential of being a high explosive, and, indeed, was the cause of the disaster reported in 1990 (3). The combination of AVPb(N03)zalso is potentially dangerous and is actually used as an initiating mixture in explosive trains (7). In the case of AL/NH4NO3the value for the energy liberated is 6.92 kJ/g of reactants, but this energy is distributed among the 9 moles of gaseous products. Volume 70 Number 5 May 1993
393
Table 3. Characteristics of Some Explosive Reactions
Mixtures
TiIAgN03 AI/NH.N03 ZnINHdN03 AIINH4C104
A# of reaction (Balanced eqn) kJ; exothermic
Moles of gaseous products /mole of reactants
2589 2033
1 per 5 moles 9 per5 moles
2589
467
3 per 2 moles 18per16 moles 11 per mole 12 per mole
156 518
9334
Afflmole of Afflgram o f Moles of gas gaseous reactants (kJ) /gram of molecules reactants produced (W)
226
5.35 6.92 3.21 9.57
0.0020
0.0306 0.0206 0.0185
the reaction that drives the solid fuel (composite propellant) booster rockets of the space shuttle on takeoff. This reaction is NOT an explosion, but an extremely fast, and exothermic, controlled bum. The balanced reaction is : 6NH4ClO4mtid)+ 10mlwlidl
Those huge billowing white clouds that you see as the shuttle l i b off are due to 6.40 0.0348 182 1998 PETNa the &03that is produced in the reaction. HCl gas also is polluting the air above 153 6.20 0.0405 1836 HMX' Cape Canaveral! 7 h e true high explosives, PETN and HMX, behave uniquely, because a small mass of crystalline solid This reaction is included here because is convened totally to gaseous products in one fast step: NH4C104itself is a powerful, potentially 11 moles 1 PETN -t 2N2 + 4H20 + 2CO + 3C0& dangerous oxidizing agent, which decom1 HMX + 4N2+4Hz0+ 4CO: 12 moles it is the sudden generation of these large volumes of gases psr mole of reactant that makes these two poses to N2, H20, C12, and O2 at less than ~ m p o u n d shigh explosives and sets them apan from the other explosive mixtures. Bear in mind that the 400 'C. In 1988, the Nevada plant in which temerature of these reactions is behveen 2000 ' and 3000 'C, so one mole of gas will occupy a b u t 200 it was being manufactured for rocket fuel was destroyed totally in a series of violent explosions and fires (9). Consider the well-known reaction between zinc dust and The principles involved in the reactions of N&C1O4 are ammonium nitrate, much loved by "Chemical Magicians" identical with those controlling the behavior of NHfi03. as Green Spontaneous Fire (8).The simplified reaction is : Conclusion Zn(sotid~+ N H ~ N O ~-t I ~ZnO~mlid) ~ I ~ ~ +I 2Hz01g-I + Nxw) ( 5 ) It is quite clear that all of these reactions involving niA@ of reaction is -467 kJ/mole of N2 gas produced, and trates and metals are extremely exothermic and danger-156 kJ/mole of gaseous molecules produced. The energy ous. But what makes them explosive is the rapid and vioreleased, 3.21 kJ/g, is relatively small, and this is spread lent expansion of the very stable gaseous nitrogen that is over three moles of gaseous products. These figures reveal formed. In all cases, the large exothermic enthalpies also why this mixture is very dangerous. It can inflame sudderive from the extreme stabilities of the metal oxides that denly and burn vigorously, but it does not explode vioare formed. lentlv .-.... Perhaps all of this is best summed up by saying simply NO& esoeciallv how it is the formation of the very stable that nitrates. ammonium nitrate in ~articular.are daneerH20molehe that releases much of the e n e m that drives ous compow;ds and must be treatedhth respect. Any mixthe reactions invulvingammonium nitrate. The H20 moleture of metal powder and a nitrate is potentially lethal, cules greatly contribute to the volume of gas evolved dethus aircraft, built of aluminum alloys and c a w n g nispite the relatively slow rate of explosion. It is the pmductrates, are ~otentiallvflvinn bombs! The world's airlines tion of water that helps make NH4N03 so effective an understand'this we1l;and they refuse to carry ammonium explosive! nitrate, that "harmless" fertilizer, on any of their aircraft. Characteristics of Explosives Caveat Table 3 compares some of the characteristics of the This is not meant to be an article about explosives. It is metahitrate combinations with those of conventional about ammonium nitrate, the not-so-friendly fertilizer. high explosives. Each has its unique thermodynamic characteristics that Acknowledgment determine its use. Ti/AgN03concentrates all of its exotherThis article is dedicated to the students in my secondmic heat of reaction into one mole of gas; AI/NHfi03 is vear inorganic chemistrv course who, in the final examinaextremely exothermic. Al/NH4C104has the largest @ per tion, without forewarni&, were asked to decide whether a mole of gaseous products while its rate of burning is low, of mixture of titanium and silver nitrate was a potential high the order 1 mls, which accounts for its use as a solid proexplosive. Many succeeded in reaching the correct conciupellant in rockets. Both PETN and HMX produce very sion. large volumes of gas at supersonic velocities with large It is a pleasure to thank the reviewer for valuable and exothermic heats of reaction per gram of reactant. constructive comments. The characteristics of the reaction that determine the qualities of the explosion are a complex combination of Literature Cited number of moles of gas liberated per gram of explosive 1. Nage1,M.C. J. Cham. Edm. 1888.66.248; Bretheriek.L. J. Cham. Edue 1989,66, 24220; Jones, D.Chamkfry inBritain lSJZ,2812), 178.. mixture, rate of liberation of the gas, the energy liberated, 2. TheEmwmisf May 18,1991,p 44. (Item: Crime, Hot Seam) and the rate at which this occurs, which, in turn, deter3. Shanley, E. S. Chem Eng. Npws A p d 16,1990, p 2 (Letter: "Chemical Safety".) 4. Meyer. R.&pbsim.s. 2nd ed.;VerlagChemie:Weinheim. 1981. pp 1&20. mines the temperature of the gas and the volume of the 5. Harvey, K B.; Porter, G. B. Intmduefion fo Physimi Inownonlr Chamishy; Addison gas. Ultimately, all depends on the rate of increase of volWesley: hading, MA, 1963, pp 404412. ume of the gas, because this generates the shock wave. 6. AECICold~gu~ofE~plosiiiondA~w~onlrs,6thed., 1981,Jahaonesbqpp 19.23, 27,28, and 35: Hania, B. w J Cham. Edue lW7,M , 541.. la). Fordham, S. Hi8hEzpbloiwsondPmpeilonfs:2nd ed., P-mon: W a r d , 1930. 7. Nevada and the Space Shuttle lb) Hams. B. W J . Chem.Edve 1987,€4,541. 8. 8hakhashul.B. 2.C h l c o l Lkmonlrhotions;Univ. dWiamnrin:Madisan, 1983;VoL As a final exercise, "the interested student" should calcu1,pst. late &for the reaction Al + NH4C104going to &03. HCI, 9. Seltzer. R. Cham. Eng News May% 1 9 8 8 , 2 ~1: May 16, p5; June 13, p %July 4, p 5; Augvst 8, pp 7-15: September 19, p 7. NP,and H20 as products. (The answer is -9334 kJ1. This is ~~
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Journal of Chemical Education
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