THERMODYNAMICS OF METAL AGENT FIRE EXTINGUISHMENT 

Robert E. Tapscott 

GlobeTech Inc. 

8200 Montgomery NE, Suite 218 

Albuquerque, NM 87109-1602 

Email: GlobeTech@speakeasy.net 

Phone: +1-505-883-9399 

Fax: +1-505-883-9399 

J. Douglas Mather 

Center for Global Environmental Technologies 

University of New Mexico 

Albuquerque, New Mexico 

Phone: 505-272-7259 

Email: mather@nmeri.unm.edu

ABSTRACT 

The thermodynamics of oxides, carbonates, halides and related metal compounds involved in 

extinguishment by dry chemicals and metal agents has been analyzed. It is shown that lattice 

energies play an important role in determining the extinguishment activities and properties of 

metallo fire extinguishants. In particular, lattice energies can explain why some metallic 

compounds are inactive as fire extinguishing compounds and others reach a saturation point 

where additional agent give little improvement in performance. 

INTRODUCTION 

For some scenarios, certain metal compounds appear to be more effective suppressants than 

halons. Three types of mechanisms have been proposed for flame extinguishment by solid 

agents — (1) thermal (temperature lowering), (2) heterogeneous chemical, and (3) homogeneous 

chemical. Studies indicate that thermal mechanisms, such as radiation and heat absorption by 

decomposition, and heterogeneous mechanisms are insufficient to account for observed 

suppression characteristics. Homogeneous reactions (e.g., Equations 1 and 2) are believed by 

most to be the primary suppression mechanism. 

I II III IV V VI VII VIII

Li Be B 

Na Mg Al Si 

K Ca Ga Ge As

Rb Sr Transition Metals In Sn Sb Te

Cs Ba Tl Pb Bi Po 

Fr Ra 

Figure 1. Metals and Metalloids. 
MOH + •H → •M + H2O (1)

MOH +·H• → ·MO + H2 (2)

Three phenomena must be explained: 

1. For some main group metals, suppression ability increases with increasing 

atomic mass. For example, extinguishment ability appears to increase in the 

order Li < Na < K < Rb and Si < Sn. 

2. Some metal and metalloid compounds, e.g. compounds of Group II, show no 

suppression characteristics. 

3. Some transition metal compounds are exceedingly effective, but show 

saturation. 

It appears that many of these characteristics might be explained from thermodynamic

considerations: 

1. Bond strengths often decrease with increasing atomic weight. Since 

activation energy often increases with increasing bond strength, reaction rates 

for suppression chemical steps may increase with increasing molecular mass. 

2. Large lattice energies may enhance particulate formation and decrease agent 

volatilization, leading to inactive suppression and to saturation. Since lattice 

energies often decrease with increasing atomic weight, lattice effects could 

also explain the increase in fire suppression activity for higher atomic weight 

elements. 

As examples of changes with atomic weight, note the trends for alkali metal and alkaline earth 

hydrides in Table 1 and Figure 2. 

Table 1. Thermodynamics of Alkali Metal Hydrides 

Metal M—H Bond Energy, kJ/mol MH(s) Atomization Energy, kJ/mol 

Li 238 470

Na 202 382

K 184 365

MH(s) -> M(g) + H(g)

0

50

100

150

200

250

300

350

400

450

500

Li Na K

M—H(g) -> M(g) + H(g)

kJ/mol

Figure 2. Atomization and Bond Energy for Alkali Metal Hydrides 

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Here we investigate these factors. Thermodynamic data are taken from Reference 1 and 2. 

MAIN GROUP METALS AND METALLOIDS 

Alkali Metals 

For alkali metal compounds, the primary suppression reactions proposed [3] are 

MOH + •H → •M + H2O (1)

•M + •OH + m → MOH + m* (3) 

This can be compared with the principal reactions for halon fire suppression, which are believed 

to be 

RBr + H → •R + HBr (4) 

•H + HBr → H2 + •Br (5)

•Br + •H + m → HBr + m* (6) 

For both mechanisms, hydrogen atoms are removed from the free-radical pool. Kinetic 

modeling [4] shows sodium compounds to be better than lithium compounds for Reactions 1 and 

3, in agreement with experimentation, but fails to explain the improved performance of 

potassium compounds. (Although it is proposed that Reaction 3 is faster for potassium.) 

The thermodynamics for Reaction 1 are surprising. As expected, the reaction is exothermic for 

all of the alkali metals for which data are available (data are unavailable for cesium), and is least 

exothermic for lithium. What is surprising is that the reaction exothermicity decreases from

sodium to potassium to cesium (Table 2 and Figure 3). Moreover, one would predict that the MOH bond strength would significantly decrease in the order Na > K > Cs, but this is not the case. 

Table 2.Thermodynamics for Alkali Metal Compounds 

Metal MOH(g)+H(g)→M(g)+H2O(g) M—OH 

∆Hf, kJ/mol Bond Energy, kJ/mol 

Li -53 447

Na -141 358

K -139 361

Cs -124 376

MOH(g) -> M(g) + OH(g)

-200

-100

0

100

200

300

400

500

Li Na K Cs

MOH(g)+H(g)->M(g)+H2O(g)

kJ/mol

Figure 3. Thermodynamics for Alkali Metal Compounds 

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Thus, like kinetic calculations, thermodynamic considerations of the reaction involved do not 

correctly predict effectiveness. The question is, why are the results different than expected? The 

problem is almost certainly that we have failed to take into account the possibility of multiple 

bonding. Hydrogen, used in the first examples, has no surplus electrons; however, oxygen does, 

so that there is a real possibility of dπ-pπ bonding for the heavier elements Figure 4. 

M

←

←

OH

Figure 4. pπ→dπ Bonding. 

Are there other things that should be considered? One of these is lattice energy. Excluding 

lithium, there is relatively little difference in the dissociation energies of the chloride salts (used 

as dry powder extinguishing agent). The correct trend is, however, observed for the oxides, 

which are a likely product (Table 3 and Figure 5). Thus, there is a possibility that particulate 

formation may be removing more sodium than potassium from flames. 

Table 3. Atomization Energies for Alkali Metal Compounds 

Metal MCl(s), kJ/mol M2O(s), kJ/mol of M 

Li 690 602

Na 640 442

K 647 396

Cs 641

MCl(s)-> M(g) + Cl(g)

0

100

200

300

400

500

600

700

800

Li Na K Cs

1⁄2 M2O -> M(g) + 1⁄2 O(g)

kJ/mol

Figure 5. Atomization Energies for Alkali Metal Compounds 

Alkali Metals 

Compounds of the alkaline earths show little if any extinguishing ability other than that 

attributable to thermal effects. Examination of the thermodynamics shows that Reaction 1 is 

exothermic for the alkaline earths, as for the alkali metals, but considerably less so (Table 4 and 

4
Figure 6). This decrease in exothermicity may explain, at least in part, the decreased suppression 

ability of the alkaline earth compounds. 

Table 4. Thermodynamics for Alkaline Earth Compounds 

Metal MOH(g)+H(g)→M(g)+H2O(g) M—OH 

∆Hf, kJ/mol Bond Energy, kJ/mol 

Be -28 472

Mg -95 405

Ca -57 442

Sr -54 415

Ba -90 379

MOH(g) -> M(g) + OH(g)

-200

-100

0

100

200

300

400

500

600

Be Mg Ca Sr Ba

MOH(g)+H(g) -> M(g)+H2O(g)

kJ/mol

Figure 6. Thermodynamics for Alkaline Earth Compounds. 

Due to their higher oxidation states (larger number of bonds and/or ionic charge), the 

atomization energies of alkaline metal compounds in the solid state are considerably higher for 

than atomization energies for corresponding compounds of the alkali metals (per mole of metal 

released, Table 5 and Figure 7). The higher lattice energies for the alkaline earths most likely 

explains their lack of chemical suppression activity. 

Table 5. Atomization Energies for Two Alkaline Earth Compounds 

Metal MCl2(s), kJ/mol MO(s), kJ/mol 

Be 1066 1176

Mg 1032 999

Ca 1218 1064

Sr 1281 977

Ba 1236 1006

MCl2(s) -> M(g) + 2Cl(g

0

200

400

600

800

1000

1200

1400

Be Mg Ca Sr Ba

kJ/mol

MO(s) -> M(g) + O(g)

Figure 7. Atomization Energies for Two Alkaline Earth Compounds 

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Group III Compounds 

None of the Group III elements appear to have an intrinsic fire suppression ability. Neither 

aluminum nor boron form a stable M—OH species, so that reactions similar to that shown in 

Reactions 1 and 3 are impossible. The only species formed are O=M—H. This behavior is not 

surprising for elements that have higher coordination numbers. The thermodynamics for a 

reaction similar to that in Reaction 1 are shown in Table 6. The reactions are strongly 

exothermic, due to the large affinity of Group III 

O

M H

Figure 8. Structure of Oxyhydrides for Group III Elements 

Table 6. Reaction Enthalpy for Reaction of Group III Oxyhydrides with Hydrogen Atoms 

Metal O=MH(g)+H(g)→M(g)+H2O(g)

∆Hf, kJ/mol

B 180

Al 47

Moreover, the Group III compounds have large lattice energies (Table 7}. Certainly aluminum

compounds are likely to give refractory aluminum oxide, rather than gas phase species that could 

be involved in suppression. 

Table 7. Atomization Energies for Some Group III Compounds 

Metal MCl3(s), kJ/mol M2O3(s), kJ/mol of M 

B 1323 1566

Al 1396 1539

Group IV Compounds 

There is significant evidence of chemical suppression activity for tin compounds. There is also 

evidence that silicon has no inherent fire suppression capability. Insufficient data are available 

to make a judgment for germanium or lead compounds. Almost certainly Si—OH would 

rearrange to O=Si-H due to the great affinity of silicon for multiple bonding to oxygen. In any 

respect, highly refractory SiO2 would form almost immediately. The dissociation energies for 

oxides of the Group IV compounds are shown below. (Note that CO2 is a gas, not a solid.) 

Although the lattice energy for SnO2 is relatively high, a lower oxidation state is available and 

the product SnO has an atomization energy of only 551 kJ/mol. The lower oxidation state is 

more stable at higher temperatures. 

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Table 8. Atomization Energies for Group IV Oxides 

Metal MO2(g,s), kJ/mol 

C 1608

Si 1861

Ge 1452

Sn 1378

Pb 965

MO2 -> M(g) + 2O(g)

0

500

1000

1500

2000

C Si Ge Sn Pb

kJ/mol

TRANSITION METALS 

There are three transition metals that are known or believed to exhibit significant catalytic fire 

suppression capabilities — chromium, iron, and, possibly, manganese. The reason that these 

specific metals possess catalytic capabilities, while others do not (or are believed to not exhibit 

such capabilities) is unknown. The saturation observed with iron pentacarbonyl, however, is 

likely to be observed for any metal that has a very high melting (low volatility) solid oxide. 

REFERENCES 

1. Stull, D. R., and Prophet, H., JANAF Thermochemical Tables, United States Department of 

Commerce, National Bureau of Standards, 2nd Edition, June 1971. 

2. NIST Chemical Webbook, http://webbook.nist.gov/ 

3. Rosser, W. A., Inami, S. H., and Wise, H., "The Effect of Metal Salts on Premixed 

Hydrocarbon-Air Flames," Combustion and Flame, Vol. 7, pp. 107-119, 1961. 

4. Williams, B. A., and Fleming, J. A., “Suppression Mechanisms of Alkali Metal 

Compounds,” Proceedings, Halon Options Technical Working Conference, Albuquerque, 

New Mexico, 27-29 April 1999, pp. 157-169. 

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