What is the most energetic chemical reaction?
-----------Warren D. Smith 1998--------------

There are several interesting versions of this question.
First, there is the question (of interest for those
who want high-performance rocket fuels) of what chemical reaction
yields the largest amount of energy per unit mass.
Indeed if energy E is yielded per mass M then a
rocket could be built with exhaust velocity V approaching
sqrt(2*E/M) if all the energy went into kinetic energy.
(Unfortunately some energy is wasted as vibrational
and/or rotational modes of the product molecule, so 
that V is not attainable.) 

If we restrict ourselves to element+element --> binary compound,
then here are the candidates:

reaction        enthalpy(kJ) per mol product    enthalpy(kJ) per gram
X+oxygen(gas):
2 H2 + O2 --> 2 H2O(gas)         241.826              13.42
4 Li + O2 --> 2 Li2O(solid)      597.9                20.01
2 Be + O2 --> 2 BeO(solid)       609.4                24.36
4 B + 3 O2  --> 2 B2O3(solid)   1273.5                18.29
C + O2 --> CO2                   393.51                8.94
2 Na + O2 --> 2 Na2O(solid)      414.2                 6.68
2 Mg + O2 --> 2 MgO(solid)       601.60               14.93
4 Al + 3 O2  --> 2 Al2O3(solid) 1675.7                16.43
Si + O2 --> SiO2(solid)          910.7                15.16  
P + O ??

X+fluorine(gas):
H2+F2 -> 2HF(gas)                273.30              13.66
2 Li + F2 --> 2LiF(solid)        616.0               23.75
Be + F2 --> BeF2(solid)         1026.8               21.94
2 B + 3 F2 --> 2BF3(gas)        1136.0               16.83
C + 2F2 --> CF4(gas)             933.6               10.66
2 Na + F2 --> 2NaF(solid)        576.6               13.73
Mg + F2 --> MgF2(solid)         1124.2               18.04
2Al + 3F2 --> 2AlF3(solid)      1510.4               17.99
Si + 2F2 --> SiF4(gas)          1615.0               15.52

Note that the biggest answers we are getting are NOT what one might 
have expected by thinking about what are the current favorite rocket fuels!
It is actually possible to get more energy than the above by using
certain unstable compounds as fuels and/or oxidizers. For example,
ozone O3 decomposes to ordinary O2 with release of 142.7 kJ per mol of ozone.
Then
  3 Be + O3 --> 3 BeO(solid)     656.97              26.27
would lead the pack with a remarkable 26.27 kJoules per gram,
for a theoretical (unattainable) rocket exhaust
velocity of 7248 meter/second.
Unfortunately, in a real rocket, one probably would want the exhaust
to be a GAS, not a solid, and would want liquid oxygen (or ozone)
not gaseous, as the reactant. This reduces the enthalpy by about 10%.
Some similarly unstable fuels include
  C3H6 cyclopropane  53.3 kJ/mol
  C2H2 acetylene    228.2 kJ/mol
  C2H4 ethylene      53.5 kJ/mol
  BH3(gas)borane    100.0 kJ/mol
It also does not help that Be is rare and Be compounds are toxic.
When one takes heats of vaporization into account (for the reactions
with solid products) and considers rarity and toxicity and
the number of "wasted" (non-translational) degrees of freedom of the
product molecule, the space shuttle's rocket fuel 2H2+O2, ends up
looking like a pretty good choice. (It has theoretical rocket
exhaust velocity of about 5000 meter/sec if non-translational
degrees of freedom are ignored.)  Perhaps the easiest way to
upgrade space shuttle performance would be to substitute ozone
for oxygen, which would increase the exhaust velocity by 9%.
Adding some borane to the liquid hydrogen might also provide
some performance boost (I'm not sure) with the additional benefit
of increasing the boiling point of the resulting mixture.

Second, there is the same question, but where there is
now only allowed to be ONE reactant. (I.e.: the most energetic
high explosive, per unit mass).

Third, there is the question of 
what chemical reaction yields the largest amount of energy 
per degree of freedom (tranalational, rotational, and vibrational) of
the product molecule(s). This reaction would lead to the
highest chemically attainable temperature.