If one examines the composition of propellants and explosives, what you find is that the successful and desirable compositions are those substances that decompose to produce many more moles of decomposition products than moles of starting materials.  As a result, modern propellant compositions have not just a preponderance of nitrogen atoms, but also more skeletal C-N or N-N linkages that replace C-C linkages. Dinitrogen as a decomposition product is more atom efficient in producing PV work than is CO2 or H2O if only because a molar volume of N2 contains only 2 moles of atoms as opposed to 3. 

Designers of explosives and propellants are principally concerned with doing work (W=Fd=PV) against the environment. It could be moving soil, forming a shock wave, or a accelerating a projectile out of a tube. Some particular mass needs to be accelerated over a distance and extracting the last bit of work from the expanding gases is desirable.

PV work is performed by evolving lots of -kJ/mol from heat of formation and arranging for the expanding gas to do something useful. In the case of propellants, dinitrogen formation yields a healthy heat of formation produced from making a triple bond. Hot gases want to expand and move whatever they are in contact with. The more molar volumes of gas generated, the more work that can be done. 

Some of the above line of thinking applies to the combustion of hydrocarbons as well, though the necessary formation of triatomic gases lowers the atom efficiency. The combination of C=O and H-O bonds being formed leads to a net evolution of heat compared to heat absorbed in breaking C-C, C-H, and O-O bonds. Properly chosen fuels and oxidizers provide a net increase in moles of gaseous products leading to an increase in molar gas volume.

Now, consider the case of the combustion of hydrogen and oxygen to produce water: 2 H2 + O2 –> 2 HOH.   In this reaction three moles of gas react to produce only 2 moles of  gas. There is a net loss in molar volume of 1/3 at constant presssure.  Obviously H2 reacts violently with O2 to produce PV work.  Hydrogen can be used to power an Otto cycle engine. But the net loss of molar volume across the reaction would appear to be a drawback to this system compared to others. The question I have is, how does this figure into the overall efficiency of H2 as a fuel?? 

Hydrogen is known to be problematic in engines due to what is called a cooling effect.

One of the key issues to consider with hydrogen economics is the fact that every last molecule has to be manufactured from hydrogen rich feedstocks using energy input. Hydrocarbons have to be cracked in some way, water has to be electrolyzed, or metals have to be oxidized with acid to produce dihydrogen. 

Given that H2 has to be manufactured by cracking hydrocarbon resources or electrolysis of water, does it make sense to use H2 as an automotive fuel? Why not just combust the hydrocarbon that was cracked to give up the H2 in the first place? Better yet, combust H2 at a centrally located gas turbine power plant and distribute the energy as electricity.

Hydrogen isn’t easily liquified (like propane) and the compressed gas requires heavy containment. 

With xtal ball in hand, the more I peer into the next 50 years, the more the future appears to be electrically powered. Todays hydrogen and ethanol schemes found in the popular media result from our collective unwillingness to address the real problem: How do we modify our behaviour to consume fewer kilowatt-hours (or BTU’s) per capita?

The answer is that we need to live closer to work, drive fewer miles, divert fewer hydrocarbons into disposable products, and generally consume fewer kg of resources per capita. Hydrocarbons are a very valuable resource- we’re fighting in the middle east over access to oil output in that part of the world. 

Petroleum distillates have a wonderful combination of attributes that make them valuable. Petroleum distillates have high energy density, they are liquid in ordinary conditions and hence can be pumped and atomized, they offer a choice of flash points, and are reasonably safe for people to handle. This is a splendid set of properties! We should be more appreciative and take better care of how we use it.

For Americans, a glimse of the future can be had for the price of a plane ticket to Japan or Europe. Higher population density, smaller portions of most things, and a larger fraction of income spent on energy.