is a higher value than for any other chemical fuel burned in O2.
If hydrogen could be stored and handled more easily, we probably
would use it instead of more conventional hydrocarbon fuels.
If the water in the combustion remained as a gas rather than condensing
to a liquid, less heat would be given off (right):
H2(g) + ŻO2(g)
DH = -58 kcal mole-1
The difference is the enthalpy or heat of vaporization:
= +10kcal mole-1
During vaporization at 25oC, water
molecules absorb 10 kcal of heat energy per mole, so the enthalpy
increases. This is necessary for liquids in general because gas
molecules move faster and have more energy (and enthalpy) than molecules
in a liquid. The same amount of heat energy is given off again when
water vapor condenses:
= -10 kcal mole-1