12. Heat, Energy, and Chemical        Bonds   Previous PageNext Page
       Heat and Chemical Reactions

This 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) « H2O(g) DH = -58 kcal mole-1 H2O

The difference is the enthalpy or heat of vaporization:

H20(l) « H20(g) DH = +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:

H2O(g) « H2O(l) DH = -10 kcal mole-1

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