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Hydrogen gas is highly flammable and will burn at concentrations as low as 4% H2 in air. The enthalpy of combustion for hydrogen is −286 kJ/mol; it burns according to the following balanced equation.
- 2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)
When mixed with oxygen across a wide range of proportions, hydrogen explodes upon ignition. Hydrogen burns violently in air. It ignites automatically at a temperature of 560 °C. Pure hydrogen-oxygen flames burn in the ultraviolet color range and are nearly invisible to the naked eye, as illustrated by the faintness of flame from the main space shuttle engines. Thus it is difficult to visually detect if a hydrogen leak is burning. Another characteristic of hydrogen fires is that the flames tend to ascend rapidly with the gas in air.
Hydrogen is not an energy source, except in the hypothetical context of commercial nuclear fusion power plants using deuterium or tritium, a technology presently far from development. The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth.Elemental hydrogen from solar, biological, or electrical sources costs more in energy to make than is obtained by burning it. Hydrogen may be obtained from fossil sources (such as methane) for less energy than required to make it, but these sources are unsustainable, and are also themselves direct energy sources.
The energy density per unit volume of both liquid hydrogen and hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel mass is higher.Nevertheless, elemental hydrogen has been widely discussed in the context of energy, as a possible future carrier of energy on an economy-wide scale. Hydrogen used in transportation would burn relatively cleanly, with some NOx (oxides of Nitrogen) emissions, but without carbon emissions.However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.
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