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Hydrogen : Hydrogen, Chemical Properties |
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Hydrogen is a combustible gas, burning in air or oxygen to form water: 2H2 + O2 = 2H2O Hydrogen and oxygen combine slowly at 180°, or in bright sunlight at the ordinary temperature. Explosion occurs with moist gases at 550° - 700°, but if the gases are exceedingly pure and dry they may be heated by an incandescent silver wire without explosion, though combination slowly occurs (Baker, 1902): the water produced appears to be so pure as to exert no catalytic influence on the reaction. The mixture 2H2 + O2 ignites at 526° on adiabatic compression, some combination occurring before the explosion itself (pre-flame period): the mixture 3H2 + O2 ignites at 544°, and H2 +4O2 at 478°, respectively (Dixon and Crofts, 1914). Thomas Thomson in 1817 gave 538° as the ignition temperature of hydrogen in air. Hydrogen also readily combines with fluorine and chlorine, less readily with bromine, iodine, sulphur, phosphorus, nitrogen, and carbon. With a few metals, such as lithium, sodium, and calcium, it forms hydrides, such as NaH. In these hydrides, which when pure are white salt-like compounds (KH explodes in air), readily decomposed by water: NaH + H2O = NaOH + H2, the hydrogen behaves to some extent like a halogen or electronegative element. On the electrolysis of fused lithium hydride, the hydrogen is liberated at the positive electrode, not the negative as is the case when water is electrolysed (Moers, 1920). Hydrogen is also evolved at the anode in the electrolysis of a solution of calcium hydride, CaH2, in fused potassium and lithium chlorides (Bardwell, 1924). By reason of its tendency to unite with oxygen, hydrogen acts as a reducing agent. Hydrogen, when passed over many heated metallic oxides (copper, iron, lead), reduces them to the metallic condition, and water is produced: CuO + H2 = Cu + H2O. Reduction is in this case the withdrawal of oxygen. Some oxides, e.g., of zinc and aluminium, are not reduced by hydrogen. |
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