Hydrogen and Hydrides

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Hydrogen and Hydrides

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Hydrogen and its isotopes Hydrogen
compounds saline hydrides metallic
(interstitial) hydrides covalent
hydrides boron hydrides
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Hydrogen
H hydrogen 1H D deuterium 2H 0.0156 T tritium 3H
10-15
6Li 1n ? 7Li ? 3H 4He
3H ? 3He ß- t½ 12.3 yr
D2O heavy water nmr solvent chemical
biochemical tracer
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Hydrides
covalent
borderline
metallic
saline
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Saline Hydrides
saline
EuH2
YbH2
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Saline HydridesPhysical Properties
Good electrical conductor when molten.
H2 released at the anode (unlike H which is
reduced)
Why are saline hydrides (H-) so uncommon?
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Saline HydridesComparison of Hydride to Halides
Formation of H- is very endothermic. Only very
electropositive elements form saline hydrides.
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Saline HydridesReactivity
Reactive with H sources (H2O, NH3) usually
liberating dihydrogen.
Good reducing agents
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Metallic (Interstitial) Hydrides
2 M xH2 ? 2 MHx
metallic
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Metallic (Interstitial) HydridesPhysical
Properties
Non-stoichiometric, with limiting stoichiometry
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Metallic (Interstitial) HydridesBonding
Displays both ionic and covalent properties
Metal-hydrogen alloy
Proton in metal matrix with conduction band
electrons around. e- in metal d-orbital (LUMO)
Migration towards cathode (unlike saline hydrides)
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Metallic (Interstitial) HydridesBonding
Magnetic susceptibility increases with increasing
hydrogen content (supports hydride formation vs
H2)
Hydride ions with e- from conduction band of
metal.
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Metallic (Interstitial) HydridesReactivity
Some release H2 fairly easily when external
pressure is released.
H2 storage more hydrogen is stored in metal
than in liquid H2.
Safety - H2 stored this way is not as
reactive and explosions are less hazardous.
Reversible lowering external pressure releases
a constant stream of H2.
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Metallic (Interstitial) HydridesReactivity
Hydrogen reactions with metal causes the metal to
powder or become brittle.
Concern for metal fatigue in nuclear reactors.
Reactive with water to give metal oxide or metal
hydroxide plus hydrogen gas.
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Hydrides
covalent
borderline
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Covalent HydridesPhysical Properties
Molecular are volatile CH4, NH3, H2O, HF,
SiH4 low mp, low bp, weak van der Waals forces
Macromolecular are non-volatile BeH2, BH3, AlH3,
GaH3
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Covalent HydridesPreparation
Decomposition of a corresponding organometallic
compound
Hydride reaction
2 ZnI2 2 LiAlH4
ZnH2 AlH3 2 LiI
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Covalent HydridesReactivity
LiAlH4 is a good reducing agent others are poor
reducing agents
Many react fairly easily with dilute acids
InH3, GaH3 are almost inert toward strong acids.
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