Hydrogen and Hydrides Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

6/28/2015 Hydrogen and its isotopes Hydrogen compounds saline hydrides metallic (interstitial) hydrides covalent hydrides boron hydrides

6/28/2015 Hydrogen Hhydrogen 1 H Ddeuterium 2 H0.0156% Ttritium 3 H % D 2 Oheavy water nmr solvent chemical & biochemical tracer 6 Li + 1 n → 7 Li → 3 H + 4 He 3 H → 3 He + β − t ½ = 12.3 yr

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Hydrides saline covalent metallic borderline

Saline Hydrides saline EuH 2 YbH 2 M + H 2 → MH 2 Δ

6/28/2015 Saline Hydrides Physical Properties Good electrical conductor when molten. H 2 released at the anode (unlike H + which is reduced) NaH(s) → Na + (ℓ) + H - (ℓ) → ½ H 2 (g) + e - Δ → Na(s) e-e- Why are saline hydrides (H - ) so uncommon?

6/28/2015 Saline Hydrides Comparison of Hydride to Halides ½ H 2 (g) → H(g) ΔH = 52 kcal/mol e - + H(g) → H - (g) ΔH = -16 e - + ½ H 2 (g) → H - (g) ΔH = +36 ½ Br 2 (g) → Br(g) ΔH = 27 kcal/mol e - + Br(g) → Br - (g) ΔH = -78 e - + ½ Br 2 (g) → Br - (g) ΔH = -51 Formation of H - is very endothermic. Only very electropositive elements form saline hydrides.

6/28/2015 Saline Hydrides Reactivity Reactive with H + sources (H 2 O, NH 3 ) usually liberating dihydrogen. Good reducing agents

Metallic (Interstitial) Hydrides metallic 2 M + xH 2 → 2 MH x

6/28/2015 Metallic (Interstitial) Hydrides Physical Properties Non-stoichiometric, with limiting stoichiometry

6/28/2015 Metallic (Interstitial) Hydrides Bonding Proton in metal matrix with conduction band electrons around. e - in metal d-orbital (LUMO) Displays both ionic and covalent properties Metal-hydrogen alloy Migration towards cathode (unlike saline hydrides)

6/28/2015 Metallic (Interstitial) Hydrides Bonding Hydride ions with e - from conduction band of metal. Magnetic susceptibility increases with increasing hydrogen content (supports hydride formation vs H 2 )

6/28/2015 Metallic (Interstitial) Hydrides Reactivity Some release H 2 fairly easily when external pressure is released. H 2 storage – more hydrogen is stored in metal than in liquid H 2. Safety - H 2 stored this way is not as reactive and explosions are less hazardous. Reversible – lowering external pressure releases a constant stream of H 2.

6/28/2015 Metallic (Interstitial) Hydrides Reactivity 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.

Hydrides covalent borderline

6/28/2015 Covalent Hydrides Physical Properties Molecular are volatile CH 4, NH 3, H 2 O, HF, SiH 4 low mp, low bp, weak van der Waals forces Macromolecular are non-volatile BeH 2, BH 3, AlH 3, GaH 3

6/28/2015 Covalent Hydrides Preparation Decomposition of a corresponding organometallic compound [(CH 3 ) 3 C] 2 Be BeH (CH 3 ) 2 C=CH °C (CH 3 CH 2 ) 2 Mg MgH H 2 C=CH 2 Δ Hydride reaction 4 LiH + AlCl 3 LiAlH LiCl 2 ZnI LiAlH 4 ZnH 2 + AlH LiI

6/28/2015 Covalent Hydrides Reactivity LiAlH 4 is a good reducing agent others are poor reducing agents Many react fairly easily with dilute acids InH 3, GaH 3 are almost inert toward strong acids.

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