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CH10. Hydrogen

CH10. Hydrogen. Preparation. Historic preparation: metal + acid  H 2 + M x+ Lab preparation: 2Fe + 6HCl  2FeCl 3 + 3H 2 Zn  Zn 2+ E  = +0.76V Fe  Fe 3+ +0.04 Cu  Cu 2+ -0.34 Industrial preparation

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CH10. Hydrogen

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  1. CH10. Hydrogen

  2. Preparation Historic preparation: metal + acid  H2 + Mx+ Lab preparation: 2Fe + 6HCl  2FeCl3 + 3H2 Zn  Zn2+ E = +0.76V Fe  Fe3+ +0.04 Cu  Cu2+ -0.34 Industrial preparation CH4(g) + H2O(g)  CO (g) + 3H2(g) steam reforming C(s) + 2H2O(g)  CO (g) + 2H2(g) water-gas shift reaction Cu(m) does not reduce acid, even 6M HCl (penny experiment) catalyst 1000 C catalyst 1000 C

  3. Industrial applications of H2

  4. H2 H2 activation

  5. H2 activation on a catalytic surface homolytic cleavage B(H-H) = 436 kJ/mol

  6. Saline, metallic, and molecular hydrides

  7. Saline hydrides Compounds with Group 1 and 2 metals, M+H (ionic) • LiH lithium hydride • rocksalt structure, r(H) = 1.2 – 1.5 Å • CaH2 calcium hydride • CaH2 (s) + H2O (l)  Ca(OH)2 (s) + 2H2 (g) • “saline” because pH increases in this reaction • used to dry organic solvents, but only when water content is low • Saline hydrides are very strong reducing agents • 2H  H2 + 2e E ≈ +2.2V • => very exothermic reactions with air/water

  8. Molecular compounds

  9. Structures: • e deficient 3c – 2e bonds • B2H6 diborane Gf = +87 kJ/mol • also Al2(CH3)6 Al2H2(C2H5)4 • BH4 (tetrahydroborate) Td anion, mild reducing agent • AlH4 ,AlH63 (Oh) stronger reducing agent • Reactions: • B2H6 B(OH)3 explosive with green flash • B2H6 + 2 NR3 2 H3BNR3 • H3BN(CH3)3 + F3BS(CH3)2 H3BS(CH3)2 + F3BN(CH3)3 • (BH3 is a soft Lewis Acid) O2 or H2O Group 13 hydrides

  10. Group 14 hydrides • All form EH4 (Td) molecules • Gf CH4 -51 kJ/mol • SiH4 +57 (endoergic) • Si (m) + 2H2 (g) No Rxn • Silane prepn: SiCl4 + LiAlH4 SiH4 + LiAlCl4 (metathesis) • H transfers to more electronegative element (LiH will also react with SiCl4) • Bond E Si-Cl 381 Si-H 318 kJ/mol • Al-H <318? Al-Cl 421

  11. Silane reactions 1. Under an inert atm, silane is stable at RT, thermolysis at 500C SiH4 Si (cryst) + 2H2 indirect band gap (semiconductor substrate)  SiHx (amorph) + (2 - x/2) H2 direct band gap (photovoltaics) for comparison, CH4 “cracks” above 2000 C, or 800 C with a catalyst 2. In air SiH4 + 2 O2 SiO2 + H2O for comparison, methane needs ignition source, but not SiH4 Silane oxidation can be very exothermic and explosive  e discharge

  12. . Higher silanes known, but decreasing stability • SiH4 + 2AgI  SiH3I + HI + 2 Ag (m) • 2 SiH3I  Si2H6 (decomposes at ≈ 400 C) • Si4H10 has neo- and iso- forms identified, but decomposes rapidly at RT 250 C Na/Hg Silane reactions

  13. Hydrogen bonding • Relatively strong intermolecular interaction where H is bonded to N, O, or F • Strongest case is in HF2 bifluoride anion • [F – H – F] • B(H-F) = 165 kJ/mol

  14. Hydrogen bonding • 2H2O(l)  H3O+ (aqu)+ OH- (aqu) Kw = 10–14 at STP • 3HF(l)  H2F+ (solv) + HF2– (solv) K ~ 10–11 • H3O+ does exist, for example in hydronium perchlorate • H3O+ClO4 (s) • but in aqu solution H+(OH2)n n > 1 • and in HF(l); F(HF)n n > 1 • LiF • KF(HF) • NBu4+ F(HF)n (l) n~4-10 • (ionic liquid)

  15. Hydrogen bonding

  16. H-bonding in DNA double helical structure (James Watson and Francis Crick, 1953) Guanine – cytosine Adenine - thymine

  17. Ice structure Ice 1H (hexagonal ice)

  18. Metallic hydrides • non-stoichiometric solid compounds with d and f block metals • Ex: PdHx O < x < 1 x depends on P/T • Ex: ZrHx x = 1.3 – 1.75 fluorite structure with anion vacancies • This non-stoichiometry is often associated with hydride vacancies

  19. PdHx Other H storage alloys - http://www.ergenics.com Pd (m) has an unusual ability to absorb hydrogen. H2 chemisorbs on the metal surface, dissociates into atomic H, and diffuses into the fcc Pd lattice (a = 3.8907 Å). The reaction can be summarized:     Pd + 2/x H2  =  PdHx In PdHx, H atoms occupy only the largest available (Oh) sites. What is the maximum possible value for x ? Pd swells when fully loaded with hydrogen, so that PdH0.97 has a = 4.03 Å. Which one do you think contains a higher concentration of H, PdH0.97 or liquid H2 (r = 0.07 g/ml)?

  20. Hydrogen purifier

  21. Metal hydride electrode • One use of metallic hydrides is hydrogen storage • Another is as the anode of the NiMH battery • MH + OH e + M + H2O negative electrode • e + NiOOH  Ni(OH)2 + OH positive electrode • (same positive electrode as the NiCd battery) • separator is OH permeable, aqueous alkaline electrolyte • M = LaNi5H6 or FeTiH2 type hydride, a common one is actually a complex alloy of V, Ti, Zr, Co, Cr, Fe

  22. Hydrogen as a fuel Fuel Form Energy Density kWh/kg kWh/L H2 gas 33 0.5   liquid 33 2.4   MHx 0.6 3.2 CH3OH liquid 5.6 4.4 Gasoline liquid 12.7 8.8 Pb/acid battery 0.03* 0.09* NiMH battery 0.05* 0.18* Li-ion battery 0.14* 0.30* *for full device www.ballard.com

  23. Hydrogen fuel cells 2 H2 + O2 2 H2O + energy Rocket booster energy = heat / pressure Fuel cell energy = electric power 2H2 4H+ + 4e anode O2 + 4H+ + 4e 2H2O cathode (theoretical cell E = 1.23V) Catalysts are Pt based Separator is either proton conductive polymer (PEM) or O2- conductive oxide (SOFC) catalyst

  24. PEM Fuel Cells Fuel cell stack

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