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CH10. Hydrogen
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
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H2 activation on a catalytic surface
homolytic cleavage
B(H-H) = 436 kJ/mol
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Saline, metallic, and molecular
hydrides
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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
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Molecular
compounds
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Group 13 hydridesStructures:
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 , AlH6
3 (Oh) stronger reducing agent
Reactions:
B2H6 B(OH)3 explosive with green flash
B2H6 + 2 NR3 2 H3BNR3
H3BN(CH3)3 + F3BS(CH3)2 H3BS(CH3)2 + F3BN(CH3)3
(BH3 is a soft Lewis Acid)
O2 or H2O
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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
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Silane reactions
1. Under an inert atm, silane is stable at RT, thermolysis at 500C
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
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Silane reactions
3. 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
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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
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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)
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Hydrogen bonding
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H-bonding in DNA
double helical structure
(James Watson and
Francis Crick, 1953)
Guanine – cytosine
Adenine - thymine
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Ice structure
Ice 1H (hexagonal ice)
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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
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PdHx
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)?
Other H storage alloys -http://www.ergenics.com
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Hydrogen purifier
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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
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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*
www.ballard.com*for full device
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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
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PEM Fuel Cells
Fuel cell stack