CH10. Hydrogen
1
Preparation
metal + acid  H2 + Mx+
Historic preparation:
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)
Cu(m) does not reduce
acid, even 6M HCl
(penny experiment)
catalyst

1000 C
CO (g) + 3H2(g)
steam reforming
catalyst
C(s) + 2H2O(g)

1000 C
CO (g) + 2H2(g) water-gas shift reaction
1
Industrial applications of H2
3
H2 activation
H2
4
2
H2 activation on a catalytic surface
homolytic cleavage
B(H-H) = 436 kJ/mol
5
Saline, metallic, and molecular
hydrides
6
3
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
7
Molecular
compounds
8
4
Group 13 hydrides
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:
O2 or H2O

B(OH)3
B2H6
B2H6 + 2 NR3

explosive with green flash
2 H3BNR3
H3BN(CH3)3 + F3BS(CH3)2  H3BS(CH3)2 + F3BN(CH3)3
(BH3 is a soft Lewis Acid)
9
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
10
5
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)
e discharge

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
11
Silane reactions
3. Higher silanes known, but decreasing stability
SiH4 + 2AgI
2 SiH3I
250 C

Na/Hg

SiH3I + HI + 2 Ag (m)
Si2H6
(decomposes at ≈ 400 C)
Si4H10 has neo- and iso- forms identified, but decomposes
rapidly at RT
12
6
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
13
Hydrogen bonding
2H2O(l)

H3O+ (aqu) + OH- (aqu)
3HF(l)

H2F+ (solv) + HF2– (solv)
Kw = 10–14 at STP
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)
14
7
Hydrogen bonding
15
H-bonding in DNA
Guanine – cytosine
double helical structure
(James Watson and
Francis Crick, 1953)
Adenine - thymine
16
8
Ice structure
Ice 1H (hexagonal ice)
17
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
18
9
Other H storage alloys http://www.ergenics.com
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)?
19
Hydrogen purifier
20
10
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
+ NiOOH  Ni(OH)2 +
positive electrode
e
OH
(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
21
Hydrogen as a fuel
Fuel
Form
Energy Density
kWh/kg kWh/L
H2
gas
liquid
MHx
33
33
0.6
0.5
2.4
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
22
11
Hydrogen fuel cells
catalyst

2 H2 + O2
2 H2O + energy
Rocket booster
energy = heat / pressure
Fuel cell
energy = electric power
2H2  4H+ + 4e
O2 +
4H+
+
4e
 2H2O
anode
cathode
(theoretical cell E = 1.23V)
Catalysts are Pt based
Separator is either proton conductive polymer (PEM) or O 2conductive oxide (SOFC)
23
PEM Fuel Cells
Fuel cell stack
24
12