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CH10. Hydrogen

PreparationHistoric 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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Industrial applications of H2

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H2 activation

H2

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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 hydridesCompounds 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 hydridesAll 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 reactions1. 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 bondingRelatively 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 DensitykWh/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