1

1

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

2

3

Industrial applications of H2

4

H2 activation

H2

3

5

H2 activation on a catalytic surface

homolytic cleavage

B(H-H) = 436 kJ/mol

6

Saline, metallic, and molecular

hydrides

4

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

5

9

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

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

6

11

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

12

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

7

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)

8

15

Hydrogen bonding

16

H-bonding in DNA

double helical structure

(James Watson and

Francis Crick, 1953)

Guanine – cytosine

Adenine - thymine

9

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

10

19

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

20

Hydrogen purifier

11

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*

www.ballard.com*for full device

12

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