Chapter 10
Hydrogen
Hydrogen and hydride ions
Isotopes of hydrogen
Dihydrogen
Polar and non-polar E-H bonds
Hydrogen Bonding
Classes of binary hydrides
Major Uses of Hydrogen
Feedstock
for CH3OH
-C-C- NH3 for
hydrogenation of fertilizers
unsaturated fats
and oils
H2
Metal
Fuel
Production
1
Water of crystallization
• Solids that consist of molecules of a compound along with
water molecules are named hydrates.
• Contain water bound to cations to anions or other electron
rich atoms via hydrogen bonds.
– [M(OH2)6]n+ etc.
• Water of crystallization found in some crystals and hydrated
metal halides e.g. CuSO4.5H2O
• CoCl2(H2O)6 has a coordination sphere around the metal as
trans-[CoCl2(H2O)4] and two equivalents of water of
crystallization that are not bound to Co.
Hydrated Protons
H(g) H+(g) + e- Ionization Energy 1312 kJ mol-1
[H3O]+ is the hydrated proton or oxonium ion
• DhydHo(H+, g) = 1091 kJ mol-1
[H5O2]+
2
Hydride Ion
H(g) + e- H-(g) DEAH(298K) = -73 kJ mol-1
In the solid state, all alkali metal hydrides crystallize with the
NaCl structure type.
•From the crystal structure, the radius of H- can be estimated
by: internuclear distance = rcation + ranion
Isotopes of Hydrogen
Three common isotopes: protium, deuterium, and tritium
3
Kinetic Isotope Effect
The zero point energy
(corresponding to the
lowest vibrational state)
of a CD bond is lower
than that of a CH bond
and this results in the
bond dissociation
enthalpy, D, of the CD
bond being greater than
that of the CH bond.
Selected properties of H2O and D2O (‘heavy water’).
Why is the boiling point of D2O greater than that of H2O?
What about tritium? Where does it come from?
4
Dihydrogen
Selected physical properties of H2.
Dihydrogen production
Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)
2Al(s) + 2NaOH(aq) + 6 H2O(l) 2Na[Al(OH)4](aq) + 3 H2(g)
CaH2(s) + 2H2O(l) Ca(OH)2(aq) + 2H2(g)
Cu/ZnO catalyst
CO + 2H2 CH3OH Haber Process
~550 K, 50 bar
Reagents above are known as synthesis gas. and the
mixture is manufactured by the water-gas shift reaction.
Ni catalyst
CH4 + H2O CO + 3H2
~1200 K
iron oxide catalyst
CO + H2O CO2 + H2
~700 K
hydrogen economy – formation of H2 from H2O requires a large energy input
5
Hydrogen Economy
Photocatalyst
[Ru(bpy)3]3 (bpy = 2,2'-bipyridine)
6
Reactivity
Interaction of an H2 molecule with a metal
surface to give adsorbed hydrogen atoms
Polar and non-polar bonds
The direction of the dipole
moment in a polar EH bond
depends upon the relative
electronegativity values.
• The direction in which the
arrow points ( to ) is
defined by SI convention.
The difference in electronegativity values between E and H means the bond may be:
•Non-polar, or polar in either of the senses given the diagram above.
•When E is a p-block element, B-H, C-H, Si-H bonds are essentially non-polar
•When E is a metal (electropositive) H carries a partial negative charge.
•When E is N, O, or F the H atom carries a partial positive charge
7
Hydrogen bonding
Hydrogen bonding in formic acid
monomer
solid state
dimer
8
Trends in (a) melting and (b) boiling points for some p-block hydrides, EHn.
DvapH (measured at the boiling Trouton’s Rule
point of the liquid) for some p-
block hydrides, EHn.
An interesting and useful approximation:
• The ratio of the heat of vaporization and the
boiling point is (approximately) constant
DvapS = DvapH/bp ~ 88 J K-1 mol-1
• Boiling point of cyclohexane is 69°C.
Therefore,
DvapH = (69 + 273)(88) ~ 30 kJ/mol
which is within 2-3% of the experimental value
• Works well for unassociated liquids and gives
useful information about degree of association.
9
Solid state structure of HF [HF2] ion
stabilization in the solid state of [H5O2] and
[H7O3] by hydrogen bonding to crown ethers
Hydrogen-bonded chains in the solid-
state structure of Me2NNO2
10
Dihydrogen Bond
Hydrogen bonding in biological systems
11
Binary Hydrides
Four major classes: metallic, saline
(salt like), molecular, covalent (with
extended structures)
Molecular hydrido complexes
[CoH5]4 [FeH6]4
[ReH9]2 [Pt2H9]5
12
Polymeric Hydrides
polymeric chain structure of BeH2
13
