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Hydrogen
The Traditional View of Cosmology: The Big Bang!!
Origin of the Universe
Hydrogen
Most abundant element in the Universe; 3rd most abundant element in the Earth's crust, found in minerals, oceans and all living things.
Hydrogen is a unique element, does not belong to any groups in the periodic table.
Hydrogen Isotopes
Deuterium was discovered in 1932 by Harold C. Urey, deuterium is a stable isotope of
the element hydrogen. An atom of deuterium consists of one proton, one neutron and one electron. About .015% of natural hydrogen is composed of deuterium.
Tritium was discovered in 1934, and is an unstable isotope of the element hydrogen. An atom of tritium consists of one proton, two neutrons and one electron.
Tritium is radioactive and has a half-life of about 12.5 years.
Isotopes of Hydrogen
The three H isotopes have different nuclear spin which give rise to easily observed changes in IR, Raman and NMR spectra of molecules containing these isotopes.
Because the relative mass differences between H's isotopes are so large, there is a significant dissimilarity in physical properties.
Heavy water D2O: can be separated from H2O by distillation.
Isotopes No. of
Neutrons Nuclear
Spin Molar
Mass
(g/mol) Boiling
Point
K Bond
Energy
(kJ/mol)
H 0 1.01 --- ---
D 1 1 2.02 --- ---
T 2 3.03 --- ----
H2 -- -- 2.02 20.6 436
D2 -- -- 4.03 23.9 443
T2 -- -- 6.03 25.2 447
H2O -- -- 18.02 373.2 463.5
D2O -- -- 20.03 374.4 470.9
Kinetic Isotope Effect
Bond dissociation energy of D2 is about 7 kJ/mol greater than H2. Therefore reaction rates are often measurably different for processes in which E-H and E-D bonds are broken, made or rearranged.
For C-H vs C-D bonds, the difference in reaction rates kH/kD can be as high as about 7. This is usually referred as the kinetic isotope effect.
The detection of this kinetic isotope effect can often help to support a proposed mechanism.
ortho and para Hydrogen 正、仲氢
These differ in the magnetic interactions of the protons due to the spinning motions of the protons.
In ortho-hydrogen, the spins of both protons are aligned in the same direction—that is, they are parallel.
In para-hydrogen, the spins are aligned in opposite directions and are therefore antiparallel.
Physical Properties of ortho & para Hydrogen
Normal hydrogen at room temperature contains 25% of the para form and 75% of the ortho form.
The ortho form cannot be prepared in the pure state.
Since the two forms differ in energy, the physical properties also differ.
The melting and boiling points of parahydrogen are about 0.1 deg C lower than those of normal hydrogen.
Where do we get it from
In the laboratory,
CaH2 + 2H2O Ca(OH)2 + 2H2
This is efficient in that 50% of the hydrogen produced comes from water.
Another approach Boyle's early synthesis, the reaction of iron filings with dilute sulphuric acid.
Fe + H2SO4 FeSO4 + H2
Industrial methods for the production of hydrogen depend upon local factors such as the quantity required and available raw materials. Two processes in use involve heating coke with steam in the water gas shift reaction or hydrocarbons such as methane with steam.
CH4 + H2O CO + 3H2 1100oC
C(coke) + H2O CO + H2
In both these cases, further hydrogen may be made by passing the CO and steam over hot (400°C) iron oxide or cobalt oxide.
CO + H2O CO2 + H2
What about very pure H2
Cathode. 2H2O(l) + 2e- 2OH- + H2
Anode. 2OH-(aq) H2O(l) +1/2O2 +2e-
This process is energy intensive, and requires electrolysis of basic solutions.
Industrial Production of Hydrogen
Water gas reaction
C + H2O CO + H2 (endothermic)
Methane reforming
CH4 + H2O CO + 3 H2 (endothermic)
Thermal Decomposition Of Water
Thermal decomposition, also called thermolysis
@ 2200 °C about three percent of all H2O molecules are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H2, O, O2, and OH. Other reaction products like H2O2 or HO2 remain minor
Nuclear-thermal
Solar-thermal
Water Splitting
1 Electrolysis
1.1 High pressure electrolysis
1.2 High-temperature electrolysis
2 Photoelectrochemical water splitting
3 Photocatalytic water splitting
4 Photobiological water splitting
5 Thermal decomposition of water
5.1 Nuclear-thermal
5.2 Solar-thermal
Properties of Dihydrogen
Dihydrogen is a colorless gas at room temperature.
Boiling point -253 °C
Melting Point -259 °C
Bond Dissociation Energy 436 kJ/mol
The H-H bond is stronger than the bonds hydrogen has with most other non-metals (e.g., H-N: 390 kJ/mol)
Bond Length 0.74
Electronegativity 2.2
Similar to B, C, Si. Therefore E-H bonds involving these elements are not expected to be polar.
Uses
commercial fixation of nitrogen from the air in the Haber ammonia process
hydrogenation of fats and oils
methanol production, in hydrodealkylation, hydrocracking, and hydrodesulphurization
rocket fuel
welding
production of hydrochloric acid
reduction of metallic ores
for filling balloons (hydrogen gas much lighter than air; however it ignites easily)
One of hydrogen's isotopes, tritium (3H) is radioactive. Tritium is produced in nuclear reactors and is used in the production of the hydrogen bomb. It is also used as a radioactive agent in making luminous paints and as a tracer isotope
Hydrogen as a modern fuel.
This car is fueled by hydrogen.
Fuel tank has 70 layers of fiberglass and Al.
What about the Challenger and Hindenburg
Compare and contrast the properties of H2 and gasoline.
Chemical reactions
Reaction of hydrogen with air
Hydrogen is a colorless gas, H2, that is lighter than air. Mixtures of hydrogen gas and air do not react unless ignited with a flame or spark, in which case the result is a fire or explosion with a characteristic reddish flame whose only products are water, H2O.
2H2(g) + O2(g) 2H2O(l)
Reaction of hydrogen with water
Hydrogen does not react with water. It does, however, dissolve to the extent of about 0.00160 g kg-1 at 20°C (297 K) and 1 atmosphere pressure.
Reaction of hydrogen with the halogens
Hydrogen gas, H2, reacts with fluorine, F2, in the dark to form hydrogen(I) fluoride.
H2(g) + F2(g) 2HF(g)
Hydronium Ion
H (g) H+ (g) + e- I.E. = 1311 kJ/mol
Li (g) Li+ (g) + e- I.E. = 520 kJ/mol
Na (g) Na+ (g) + e- I.E. = 490 kJ/mol
I.E. is three times larger than the value of alkali metals. Therefore, H+ (g) does not exist under ordinary conditions.
Ionic Radii: H+ 10-5 Li+ 0.9 Na+ 1.2
Due to its large charge/size ratio, H+ has a high polarizing power.
In solutions, it is solvated by solvent molecules.
In aqueous solution, exits as [H(H2O)n]+ (aq).
In liquid ammonia, form NH4+
Hydride Ion
H2(g) + e- -------> H- (g) DH0 = +145 kJ/mol
F2(g) + e- -------> F- (g) DH0 = -249 kJ/mol
Cl2 (g) + e- -------> Cl- (g) DH0 = -228 J/mol
Only most electropositive metals (e.g. Group 1 and 2 metals) can stabilize H- in an ionic lattice.
Ionic Radii: H- 1.30 (in LiH) ~ 1.54 (in CsH)
F- 1.33
Cl- 1.67
H-: low charge/size ratio; easily polarizable (i.e. it can easily distorted by any nearby cation).
Strong basic character, reacts violently with H+ source.
Hydrides.
Hydrides are binary compounds of hydrogen and another element.
Although all hydrogen compounds could be termed as hydrides,
not all hydrogen containing compounds display hydridic character.
Hydridic compounds are those that:
React as H- donors or,
Clearly contain anionic hydrogen.
Hence NaH is hydridic, methane and HCl are not.
This is parallel with the earlier statements…
The loss of an electron to give H+.
Acquisition of and electron to give H-.
The formation of a single covalent bond.
Classification of Hydrides
It is useful to classify hydrogen compounds as :
Ionic or covalent
Stoichiometric or nonstoichiometric
Binary or complex.
Be and Mg are extended and polymeric.
Salt-like = ionic
Ionic Hydrides
The most electropositive elements react directly with H2 to form
stoichiometric hydrides.
These compounds are referred to as ionic or salt-like hydrides.
When heavier metals are involved they are truly hydridic….
Also, Be, Mg, and Li have slightly more covalent character and are better described as a polymer.
Ionic hydride preparation and structure.
Typically these compounds are prepared by direct interaction with the metals at 300-700oC.
2 M(l) + H2(g) 2MH(s)
M(l) + H2(g) MH2(s)
The rates of these reactions are Li> Cs> K> Na.
All produce pure white solids that appear grey when impure.
Crystal Structures of Metal hydrides
Alkali metal hydrides and LiH and CsH take on the NaCl Structure.
What about others
MgH2 adopts the rutile金红石
structure.
CaH2
SrH2
BaH2
All take on a PbCl2-like
distorted hcp六方紧密堆积 array.
Reactions of Ionic metal hydrides
1. All thermally decompose to give metal and
hydrogen.Only LiH is stable to its melting point
of 688oC.
Note that LiH is unreactive at moderate temperatures toward oxygen and chlorine.
Generally ionic hydrides are highly reactive toward air and water.
MH(s) + H2O H2(g) + MOH(s)
MH2(s) + H2O H2(g) + MOH2(s)
Ionic hydrides are powerful reducing agents and good hydrogen-transfer agents.
NaH + B(OCH3)3 Na[HB(OCH3)3]
NaH + TiCl4 Ti0 + 4NaCl +2H2
Covalent hydrides
Covalent hydrides include:
Neutral binary XH4 compounds of Group 14, like methane.
Slightly basic binary XH3 compounds of Group 15, NH3 and PH3.
Weakly acidic or amphoteric, binary XH2 of Group 16, H2O and H2S.
Strongly acidic binary HX compounds of Group 17, HCl and HI.
Covalent hydrides of boron.
Hydridic, complex compounds of hydrogen.
Examples include LiAlH4 and NaBH4. These are powerful reducing agents
despite the covalent nature of the Al-H and B-H bonds.
Some interesting notes about LiAlH4 and NaBH4.
These two compounds are ionic in nature BUT they possess tetrahedral anions containing covalent bonds to H
How are LiAlH4 and NaBH4 prepared
Both these reactions are carried out in ether.
8LiH + Al2Cl6 2LiAlH4 + 6 LiCl
2 NaH + B2H6 2NaBH4
The anions are powerful hydrogen transfer agents.
2LiAlH4 + 2 SiCl4 2SiH4 + 2 LiCl + Al2Cl6
I2 + 2 NaBH4 B2H6 + 2NaI + H2
Gas phase BH3
B2H6
Some more details about covalent hydrides.
Covalent hydrides can be divided into three subcategories which are reliant
on the nature of the H atom.
The H-atom is neutral.
The H-atom is positive.
The H-atom is negative.
These are a generalization of the statements that we made before.
By far the majority of covalent hydrides fall into the first category.
Given their low polarity these compounds are only held together by
weak intermolecular forces … termed dispersion forces.
This results in low boiling points.
SnH4 -52oC , PH3 -90oC
Carbon-based systems comprise the largest set of hydrides.
Hydrides of the Alkali and Alkaline Earth Metals
The alkali and alkaline earth metals react exothermically with hydrogen at elevated temperature to form ionic hydrides:
2 Na + H2 2 Na+H–
Ca + H2 Ca2+H2–
The chemical properties of the hydrides are determined by their high basicity and reducing power. The hydride ion reacts irreversibly with compounds containing acidic protons, liberating hydrogen:
M+H– + Hd+Xd– M+X– + H2
X = halogen, OH, OR, HNR, SR
Three Elements dominate hydride chemistry.
C, Si, and B
In summary.
Si and C: X-H bonds are “normal’ in that they are covalent in the traditional sense of e- sharing.
B: bonds in these systems are unusual because H can bridge B atoms.
I2 + 2 NaBH4 B2H6 + 2NaI + H2
1976 Nobel Prize for Chemistry
Professor William N. Lipscomb, Harvard University, USA,
for his studies on the structure of boranes illuminating
problems of chemical bonding
The electronic structure of diborane.
The electron configuration of B is 1s2 2s2 2p1
It has three valence electrons and is expected to form BH3.
BUT IN THIS SITUATION THE OCTET RULE IS NOT FULFILLED
To achieve a pseudo-full octet boron is hybridized to an sp3 orbital organization. It forms two 2-center-2-electron bonds (B-H) with two hydrogen atoms as well as two 3-center-2-electron bonds (B-H-B).
Transition Metal Hydrides
Transition metal hydrides are diverse in structure and properties.
Many contain Metal-hydrogen bonds including:
1. Stoichiometric Binary anions: [ReH9]2- and [FeH6]4-
2. Complex Stoichiometric compounds with covalent bonds to hydrogen.
like H(Mn(CO)6) and Re2H8(PR3)4.
3. Non-stoichiometric compounds formed by exposure of metals to H2.
Reaction of H2 with transition metals yields complex substances.
Typically these compounds are black or grey and have formulas like LaH2.87, YbH2.55, TiH1.7, ZrH1.9. Under extreme circumstances limited compositions can be attained.
Right now we do not understand the dynamics of these systems.
H2 as a ligand
H2 can behave as a ligand and occupy a coordination space around a metal center. This occurs for metals in low oxidation states.
Hydrogen can take on a “ side on” orientation with respect to the metal resulting in weak donation of its electrons into an empty on the metal center OR through acceptance of electrons from a filled orbital on the metal into the * orbital of the H2 molecule. This weakens the H-H bond.
Unless the system is very carefully balanced a system will tend toward the dihydride.
The bonding configuration of M-H2
Bonding in H2 complexes does not require any complicated explanation. In a similar way to that seen in boranes a three-center two-electron bond is formed. The H2 molecule thus acts a neutral two electron sigma donor. It is also possible to understand the bonding as back-donation of electrons from a filled metal orbital to the sigma-* orbital on the H2.
Both are shown schematically below.
Lanthanide Metal Hydrides
Preparation: Heat at 300-350°C,
Ln + H2 LnH2
Properties of LnH2
black, reactive, highly conducting, fluorite structure
This is most thermodynamically stable of all binary metal hydrides and is viewed as Ln3+(H-)2(e-) with e- delocalized in a metallic conduction band.
More H can often be accommodated in interstitial sites and it is frequently non-stoichiometric
e.g. LuHx where x = 1.83-2.23 & 2.78-3.00
A high pressure of H2 yields LnH3 reduced conductivity and is found to be salt-like Ln3+(H-)3
Hydrogen Storage in Liquid
Luo W.; et al; J. Am. Chem. Soc., 2011, 133, 19326.

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