Download 1 Assignment 5 Hydrogen – The Unique Element

Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
Assignment 5
Hydrogen – The Unique Element
Elemental Hydrogen and its reactivity
Although hydrogen is the simplest of all the elements, it possesses a
very diverse chemistry.1 It forms compounds with nearly every other
element in the periodic table! As it does not belong to a particular
group, it is difficult to draw comparisons between it and other
elements. It exists in its elemental form as a diatomic gas, which is
Hydrogen was discovered
by the British scientist
Henry Cavendish in 1766.
The name comes from the
two Greek words "hydro"
and
"genes"
meaning
"water" and "generator".9
highly explosive and flammable in air, Figure 1. Interestingly, the cores of the largest planets in our
solar system – Jupiter and Saturn – are made up of hydrogen under extreme (very high pressure)
conditions.2 Scientists think that under these conditions hydrogen is metallic in nature and therefore,
these planets have magnetic fields similar to earth’s although earth’s magnetic field is due to its iron
core. Of course on earth, hydrogen is a non-metal.
Figure 1: Colourised photograph of the Hindenburg (hydrogen-filled
zeppelin) burning in 1937.3 Equation for burning of hydrogen added
by author. [Note: Image reproduced without copyright permission]
H2 + 1/2 O2
'spark'
H2 O
Hydrogen does not exist stably in its atomic form but can exist as both H+ (known as a proton)
or H- (the hydride ion). There are two less common isotopes of hydrogen (1H) that have a higher mass
– they are heavier isotopes – and are called deuterium (2H or D) and tritium (3H or T).4 Deuterium is of
interest to chemists as it can be used to make ‘labelled’ compounds, which are used in NMR
spectroscopy.* Tritium is formed through nuclear reactions in the upper atmosphere. It is the most rare
isotope of hydrogen and is radioactive.
*
NMR spectroscopy is used widely in synthetic chemistry to obtain structural information on new compounds e.g. new
medicines and plastics. In most cases, the new compound is dissolved in a deuterated solvent because this stops the 1H
NMR spectrum becoming ‘swamped’ by signals from 1H present in the solvent. [Box 2.4, Housecroft and Sharpe, 2nd
edition, pages 66-67] In short, in NMR experiments, we want to see just the 1H signals from the new compound and not
from the solvent dissolving it.
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Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
Hydrogen can be made in a number of ways, equations 1-4.5 On a small scale, it can be formed
through the reaction of an active metal with an acid, equation 1. That is, reaction of hydrochloric,
sulfuric or nitric acid with a metal that have a negative standard potential e.g. Zn2+/Zn Eo –0.76 V,
Cr3+/Cr Eo –0.74 V, Fe3+/Fe Eo –0.44 V.6 On a larger scale and in industry, hydrogen is produced via a
process known as steam reforming, equation 2. Industry is also interested in using coal to produce H2
industrially, as coal is cheaper than methane and easier to transport, equation 3. Both
equations/processes 2 and 3 require high temperatures and catalysts to generate H2. This is probably a
reflection of the immense thermodynamic stability of water (the source of hydrogen in these reactions).
Zn(s) + H2SO4(aq) → Zn2+(aq) + SO42-(aq) + H2(g)
(equation 1)
CH4(g) + H2O(g) → CO(g) + 3 H2(g)
(equation 2)
C(s) + 2 H2O(g) → CO2(g) + 2 H2(g)
(equation 3)
H2O(l) → H2(g) + ½ O2 (g) Ecello –1.23 V
(equation 4)
Many researchers are interested in forming hydrogen through electrolysis, equation 4, as
renewable energy e.g. solar-power could be used and lead to a clean fuel for transport as water is
reformed as the only by-product, Figure 2.
Figure 2: Simple schematic representations of the hydrogen cycle and a hydrogen fuel cell.7,8 [Note: Image
reproduced without copyright permission]
Hydrogen can be used in many other areas, in addition to its use as a clean fuel, as detailed in the
bullet list on the next page:9,10
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Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
•
Fixation of nitrogen in the Haber process to produce ammonia.†
•
Production of hydrochloric acid.
•
Manufacture of margarine via hydrogenation of oils and fats. Note: this is a reduction process.
Double C=C bonds are converted to single C-C bonds.
•
Production of metals through reduction of metallic ores.
o
•
As can be seen from the above two points (and in methanol synthesis below) –
Hydrogen is a very important reducing agent.
It is an important feedstock in methanol production. CO(g) + 2 H2(g) → CH3OH(g), which
requires a catalyst and is performed at high temperature.10
o
Methanol in turn is a feedstock for other processes.
•
As a rocket fuel!
•
In welding - ‘atomic hydrogen welding’ can be used to melt and weld tungsten metal.11
Tungsten is the metal with the highest melting point known and cannot be melted using
conventional means.
•
In cryogenics, as liquid hydrogen, because it has a melting point only just above absolute zero.
Note: Helium is used more commonly because of the flammability of hydrogen.
H
In coordination compounds, hydrogen is known as dihydrogen and can act as a
Lewis base by donating some of the electron density from its H-H σ bond to a
M
Lewis acidic metal centre, Figure 3.12
H
Figure 3: Schematic diagram of a metal bonding with dihydrogen
Compounds of hydrogen and their reactivity
For this part of the essay, I will focus on binary compounds of hydrogen i.e. they will only contain
hydrogen and one other element. Compounds of hydrogen fit into three categories:13
1. Molecular hydrides e.g. methane CH4, ammonia NH3 and water H2O
2. Saline hydrides e.g. LiH and CaH2
3. Metallic hydrides e.g. TiHn and NiHn
Molecular hydrides normally contain a p-block element with similar or higher electronegativity
compared with hydrogen. Binary molecular hydrides are often gases. Water is an exception because of
†
Ammonia is of crucial importance to world food production and this process uses the most hydrogen on a yearly basis!
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Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
hydrogen-bonding. The number of possible binary hydrides is expanded significantly if we consider
negatively and positively charged species, Table 1. Saline hydrides can be regarded as salts containing
H- ions and are crystalline in nature. They are formed by electropositive metals i.e. group 1 and 2
metals.
Table 1:An array of some isoelectronic hydrides of the 2s22pn elements14
Number of Hs
4
3
2
1
Group 13
BH4BH32-
Group 14
CH4
CH3-
Group 15
NH4+
NH3
NH2-
Group 16
H3O+
H2O
OH-
Group 17
H2F+
HF
Both molecular and saline hydrides are quite reactive. Group 1 and 2 hydrides react vigorously
with water to produce hydrogen gas and a metal hydroxide. This means that they can be used as drying
agents for solvents – the most commonly used in this regard is CaH2. p-Block molecular hydrides have
different reactivities depending on their group and also the first row 2p compounds are significantly
different to those formed with elements further down the periodic table. For example, the halogens
form acidic hydrides e.g. HCl, but it should be noted that HF is anomalous in that it is a weak acid.
Hydrides of the pnictogens are all Lewis bases and useful ligands in coordination chemistry. However,
in contrast to NH3, PH3 and AsH3 are very toxic and rarely used. Also, PH3 and AsH3 are readily
oxidized to give pentavalent central atoms e.g. O=PH3, whereas NH3 is stable in air. Ammonia
dissolves in water to give ammonium hydroxide solutions, whereas PH3 is not very soluble in water.
This is likely a result of the hydrogen-bonding that is possible for 2p molecular hydrides of
electronegative elements i.e. NH3, H2O and HF. Hydrides of group 13 – B, Al, Ga – are electron
deficient and therefore, exist as dimers e.g. B2H6. They are also Lewis acids and form adducts with
Lewis bases.
As mentioned above hydrogen-bonding plays an important role in the physical properties of
many molecular hydrides e.g. boiling points, pKa. It is also important in many biological settings e.g.
base-pairing in DNA. Typically, hydrogen bonds have enthalpies of between 20 and 60 kJ mol-1.15 The
presence of hydrogen bonds within certain compounds can be detected using molecular structure
determination (X-ray crystallography and neutron diffraction studies), infrared spectroscopy (where
bands become broadened if hydrogen-bonding is present) and 1H-NMR (where unusual proton
chemical shifts will be seen).15 An important manifestation of hydrogen-bonding, with particular
relevance to Newfoundland, is the structure of ice. The hydrogen-bonding leads to ice having a lower
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Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
density than liquid water and therefore, ice can float on water.
Without hydrogen-bonding, we would not see icebergs off the coast of
this province in May and June.
Figure 4: Icebergs trapped in Quidi Vidi harbour, image taken without
permission from http://www.canadian-travel.ca/
Current research surrounding the chemistry of hydrogen
As mentioned earlier in this essay, there is immense interest in hydrogen fuel cells as a source of clean
energy. In November 2010, Mitch Jacoby wrote a short report in a chemical newspaper on a nickel
complex that could boost hydrogen oxidation.16 A team of US researchers at the Pacific Northwest
National Laboratory and Villanova University made a Ni2+ coordination compound where the metal
centre was bonded to two bidentate chelating phosphine ligands. They found that it was an excellent
electrocatalyst for oxidizing hydrogen.17 This is important because expensive platinum metal is
currently used as the electrocatalyst in hydrogen fuel cells, and nickel is much cheaper. Electrocatalytic
hydrogen oxidation is when hydrogen is separated into protons and electrons and forms water upon
reaction with oxygen. The experiments that the researchers performed included X-ray crystallography,
cyclic voltammetry and computational studies.
They concluded their paper by saying that their
electrocatalyst “is 5 times faster than the best molecular H2 oxidation catalyst previously reported.”
We can only hope that advances will continue to be made in this field and soon we will have cheaper
hydrogen fuel cells and clean energy for all.
References
1) Shriver & Atkins’ Inorganic Chemistry, P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong
and M. Hagerman, W. H. Freeman and Company, New York, 5th Edition, 2010, Chapter 10, Hydrogen,
pages 274-279.
2) Inorganic Chemistry, C. E. Housecroft and A. G. Sharpe, Pearson Education Ltd., Harlow, 2nd
Edition, 2005, Chapter 9, Hydrogen, Box 9.1, page 239.
3) Picture taken from http://depletedcranium.com/what-killed-the-hindenburg-hydrogen-or-fabric/
accessed January 11th 2011.
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Chem 2210
Introductory Inorganic
A. N. Other MUN# 200XXXXXX
4) Basic Inorganic Chemistry, F. A. Cotton, G. Wilkinson and P. L. Gaus, John Wiley & Sons Inc.,
New York, 3rd Edition, 1995, Chapter 9, Hydrogen, page 273.
5) Shriver & Atkins’ Inorganic Chemistry, P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong
and M. Hagerman, W. H. Freeman and Company, New York, 5th Edition, 2010, Chapter 10, Hydrogen,
Section 10.4, Production of dihydrogen, pages 280-281.
6) Shriver & Atkins’ Inorganic Chemistry, P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong
and M. Hagerman, W. H. Freeman and Company, New York, 5th Edition, 2010, Standard potentials
taken from Resource Section 4, pages 787-799.
7) Picture taken from http://kuzwe.com/hydrogen_clean.asp accessed January 11th 2011.
8) Picture taken from http://images.yourdictionary.com/images/computer/_FUELCEL.GIF accessed
January 11th 2011. Note: Image prepared by Ballard power systems, a leading company in fuel cell
technologies based in Vancouver, BC.
9) http://www.webelements.com/hydrogen/history.html and
http://www.webelements.com/hydrogen/uses.html accessed January 10th 2011.
10) Inorganic Chemistry, C. E. Housecroft and A. G. Sharpe, Pearson Education Ltd., Harlow, 2nd
Edition, 2005, Chapter 9, Hydrogen, pages 238-239.
11) http://en.wikipedia.org/wiki/Atomic_hydrogen_welding accessed January 10th 2011.
12) Basic Inorganic Chemistry, F. A. Cotton, G. Wilkinson and P. L. Gaus, John Wiley & Sons Inc.,
New York, 3rd Edition, 1995, Chapter 9, Hydrogen, page 282.
13) Shriver & Atkins’ Inorganic Chemistry, P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong
and M. Hagerman, W. H. Freeman and Company, New York, 5th Edition, 2010, pages 276-277.
14) Personal Communication, Geoff Rayner-Canham, Memorial University of Newfoundland,
December 2009.
15) Shriver & Atkins’ Inorganic Chemistry, P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong
and M. Hagerman, W. H. Freeman and Company, New York, 5th Edition, 2010, pages 284-286.
16) M. Jacoby, Chemical & Engineering News, 2010, 88, Iss. 47 (November 22), p. 28.
17) Jenny Y. Yang, S. Chen, W. G. Dougherty, W. S. Kassel, R. M. Bullock, D. L. DuBois, S. Raugei,
R. Rousseau, M. Dupuis and M. Rakowski DuBois, Chemical Communications, 2010, 46, 8618-8620.
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