2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 19 – Hydrogen and Hydrides.
1.
Properties of hydrogen and its position in the periodic
table.
2.
Hydrides
3.
Hydrogen bonding and the structure of ice
4.
Interactions of water with solutes: hydration of cations
and anions
1. Properties of hydrogen, lithium and fluorine
„
Hydrogen has one valence electron like lithium, but is also one electron short of completing
an inert-gas configuration like fluorine.
„
This implies hydrogen might have some properties in common with all three of these
elements.
„
Hydrogen has a “split personality”
„
This is reflected in the various positions to which hydrogen is assigned in the periodic table.
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The position of hydrogen in the periodic table
„
Most commonly it is placed over the alkali metals (Group 1A), but occasionally it will
also be placed over the halogens (Group 7A).
„
„
More rarely it will be found above carbon.
Hydrogen is not particularly reactive, although reaction with oxygen is certainly
impressive, a flame, spark or catalyst is necessary to set it off.
The Hindenburg Disaster
„
Often repeated remarks concerning the hydrogen economy go something like
the following: Hydrogen would answer the pollution question... When hydrogen
is produced in sufficient amounts to achieve the economies of scale it will be
the cheapest renewable fuel ... but “remember the Hindenburg" ... It is often
suggested that the Hindenburg disaster ended the chance for practical
applications of hydrogen.
„
The Hindenburg was a rigid "airship" with a stretched outer shell of
streamlined silver-colored fabric. It was lighter than air because it contained
giant bags of hydrogen. Some 236 tons of air was displaced by the Hindenburg.
This displaced air created a lifting force and buoyed the Hindenburg upward
with a force of 236 tons.
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The Hindenburg
A size comparison of the Hindenburg
with a 747 and the Titanic. The
Titanic is only 78 feet longer than the
Hindenburg at 882 feet long.
The Hindenburg had crossed the Atlantic 21 times and used a Goodyear-formula for a
gelatin-latex membrane to contain the hydrogen in the gas cells. Much attention was
paid to the silver airship image that displayed giant swastikas on the tail section. The
silver appearance of the Hindenburg was due to a surface varnish of powdered
aluminum in a paint formula that resembles the chemistry of modern solid booster
rocket fuel.
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The Hindenburg Disaster - Titanic in the Sky
Burning of an airship that carried a crew of 59. It had capacity for 50 passengers in
individual cabins or for 70 passengers on day flights. On the evening it burned, the
Hindenburg carried 97 persons.
35 people died in the flames - and nobody knew why. Sabotage? A bolt of lightning?
The mystery surrounding the disaster has never been resolved - until now. In many
years of research, a NASA scientist at Cape Canaveral has found proof that neither
the hydrogen in the hull nor a bomb was to blame, but the fabric of the Hindenburg's
outer skin and a new protective coating. A single spark of static electricity was enough
to make it burn like dry leaves. The "infallible" German engineers had designed a flying
bomb just waiting to explode.
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2.
Hydrides
„
Part of the reason for Hydrogen’s chemical lethargy is the high H-H bond
energy 436.4 KJmol-1.
„
This is the highest homonuclear single-bond energy known.
„
Once it does react with another element to form a binary hydride - a compound
in which hydrogen is bound to one other different element - its resulting
oxidation state will usually be +1 or -1.
„
The oxidation state of hydrogen in some transition metal hydrides is not well
characterized.
„
There are three classes of Hydrides:
1.
Ionic Hydrides - contain an H+ ion,formed between H and electropositive
elements.
2.
Covalent Hydrides – molecular formed between H and nonmetals.
3.
Metallic Hydrides – formed between transition metals and H
1.
Covalent Hydrides – We learned before that a large charge density leads to a
high polarizing power and a tendency to form covalent bonds. (Because of the
very small radius of this species, its charge density is extremely large, larger
than any other ionic species).
„
Hydrides can be subdivided into two types: those that form discrete, selfcontained neutral of positively charged molecular units: for example, HCl, H2O,
H3O+, NH3, NH4+ and those that assume an extended, polymeric structure,
such as AlH3.
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2.
Ionic Hydrides – characterized by hydrogen in the -1 state, occur only with
the least electronegative metals, those of Groups 1A and 2A.
There is good evidence to show that these compounds really are significantly
„
ionic.
‰
Molten ionic hydrides like salts, conduct electricity well, implying the
existence of charged species,
‰
The melt releases hydrogen at the positive anode upon electrolysis,
consistent with a H- species.
„
Ionic hydrides are usually gray solids formed by direct combination of the
metal and hydrogen at elevated temperatures, they are used as drying and
reducing agents, as strong bases, and some safe sources of pure hydrogen.
„
Calcium hydride CaH2 is particularly useful as a drying agent for organic
solvents, reacting smoothly with water:
CaH2(s) + 2H2O (l)
„
Ca2+(aq) + 2H2(g) + 2OH-(aq)
Sodium hydride, NaH, reacts violently with water to produce H2 gas, H2(g) and
hydroxide in solution and like other ionic hydrides, is a strong base. LiH and
CaH2 are convenient portable sources of pure hydrogen. LiH also reacts with
aluminum chloride to form the complex hydride LiAlH4, which is extremely
useful as a reducing agent in organic chemistry.
3.
Metallic Hydrides – Hydrogen reacts with a variety of transition metals
including lanthanides, and actinides, to produce a third class of hydride that is
rather poorly understood.
„
These brittle solids are generally metallic in appearance, good conductors of
electricity, and of variable composition.
„
Their hydrogen-metal ratios are often not ratios of small whole numbers and
so they are referred to as nonstoichiometric compounds. e.g. TiH1.7, TiH2,
PdH0.65, LaH1.86, UH3
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„
These metallic hydrides were formerly thought to be interstitial compounds
with atomic hydrogen fitting into the holes (interstices) left in the crystal
structure of the pure metal.
„
In many cases however, the arrangement of the metal atoms in the hydride has
been found to be different from that in the pure metal.
„
Better models are emerging to propose the correct structures of these
materials but work is ongoing.
„
Metal hydrides have several important applications – the hydrides are fairly
easily formed from a direct combination of hydrogen gas and the metal. The
hydrogen uptake is reversed at higher temperatures, yielding finely powdered
metals and hydrogen gas. These compounds are a good way to store and purify
hydrogen, as well as to produce finely divided metals.
„
The future for hydrogen – fuel cells
3. Hydrogen bonding and the structure of ice – see handout.
„
Density of water as a function of temperature:
„
Melting of Ice – liquid water at 0oC still has no remnants of the solid H2O (ice)
structure. Groups of 20-30 H2O molecules still have the ice-like structure called
“iceberg” structure.
„
Ice at 00C is all “at iceberg”
„
Liquid water at 00C has some water and some “icebergs”
„
Heated liquid water – increase in the number of “loose” waters and a decrease in
the size of “iceberg”.
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4.
„
„
Hydration of ions in solution
Consider what happens when CuSO4 dissolves in water:
Cu 2+(aq) + SO42- (aq)
CuSO4 (s)
Hydrated ions will perform specific interactions between water
molecules and the ions.
δ+
H
H O
H
+H
δ
H
O H
O
H
H
O
δ
H
O
δ+
δ+
H
H
-1/2
-1/2
O
S
H
H O
H
H
O
O
δ
Cu2+
δ+
H
-
-
δO
ion-dipole
O H interaction
H
Cu2+ surrounded octahedrally by the
oxygens of 6 H2O molecules. This is a
hydrated ion.
-1/2
δ+
O
H
δO
H
O -1/2
negative charge is spread out over
the oxygen atoms.
SO42- is surrounded by H2O molecules
that hydrogen bond with with sulfate oxygens.
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