Covalent hydrogen compounds - Classes at U. of L.

bloatdecorumSoftware and s/w Development

Oct 30, 2013 (3 years and 9 months ago)

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Hydrogen The “
Groupless
” Element

Hydrogen has three isotopes:
1
H, 99.985%,
2
D, 0.015% and
3
T, ~10
-
15
%.


D
2
O melts at 3.8
°
C,boils

at 101.4
°
C and is ~10% denser than H
2
O


hence

the name “heavy water”.


The main use of D
2
O is to slow down neutrons in nuclear reactors.

Approximately 1000 tons of D
2
O is also being used as part of a neutrino

detector at the Sudbury Neutrino Observatory (SNO) in Ontario.



It slows down chemical reaction involving H
+

Useful in determining reaction mechanism in

Organic and biochemistry.


It can also be used to determine orientational

Dynamics of water.


1

Tritium

3
T is used as a radioactive tracer in medicine and


biochemistry as it emits low energy
β

radiation


which does relatively little tissue damage.


The commercial value of
3
T, however,


is that it is the fuel for the “hydrogen

Bomb” and Fusion Reactors.









3
T decays to
3
He, a rare but very useful isotope. It has a

Lower boiling point than the common
4
He, and used in

extreme low
-
temperature
apparati

for cryogenic physics.



PEM FUEL CELL


Fuel Cells

Polymer Electrolyte Membrane Fuel Cell


Alcohols can be used as fuels in
these system. The catalyst and
membrane materials are currently
being developed

Solid Oxide Fuel Cell


O
2
-

Air

Fuel

Exhaust

e
-

e
-

Fuel Cells could replace


Internal combustion engines (ICE)


Gas Motor Combined Heat and Power (CHP)


Mobile generators


Batteries


Power Stations


On
-
board Electricity Generation


(Auxiliary Power Units, APU)

H
2



PEMFC for mobile
phone with H
2

recharging
unit

Direct Methanol Fuel Cell PDA
with methanol fuel capsule

Prototype DMFC
IBM laptop and
DMFC unit

Portable electronic devices

Generator Replacements

1 kW portable H
2
-

PEMFC generator

DMFC generator with
methanol container

Demonstrated

>3000 miles in 12 days
-

Range

100 mph
-

Speed/power

-
1
-
> 35
o
C


Operating temperature range


World Record Fuel efficiency 25 km for 1 g H
2
.

Fuel Cells for Motive Power in Vehicles

Key Targets


Cost $120/kW


Lifetime, reliability

o
A conventional car has fuel efficiency
ranging between 7 to 20 km/l (18 to 50
MPG), or 14 to 5 L/100km

o
This translates to 6 to 18 m for 1 g of
fuel

o
This is a difference of 1400 to 4000 fold.

Domestic CHP Units

System generates domestic electricity needs and provides heating


overall efficiency > 80%

CERES POWER SOFC

Power Generation on the Mega Watt Scale

http://www.rolls
-
royce.com/energy/tech/fuelcells.jsp

Rolls Royce 1
MW SOFC
system

Fuel Cell Energy
Molten Carbonate

0.3 MW system

Hydrogen Stands Alone!


Actually, hydrogen belongs to group 1, but it’s not an alkali metal.


It behaves like an alkali metal and like a halogen

Electronegativity

of 2.1 which is between Boron at 2, and Carbon at 2.5.


i.e.

more electronegative than the metals and less electronegative than the

nonmetals.


As its electron configuration 1s
1

it can achieve

a “noble gas” configuration by
either

gaining

losing or sharing an electron:



As the H


H bond is extremely strong, 436 kJ/mol,

it is a relatively
unreactive

molecule.


Even thermodynamically
favoured

reactions of

hydrogen often require a catalyst to break the

strong H


H bond. Hydrogen does, however,

react with exothermically with oxygen and with fluorine



Catalytic Steam Reformation of
hydrocarbons.

Methane, CH
4
, (or some organic material) reacts with steam at 900
-
1000

°
C
to give CO and H
2
. Then CO reacts with more steam at 400
-
500
°
C in

the presence of catalyst, giving more
H
2

gas











The purest hydrogen is obtained from the electrolysis of water, but is

prohibitively expensive for large
-
scale production.

The new fuel??? Alternative to oil?


Classes of Hydrogen Compounds

There are three general classes of hydrogen compounds:


Ionic hydrides

in which hydrogen combines with elements from groups 1
-
2

(except beryllium) to form ionic compounds:



Metallic hydrides

(also called
interstitial compounds
) in which elements

from groups 3
-
10 “absorb” hydrogen. The hydrogen atoms fill holes in

the metallic lattice, distorting its structure if enough hydrogen is absorbed.


Covalent hydrogen compounds

in which hydrogen

combines with elements from groups 11
-
17 (or beryllium)

to form covalent molecules:


Ionic Hydrides

Most ionic hydrides have a crystal structure like
NaCl

(mono

hydrides) or CaF
2

(for
dihydrides
).


In this case the
cations

form the main

lattice as they are typically larger

than the hydride anions:



Ionic hydrides are strong bases,

reacting with acids (even those as weak

as water):


Ionic hydrides are typically sold as grey powders

suspended in mineral oil. The oil protects them

from reacting with moisture in the air though it

must be washed off
(with solvent)

if an accurate

amount is to be weighed. If an ionic hydride is


not stored properly, it turns white. What has

happened?

Metallic Hydrides

The hydrogen in metallic hydrides can act

as either “H
+
” or “H
-
”:






Transition metals are often used as catalysts for reactions in which

hydrogen is added to a double bond (e.g. hydrogenating vegetable oil to

make margarine). The hydrogen first reacts with the transition metal to

make a metallic hydride (more reactive than hydrogen gas).


The ratio of hydrogen : metal atoms in a metallic hydride is often fractional


not every hole in the lattice contains a hydrogen atom.

Covalent Hydrogen Compounds

Most of the “everyday” compounds containing hydrogen are covalent hydrogen

compounds.


When hydrogen is covalently bonded to a less electronegative element (like

aluminum), it has a partial negative charge and may behave like a hydride.


When hydrogen is covalently bonded to an element with similar electro
-

negativity (like carbon), it is relatively neutral and tends not to be reactive:



When hydrogen is covalently bonded to a more electronegative element (like

oxygen), it has a partial positive charge and
may

behave as an acid.