Hydrogen The “Groupless” Element

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

D2O melts at 3.8 °C,boils at 101.4 °C and is ~10% denser than H2O – hence

the name “heavy water”.

The main use of D2O is to slow down neutrons in nuclear reactors.

Approximately 1000 tons of D2O 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• 3T 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 3T, however,• is that it is the fuel for the “hydrogen• Bomb” and Fusion Reactors.

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

• Lower boiling point than the common 4He, 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

O2-

AirFuel

Exhauste-

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)

H2 – PEMFC for mobile phone with H2 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 H2- PEMFC generator

DMFC generator with methanol container

Demonstrated

• >3000 miles in 12 days - Range

• 100 mph - Speed/power

• -1 -> 35oC – Operating temperature range

• World Record Fuel efficiency 25 km for 1 g H2.

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 fuelo 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 Carbonate0.3 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 1s1 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, CH4, (or some organic material) reacts with steam at 900-1000

• °C to give CO and H2. Then CO reacts with more steam at 400-500 °C in

• the presence of catalyst, giving more H2 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 CaF2 (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.