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ChemMatters October 1985 Page 4
© Copyright 1985, American Chemical Society
Hydrogen and Helium
by Marilyn Linner
DATE: May 6, 1937
TIME: 7:00 P.M.
PLACE: Lakehurst, N.J.
The giant dirigible hovers near the mooring mast and drops its landing
lines to the crew on the ground. Suddenly, a sheet of fire bursts from its
tail. Within minutes, the 800-ft-long craft is reduced to a tangled mass of
charred wreckage; 35 people are dead, and dozens are injured.
The accident is famous: the spectacular explosion of the German
dirigible, Hindenburg. Less well known, however, is the fact that this
was one of 73 similar disasters that occurred during the era of the giant
airships’ popularity.
Today, during a televised football game, we are likely to see the
Good-year blimp floating serenely over the stadium, giving us a bird’seye view of the action. Some 300 of these blimps have been built and
operated safely.
What accounts for the difference between the Hindenburg and the
Goodyear blimp? Primarily, it is the gas. The fire that swept through the
Hindenburg was fed by 6.7 million cubic feet of its lifting gas, hydrogen.
Goodyear blimps, on the other hand, are inflated with another light gas,
helium. Its lifting power is slightly less than hydrogen’s (about 7%), but
helium doesn’t burn under any circumstances. The Hindenburg disaster
ended the use of hydrogen in airships. Helium now fills the blimps, as
well as millions of toy balloons.
Fraternal twins
These two gases—hydrogen and helium—do much more than lift
balloons. Both play important roles in today’s industrial society. Let’s
take a closer look at where they are found, how they are used, and why
they behave the way they do.
Hydrogen and helium have pronounced differences—and
similarities. Where they are alike, they are very alike; where they differ,
they are very different. Basically, they are similar in physical properties
but completely different in chemical behavior.
Both gases are colorless, odorless, tasteless, and nontoxic. They are
the most difficult to liquefy of all known gases. At normal pressure,
hydrogen becomes a liquid only when cooled to a temperature of -253
°C, and helium liquefies at about -269 °C, only four degrees above
absolute zero.
Hydrogen and helium are the smallest, highest, and simplest of all
elements. Atoms of ordinary hydrogen are made up of only one
electron and one proton and have an atomic mass of 1 atomic mass unit
(amu). Helium atoms have two electrons, two protons, and two
neutrons, and an atomic mass of 4 amu.
These simplest of atoms comprise the most abundant elements in the
universe. Approximately 90% of all atoms in the universe are hydrogen,
making up about 75% of the mass of all matter. Our sun and most other
stars are chiefly hydrogen, and astronomers have detected huge clouds
of hydrogen drifting between the stars. Helium accounts for another
23% of the mass of the universe. It is concentrated in the interior of
stars, where it is continuously being formed by the fusion of hydrogen
nuclei. This process is the source of most of the energy radiated by the
sun and other stars.
Fly away
The amounts of helium and hydrogen found on earth are much less
than one might expect from their cosmic abundance, but many
scientists believe they were plentiful when the earth was formed.
Because they are so light, earth’s gravity couldn’t hold them and they
escaped from the atmosphere into space. This process continues today;
this is why helium is relatively rare and expensive.
Hydrogen, in contrast, is the ninth most abundant element in the
earth’s crust. This is because, eons ago, hydrogen combined with other
elements to form compounds, notably water. All plant and animal
tissues contain hydrogen in organic compounds like carbohydrates,
proteins, and fats. Petroleum and natural gas are mixtures of
hydrocarbons, compounds which contain only hydrogen and carbon.
No hydrogen mines
Because earth’s gaseous hydrogen has long since escaped into space,
the only way to get pure hydrogen gas is by separating hydrogen
chemically from one of its compounds. Most commercial production
starts with natural gas, which is composed primarily of methane, a
hydrocarbon with the formula CH4. The methane is reacted with steam
in the presence of a catalyst, a substance that speeds up a chemical
reaction without entering into the reaction itself. Hydrogen atoms are
stripped from the methane molecules and combine to form molecules of
hydrogen gas, H2.
Small amounts of hydrogen can be prepared quickly by reacting a
metal, such as zinc, with an acid. Paracelsus, a 16th-century alchemist,
prepared hydrogen this way. In 1783, Jacques Charles, a French
scientist, generated gas for the first flight of a hydrogen balloon by
pouring sulfuric acid into a barrel filled with bits of iron, thus starting
experimenters on the long road that led to the explosion of the
Hindenburg.
On earth, elemental hydrogen is rare, but compounds of hydrogen
are common. Helium, however, is just plain rare. Like hydrogen, helium
is obtained from natural gas. Helium is slowly formed deep inside the
earth by the breakdown of radioactive elements such as uranium and
thorium. Nuclei of these heavy elements spontaneously break apart. In
the process, they eject alpha particles, which are composed of two
protons and two neutrons. When an alpha particle captures two
electrons, it becomes a helium atom. The helium gas leaks upward
through the rocks and is eventually trapped beneath the same domeshaped geological formations that collect natural gas. The helium that
fills the Goodyear blimp today may have been part of a Texas rock
formation millions of years ago.
Natural gas is a mixture (primarily methane, ethane, and nitrogen)
that includes helium in concentrations from a fraction of a percent to
nearly eight percent. Helium is separated by compressing and cooling
the mixture until all of the gases except one condense to liquid, leaving
gaseous helium behind.
Different family backgrounds
Although hydrogen and helium are similar in physical properties and
cosmic abundance, any likeness disappears when we look at their
chemical characteristics. Here they are as dissimilar as night and day.
Helium is safe in dirigibles and balloons because of its chemical
stability. It is completely unreactive: Helium atoms don’t combine with
each other or any other atoms. Helium belongs to a family of inert, or
unreactive, elements. Another well-known family member is neon, the
gas whose familiar red-orange glow lights up neon signs. Other
members are argon, krypton, xenon, and radon. When they were
discovered, these gases were thought to be as inactive as helium, but
scientists have subsequently shown that xenon and krypton will
occasionally combine with highly reactive elements such as oxygen and
fluorine.
Hydrogen is chemically very active and reacts with most other
elements. Under ordinary conditions, single hydrogen atoms are never
found by themselves. Hydrogen molecules are diatomic—two atoms
chemically combined—represented by the formula H2. Hydrogen is
quite versatile in the manner in which it combines with other elements.
In various reactions, hydrogen can gain or lose an electron. However, it
most often combines by sharing electrons. Common compounds, such
as methane (CH4), ammonia (NH3), and water (H2O), are formed this
way.
Return to space
The similarities and differences of hydrogen and helium are exploited
in the space program. The Apollo moon missions and the space shuttle
have both used liquid hydrogen as rocket fuel, but helium is also
essential.
A rocket propellant consists of a fuel and an oxidizer (the substance
that burns the fuel). The best propellants are fuel—oxidizer
combinations with high specific impulse, a kind of miles-per-gallon figure
for rockets. (Specific impulse is the pounds of thrust obtained by
burning a pound of fuel in one second.) Both the energy value and the
weight of a propellant affect its specific impulse. A gallon of gasoline,
for example, has about three times as much energy as a gallon of liquid
hydrogen, but weighs 10 times as much. In space flight, this weight
difference is critical. Hydrogen’s exceptional lightness, in combination
with its moderately good energy content, makes it an excellent rocket
fuel.
The space shuttle’s large external tank (Figure 1) supplies 140,000
gallons of liquid oxygen and 380,000 gallons of liquid hydrogen to the
shuttle’s three main engines. The flow of these flammable liquids to the
engines is controlled by valves that are operated by compressed gas—
nonflammable helium.
Once in orbit, the shuttle is flown to different orbital positions by a
pair of smaller engines, part of the orbital maneuvering system (OMS).
The OMS engines use monomethylhydrazine as fuel and nitrogen
tetroxide as oxidizer. This combination is very reactive, even
hypergolic—the chemicals ignite when brought in contact with each
other. How are these chemicals forced from their storage tanks to the
OMS engines? The tanks are pressurized with unreactive helium, much
like an aerosol can (of paint or hair spray) is pressurized by a propellant
gas.
Any helium left over at the end of the flight is expended in a final
safety step. When the shuttle reenters the atmosphere, air friction causes
it to heat up, thus creating one of the conditions required for a fire.
After landing, just as the shuttle rolls to a stop, helium is released into
the main engine compartment where it acts—preventively—as a fire
extinguisher.
Down to earth
Hydrogen and helium figure in practical, everyday processes, too.
Large amounts of hydrogen are used in hydrogenation, a process that
changes vegetable oils into margarine and solid shortenings like Crisco
and Spry (Figure 2). Soybean, cottonseed, and other vegetable oils are
called unsaturated fats because their molecules contain double bonds
that are chemically active. In linseed oil, this property is useful. The oil
reacts with air, forming a tough protective film over an oil painting. A
similar reaction, however, in oils used in foods like peanut butter
produces an unpleasant odor and taste referred to as rancidity. By
treating oils with hydrogen, their molecules pick up additional
hydrogen atoms, getting rid of the reactive bonds. The product becomes
a saturated fat, a solid that keeps indefinitely without refrigeration.
An unusual property of helium, its low solubility in water, makes it
useful in breathing mixtures supplied to deep sea divers. To balance the
water pressure on their bodies, divers must breathe compressed air.
This can cause two problems: nitrogen narcosis and “the blends”. At
pressures below 100 feet, nitrogen in the air dissolves in the blood and
is carried to the brain, where it interferes with the ability to think
clearly. Also, if a diver returns to the surface too rapidly, the dissolved
nitrogen can “undissolve,” forming bubbles that block the circulation.
When divers breathe a 20% oxygen-80% helium mixture instead of air,
these hazards are eliminated.
Space exploration, food processing, underwater breathing, welding,
energy research, blimps: The list of fields that depend on hydrogen or
helium goes on and on. These simplest of all elements are remarkably
similar, remarkably different, and indispensable.
SIDE BARS
Lifting power
Helium’s lifting ability is greater than you would expect from
comparing the densities of the two gases. Because hydrogen’s molecules
are composed of two atoms each, their molecular weight is 2 amu.
Helium is made up of single atoms with an atomic weight of 4 amu.
The density of helium, therefore, is twice that of hydrogen; it seems
logical that helium’s lifting power would be only half as great.
It turns out that helium’s lifting power is only slightly less than that
of hydrogen. Look at it this way. If a certain volume of gas in a balloon
displaces one pound of air, the air around the balloon exerts an upward
force of one pound. (This is Archimedes’ principle.) If the balloon is
filled with hydrogen, the volume of hydrogen displacing a pound of air
weighs 0.069 lb. The air pushing up on the balloon must lift the 0.069 lb
of hydrogen, leaving a lifting power of 0.931 lb to lift the balloon and
load (1.00 - 0.069 lb = 0.931 lb). Because the weight of helium is twice as
great, 0.138 lb of helium will displace 1 lb of air, and the air will lift
0.138 lb of helium plus 0.862 lb attached to it. Because the hydrogen
balloon lifts 0.931 lb and the helium balloon, 0.862 lb, the ratio of their
lifting abilities is 0.862/0.931 or nearly 93%.
Fuel of the future?
In many ways, an energy cycle using hydrogen, oxygen, and water is
ideal. According to one plan, electricity would be used to separate
water into hydrogen and oxygen, with the oxygen being released into
the air.
2H2O + electricity ⇒ 2H2 + O2
When the hydrogen fuel is later burned, it releases heat energy and
forms water again.
2H2 + O2 ⇒ 2H2O + heat
No compounds are added to or removed from the environment, and
there are not polluting by-products.
Roger Billings of Independence, Mo., is one person who believes in
hydrogen as a fuel. In the ninth grade, Billings began converting an
automobile engine to run on hydrogen. Three years later, at the age of
18, he made a crude version that worked on his father’s pickup truck,
winning top honors at a science fair and a scholarship to Brigham
Young University. Later, he formed his own company to perfect and
produce hydrogen-powered cars. Since that time, he has modified a
number of vehicles to run on hydrogen fuel, including a hydrogen bus
that ran a 13-mile route connecting Provo and Orem, Utah, a hydrogen-
powered postal jeep used in Independence, Mo., and a Cadillac Seville
that participated in President Carter’s inaugural parade.
What’s more, Billings and his family lived for two and a half years in
a hydrogen-powered home. Hydrogen, generated by electrolysis,
operated the stove, oven, water heater, outdoor barbecue, and a
supplemental furnace. (A heat pump provided the primary heating.)
Power for the electrolysis came from the local electric company. Billing’s
research has shown us that hydrogen fuel works and can be substituted
for hydrocarbons, such as natural gas, gasoline, and oil.
If hydrogen is such a perfect fuel, why isn’t it being used right now?
A lot of energy is required to separate hydrogen from its compounds,
making hydrogen very expensive. Much research is being done to
reduce the energy requirement.
CAPTIONS
The explosion of the German airship Hindenburg marked the end of the use of hydrogen in
dirigibles.
Figure 1. The main rockets of the space shuttle are fueled by 1.58 x 106 lb liquid hydrogen and
liquid oxygen, stored in separate chambers within the large external tank. The H2/O2
propellant combination is not the most powerful but has an excellent thrust-to-weight ratio.
Figure 2. Liquid vegetable oils are composed of unsaturated fatty acid molecules (top
diagram). When treated with hydrogen, the molecules become saturated (bottom), and the
product turns solid.
Breathing compressed air, divers cannot venture below 200 feet (62 m). When breathing a
helium-oxygen mixture, they can go to about 450 feet (140 m) because helium is relatively
insolube in the blood.
This bus, operated in Provo, Utah, in 1976, demonstrated that hydrogen can be a practical fuel.
BIOGRAPHY
Marilyn Linner is a freelance science writer from Clinton, Iowa.
REFERENCES
Davenport, Derek. “How the Right Professor Charles Went Up in the Wrong Kind of
Balloon,” Chem Matters 1983, December, 14-15.
Finkbeiner, Ann. “Star Born,” Chem Matters 1984, October, 6-9.
Hill, Ray. “Before the Oil Runs Out: The Search for Cheaper Hydrogen,” Popular Science 1981,
September.
Kaufman, Michael. “On Wings of New Technology—Dirigible Tries a Comeback,” The New
York Times 1983, May 24.
Robinson, Paul. “A Gas for When Oil Is Gone,” Sciquest 1981, February.