18.5 Occurrence, Preparation, and Compounds of Hydrogen

Occurrence and Preparation of Hydrogen

Hydrogen is the most abundant element in the universe, yet on Earth it rarely exists as a free gas. Instead, it's locked up in compounds like water and hydrocarbons, so producing pure hydrogen requires energy-intensive processes. This section covers how hydrogen is prepared, what it's used for, and the compounds it forms.

Preparation and Uses of Hydrogen

Steam-Methane Reforming (SMR) is the most common industrial method for producing hydrogen. Here's how it works:

1. Methane ($CH_4$) reacts with high-temperature steam ($H_2O$) at 700–1100°C in the presence of a nickel catalyst.
2. This produces a mixture of hydrogen ($H_2$) and carbon monoxide ($CO$), called synthesis gas (or syngas).
3. In a second step called the water-gas shift reaction, the carbon monoxide reacts with more steam to produce additional hydrogen and carbon dioxide ($CO_2$).

Electrolysis of water is a cleaner but more expensive method:

1. A direct current (DC) power source drives electricity through water using two electrodes.
2. At the cathode, water is reduced to produce hydrogen gas.
3. At the anode, water is oxidized to produce oxygen gas.

Once produced, hydrogen has a wide range of uses:

• Ammonia production through the Haber-Bosch process: hydrogen reacts with nitrogen ($N_2$) at high temperatures and pressures using an iron catalyst. Ammonia is then used to make fertilizers, plastics, and explosives.
• Hydrogenation of unsaturated fats and oils: hydrogen is added across double bonds in vegetable oils to produce saturated fats like margarine.
• Methanol production: hydrogen reacts with carbon monoxide or carbon dioxide over a catalyst. Methanol serves as a fuel additive and industrial solvent.
• Fuel cells: these convert the chemical energy of hydrogen and oxygen directly into electrical energy, powering everything from portable devices to electric vehicles.

Isotopes of Hydrogen

Hydrogen has three isotopes, which differ only in their number of neutrons:

• Protium ($^1H$): no neutrons, makes up 99.98% of natural hydrogen
• Deuterium ($^2H$ or D): one neutron, used in nuclear magnetic resonance (NMR) studies and as "heavy water" ($D_2O$) in certain nuclear reactors
• Tritium ($^3H$ or T): two neutrons, radioactive, used in nuclear fusion research and some self-luminous devices

Because these isotopes have the same number of protons and electrons, their chemical behavior is nearly identical. The differences in mass, however, matter for nuclear reactions and isotope-tracing experiments.

Compounds and Reactions of Hydrogen

Chemical Properties of Hydrogen

Hydrogen's single electron makes it versatile. It can lose that electron (like a metal), share it (covalent bonding), or even gain one (forming the hydride ion). This flexibility means hydrogen reacts with both metals and nonmetals.

Reactions with nonmetals:

• With halogens (F, Cl, Br, I), hydrogen forms hydrogen halides (HF, HCl, HBr, HI). These are exothermic reactions that produce colorless, acidic gases soluble in water.
• With oxygen, hydrogen undergoes combustion to form water, releasing significant energy:

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

• With nitrogen, hydrogen reacts at high temperatures and pressures in the Haber-Bosch process to form ammonia:

$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$

The double arrow indicates this is a reversible, equilibrium reaction.

Reactions with metals:

• With active metals like sodium and potassium, hydrogen forms ionic hydrides containing the hydride ion ($H^-$). These hydrides are strong reducing agents.

$2Na(s) + H_2(g) \rightarrow 2NaH(s)$

• Certain transition metals (palladium, platinum) absorb hydrogen into their crystal lattice, forming metal hydrides. This property makes them useful for hydrogen storage applications.

Redox behavior: Hydrogen can act as a reducing agent (donating electrons, as when it reacts with oxygen) or as an oxidizing agent (accepting electrons, as when it reacts with sodium to form $H^-$). Which role it plays depends on the reaction partner.

Hydrogen Compounds of Nonmetals

Water ($H_2O$) has a bent molecular geometry with a bond angle of about 104.5°. Its polarity and ability to form hydrogen bonds give it unusually high boiling point, surface tension, and specific heat capacity. These hydrogen bonds also make water an excellent solvent for ionic and polar substances.

Ammonia ($NH_3$) has a trigonal pyramidal shape with a bond angle of about 107°. The lone pair on nitrogen makes it a polar molecule that can form hydrogen bonds, which is why its boiling point is higher than you'd expect for its molar mass. In water, ammonia acts as a weak base by accepting a proton from water.

Hydrogen halides (HF, HCl, HBr, HI) are linear, polar molecules. In water, they behave as acids. An important trend: acid strength increases from HF to HI. This happens because as you go down the halogen group, the H–X bond gets longer and weaker, making it easier to break and release $H^+$. (HF is actually a weak acid in water due to its exceptionally strong bond, while HCl, HBr, and HI are all strong acids.)

Hydrocarbons (methane, ethane, propane) contain only hydrogen and carbon. They're nonpolar, so they have low boiling points and don't dissolve well in water. Methane ($CH_4$) has a tetrahedral geometry. All simple hydrocarbons are combustible and widely used as fuels.

Bonding in Hydrogen Compounds

Covalent bonding is the primary bonding type in hydrogen compounds with nonmetals. Hydrogen shares its single electron with another atom to achieve a stable electron configuration.

You can represent the electron arrangement in these molecules using Lewis structures, which show shared electron pairs as lines (or paired dots) and lone pairs as dot pairs on individual atoms. For example, in water, oxygen shares one pair of electrons with each hydrogen and retains two lone pairs.

Catalysts play a key role in many hydrogen reactions. In steam-methane reforming, the nickel catalyst lowers the activation energy so the reaction proceeds at a practical rate. In the Haber-Bosch process, the iron catalyst does the same. Without these catalysts, these reactions would be far too slow to be industrially useful.