lecture 9 chem

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Hydrogen

Physical Properties and Abundance

Hydrogen (H2(g)) is a colorless, odorless gas with a melting point of –239 ºC and a boiling point of –235 ºC. It is the most abundant element in the universe and is present in more compounds than any other element, including carbon. Hydrogen is typically classified as a nonmetal, but under extreme conditions (such as those found in the cores of gas giant planets), it can exhibit metallic properties.

The Sun, composed primarily of hydrogen undergoing fusion Jupiter, with a cutaway showing metallic hydrogen in its core

Preparation of Hydrogen

The most abundant source of hydrogen on Earth is water. While laboratory electrolysis can reduce water to hydrogen, this method is impractical and expensive for large-scale production. Industrially, hydrogen is produced mainly by two processes:

Water-gas shift reaction: Involves reacting carbon monoxide with water vapor to produce hydrogen and carbon dioxide.
Steam-hydrocarbon reforming: Hydrocarbons (such as methane) react with steam to yield hydrogen, carbon monoxide, and a small amount of carbon dioxide.

Industrial hydrogen production plant

Additional info: Electrolysis is more commonly used for high-purity hydrogen in laboratory settings, but not for bulk industrial production due to energy costs.

Uses of Hydrogen

Hydrogen has several important industrial and chemical uses:

Haber Process: Hydrogen is a key reactant in the Haber-Bosch process for synthesizing ammonia from nitrogen and hydrogen gases. Ammonia is essential for fertilizers and various chemicals.

Portrait of Fritz Haber, developer of the Haber process

Hydrogenation Reactions: Hydrogen is used to convert unsaturated fats (oils) into saturated fats (margarine and shortening) in the food industry.

Hydrogenation of vegetable oils to produce margarine and shortening

Alternative Fuel: Hydrogen is considered as a potential clean fuel. Its combustion with oxygen produces water and releases energy:

For comparison, the combustion of octane (a component of gasoline) is:

Advantages of hydrogen fuel:

Produces only water as a byproduct (no CO2 emissions)
High energy content per unit mass

Disadvantages:

Storage and transport are challenging due to low density and flammability
Most hydrogen is currently produced from fossil fuels, not water

Hydrogen fueling station for vehicles Hindenburg disaster, illustrating hydrogen's flammability risk

Chemical Reactivity of Hydrogen

Hydrogen's small atomic size and electron configuration (1s1) give it unique chemical properties:

As an alkali metal: Hydrogen can lose its electron to form H+ (proton), similar to alkali metals (Group 1).
As a halogen: Hydrogen can gain an electron to form H– (hydride ion), similar to halogens (Group 17).
With transition metals: Hydrogen forms nonstoichiometric metallic hydrides, where hydrogen atoms occupy interstitial sites in the metal lattice. These hydrides can have applications in hydrogen storage and advanced materials.

Yttrium trihydride (YH3) is a metallic hydride with unique optical properties Yttrium metal (Y) for comparison Yttrium dihydride (YH2) with intermediate properties

Alkali Metals

Atomic and Physical Properties

The alkali metals (Group 1: Li, Na, K, Rb, Cs, Fr) are characterized by:

Low density and soft texture (can be cut with a knife)
Low melting points compared to most metals
High electrical conductivity
Shiny, metallic luster when freshly cut

Freshly cut alkali metal showing metallic luster

Natural Sources and Extraction

Alkali metals are highly reactive and are not found in their elemental form in nature. They are typically found as salts in mineral deposits or dissolved in seawater. Common sources include:

Sodium chloride (NaCl) in seawater and rock salt deposits
Potassium salts in minerals such as sylvite (KCl)

Salt evaporation ponds, a source of sodium and potassium salts

Extraction is usually performed by electrolysis of molten salts (e.g., NaCl):

At the cathode:
At the anode:

Diagram of the Downs cell for sodium extraction by electrolysis

Uses of Alkali Metal Compounds

Alkali metal compounds have a wide range of uses:

Sodium chloride (table salt) for food and chemical industry
Sodium hydroxide (caustic soda) for soap and paper production
Lithium compounds in batteries
Potassium fertilizers in agriculture

Lithium-ion batteries, a major use of lithium compounds Milk bottles, historically sterilized with sodium compounds

Chemical Reactivity of Alkali Metals

Alkali metals are highly reactive, especially with nonmetals and water:

With nonmetals: Alkali metals react vigorously with halogens to form ionic salts (e.g., NaCl, KBr).
With water: Alkali metals react explosively with water to produce hydrogen gas and a metal hydroxide:

Alkali metal reacting with water, producing sparks and hydrogen gas Alkali metal burning in air, showing high reactivity

Additional info: The reactivity increases down the group, with cesium and francium being the most reactive.