THE GROUP 1A ELEMENTS

SECTIONS 9.2 AND 9.3

Jaron Mason

All group 1A elements have 1 valence electron.

All group 1A elements, except hydrogen, are extremely active metals (H acts as a non-metal).

The 1A metals are referred to as Alkali metals.

19.2 The Group 1A Elements

Sources and propertiesTABLE 19.3 Sources and Methods of Preparation of the Pure Alkali Metals

Element Source Method of Preparation

Lithium Silicate minerals such as spodumene, LiAl(Si2O6)

Electrolysis of molten LiCl

Sodium NaCl Electrolysis of molten NaCl

Potassium KCl Electrolysis of molten KCl

Rubidium Impurity in lepidolite, Li2(F,OH)2Al2(SiO3)3

Reduction of RbOH with Mg and H2

Cesium Pollucite (Cs4Al4Si9O26∙H2O) and impurity in lepidolite

Reduction of CsOH with Mg and H2

TABLE 19.4 Selected Physical Properties of the Alkali Metals

Element Ionization Energy (kJ/mol)

Standard reduction potential (V)

Radius of M+ (pm)

Melting point (C)

Lithium 520 -3.05 60 180

Sodium 495 2.71 95 98

Potassium 419 2.92 133 63

Rubidium 409 2.99 148 39

Cesium 382 3.02 169 29

Reaction with water Alkali metals react vigorously with water:

2M(s)+2H2O(l)→2M+(aq)+2OH-(aq)+H2(g)

Based on ionization energies, lithium would be expected to be the weakest reducing agent in water, but its standard reduction potential suggests it is the strongest. Lithium has a high energy of hydration, so the high charge density attracts more water molecules.

Lithium reacts more slowly with water than other Alkali metals, because the high melting point prevents the reaction from melting the lithium, increasing surface area.

Oxides, peroxides, and superoxides Lithium is the only Alkali metal that forms a normal

oxide with excess oxygen:4Li(s) + 1O2(g) → 2Li2O(s)

Sodium will only form Na2O when there is limited oxygen. In excess oxygen, it forms sodium peroxide:4Na(s) + 2O2(g) → 2Na2O2(s)

Potassium, rubidium, and cesium react with oxygen to form superoxides, which contain O2

-:K(s) + O2(g) → KO2(s)

Superoxides react with water or carbon dioxide to release oxygen gas:2KO2(s) + 2H2O → 2K+(aq) + 2OH-(aq) + O2(g) + H2O2

4KO2(s) + 2CO2 → 2K2CO3(s) + 3O2(g)

Predicting Reaction Products Predict the products formed by the

following reactants:A. Li3N(s) and H2O(l)

B. KO2(s) and H2O(l)

Solution:A. Li3N(s) + H2O(l) → NH3(g) + 3Li+ + 3OH-

B. KO2(s) + H2O(l) → 2K+(aq) + 2OH-

19.3 Hydrogen

Under normal conditions, hydrogen is colorless and odorless.

It is non-polar and has a low molar mass, so the boiling point (-253C) and melting point (-260C) are extremely low.

Hydrogen is extremely flammable and mixtures of hydrogen in air with 18-60% H are considered explosive.

Sources and Uses A major source of hydrogen is the reaction of methane

with water at high temperatures and pressures with a catalyst:CH4(g) + H2O(g) → CO(g) + 3H2(g)

Hydrogen is also formed in large quantities in the production of gasoline when large hydrocarbons are broken down (cracked) into smaller molecules.

A major industrial use for hydrogen is the production of ammonia through the Haber process.

Hydrogen is also used to create shortening by hydrogenating vegetable oils.

C C

H H C C

H H

H H+H2

Hybrides

Hydrogen behaves as a nonmetal, forming covalent compounds with other non-metals and salts with very active metals.

There are three types of binary compounds containing hydrogen known as hybrides:Ionic hybridesCovalent hybridesMetallic hybrides

Ionic hybrides Ionic (salt-like) hybrides are formed when

hydrogen combines with metals from groups 1A and 2A.

LiH and CaH2 are examples of ionic hybrides and contain hydride (H-) ions.

Hydride ions are a string reducing agent because of the weak 1+ charge and the strong electron-electron repulsion.

There is a violent reaction between hybrides and water, resulting in the formation of hydrogen gas:LiH(s) + H2O(l) → H2(g) + Li+(aq) + OH-(aq)

Covalent hybrides Covalent hybrides form when hydrogen reacts

with on-metals. Examples are HCl, CH4, NH3, and H2O. Water is considered the most important

covalent hybride. It has a high heat of vaporization for its molar mass and a large heat capacity, making it a useful coolant. Water is an excellent solvent for ionic and polar materials because of hydrogen bonding, so it provides an effective medium for biological processes.

Metallic (interstitial) hybrides Metallic hybrides are formed when crystals of transition

metals absorb hydrogen gas. The small hydrogen molecules dissociate at the metal’s

surface and migrate into the crystal structure. The metal-hydrogen mixtures are better considered solid

solutions than actual compounds. Hydrogen can be separated from other gasses by allowing

it to diffuse through a metal barrier into a separate area. Hydrogen can react with transition metals, but metallic

hybrides tend to have variable compositions. These nonstoichiometric hybrides have formulas such as

LaH2.76 and VH0.56 dependent on how much hydrogen is absorbed.

Absorbed hydrogen can be released by heating the metal hybride.