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The Group 1A Elements Sections 9.2 and 9.3

Jaron Mason. The Group 1A Elements Sections 9.2 and 9.3. 19.2 The Group 1A Elements. 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.

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The Group 1A Elements Sections 9.2 and 9.3

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  1. Jaron Mason The Group 1A ElementsSections 9.2 and 9.3

  2. 19.2 The Group 1A Elements • 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.

  3. Sources and properties

  4. 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.

  5. 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)

  6. 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-

  7. 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.

  8. 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. H H C C H H +H2 H H C C

  9. 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 hybrides • Covalent hybrides • Metallic hybrides

  10. 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)

  11. 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.

  12. 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.

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