Introduction aux métaux alcalins

Introduction to Alkali Metals

• Alkali metals derive their name from the Arabic word "al-kali," which entered European languages via Latin, referring to the alkaline properties exhibited by these substances.
• Alkali metals all possess a valence electron configuration of ns¹.
• Specific examples include: Lithium (Li, 2s¹), Sodium (Na, 3s¹), Potassium (K, 4s¹), Rubidium (Rb, 5s¹), Cesium (Cs, 6s¹), and Francium (Fr, 7s¹).

Properties of Alkali Metals

• Physical and chemical properties are largely determined by the ease with which they lose their single valence electron.
• Their high reactivity means they don't exist in elemental form in nature, especially due to their interactions with oxygen and water.
• Sodium and potassium are found in dissolved or solid states, typically combined as halides, nitrates, or carbonates.
• Lithium, rubidium, and cesium are primarily found in silicate minerals.
• Francium is a radioactive element with a half-life of 22 minutes found in trace amounts in uranium and thorium ores, can also be produced through nuclear reactions from actinium.
• Possess a single electron in their outermost shell making them highly reactive.
• This electron is easily removed, leading to the formation of M⁺ cations, characterized by a low ionization energy.
• Alkali metals possess an oxidation number of +1 and are highly electropositive and metallic.
• Compounds formed by alkali metals are generally ionic, lacking covalent bonds.
• Alkali metals react violently.

Preparation of Alkali Metals

• Extraction from their compounds is difficult, generally achieved through the electrolysis of their molten salts.
• The Downs process is used to produce sodium through the electrolysis of molten sodium chloride (NaCl) with calcium chloride (CaCl₂) added to reduce the melting point.

Physical Properties of Alkali Metals

• Among alkali metals, lithium is the hardest.
• Melting points decrease going down the group.
• Possess high boiling points and are excellent conductors of heat, in addition to their stability against radiation.
• Certain Na-K alloys are liquid and used as refrigerants in nuclear reactors.
• Emit characteristic colors when exposed to a flame.
• The metallic bond weakens with increasing atomic size.
• Atomic and ionic radii are large relative to other elements in their respective periods.
• Density is low, with lithium, sodium, and potassium being less dense than water.

Chemical Properties of Alkali Metals

• Alkali metals tarnish quickly in humid air, with rubidium and cesium being even more reactive.
• Francium, being radioactive, has never been isolated in visible quantities.
• They primarily exist as univalent (Na⁺) cations in compounds due to their low ionization energy, resulting in ionic compounds.
• Exhibit reducing properties because of a low ionization energy.
• React with liquid ammonia to form solutions of electrons and metal cations, useful for reducing organic compounds.
• They react with non-metals (except noble gases).
• With nitrogen, they form nitrides (e.g., Li₃N).
• With oxygen, the product varies: lithium forms oxides (Li₂O), sodium forms peroxides (Na₂O₂), and potassium, rubidium, and cesium form superoxides (MO₂).
• KO₂ is an orange-yellow solid used in closed-circuit breathing apparatuses (gas masks, submarines, spacecraft). It releases oxygen and reacts with exhaled water and carbon dioxide.

Compounds of Lithium, Sodium, and Potassium

• Lithium has a relatively high ionic potential due to its small size, which means some of its properties are similar to those of Group 2 elements, like magnesium.
• Lithium compounds are generally less soluble.
• Lithium compounds are more soluble in aprotic solvents.
• Lithium ion (Li⁺) leads to covalent bonding and ion-dipole interactions, resulting in hydrated salts like LiOH.H₂O.
• Lithium is used in ceramics, lubricants, batteries, medicine, and soaps.
• Examples include: lithium aluminum hydride (LiAlH₄) in pharmaceutical production, lithium borohydride (LiBH₄) in organic synthesis, and lithium carbonate (Li₂CO₃) for treating schizophrenia.

Sodium Compounds

• Sodium compounds are soluble in water and relatively inexpensive.
• Sodium chloride (NaCl) is a white crystalline solid.
• It is obtained from salt mines or by evaporating brines.
• Used in seasoning, food preservation, and the production of sodium hydroxide, sodium carbonate uses the Solvay process, chlorine, and hydrogen chloride.
• Sodium hydroxide (NaOH), also known as caustic soda, is a white, corrosive, hygroscopic solid that is readily soluble in water; used to make other sodium salts.

Reactions of Sodium Compounds

• Electrolysis of sodium chloride produces chlorine gas and sodium hydroxide.
• Aqueous solutions of sodium compounds are strong bases.
• Used to neutralize acids to form salts.
• Sodium hydroxide precipitates most metal ions as hydroxides.
• Reacts with acidic oxides for corresponding salts and water.
• Reacts with halogens.
• Sodium chloride is used as road de-icer.
• Sodium hydroxide is used in soap manufacturing.
• Sodium sulfate (Na₂SO₄) is found a natural source or produced from sulfuric acid and sodium chloride. Has a total reaction because of a volatile by-product. Used in the lab to produce hydrochloric acid, detergents, and paper treatment.
• Sodium bicarbonate (NaHCO₃) is a baking soda or chemical leavening used in cooking and the preparation of salts and effervescent drinks.
• Anhydrous sodium carbonate (Na₂CO₃) is used.
• Sodium cyanide (NaCN) is used in extracting gold and silver from ores.

Potassium Compounds

• Potassium mineral sources include sylvinite (KCl) and carnallite (KCl.MgCl₂.6H₂O).
• Potassium compounds are generally more expensive than sodium compounds.
• Potassium nitrate (KNO3) with hygroscopic properites, oxidant used in matches, explosives, and meat preservation.
• Potassium hydrogen phosphate (K₂HPO₄) is used as a buffering agent.

Properties of Potassium Compounds

• Alkali metal compounds are generally high-melting-point ionic solids that are white unless the anion is colored: such as yellow (K₂CrO₄) and orange (K₂Cr₂O₇), oxidants that are used in matches, fireworks, and in textile dyeing.
• Potassium permanganate (KMnO₄) -- is used as a disinfectant and oxidant in the laboratory.

Metal Oxides and Hydroxides

• Alkali metals form simple oxides (M₂O), peroxides (M₂O₂), and superoxides (MO₂).
• M₂O reacts with water.

Hydroxides

• Metal hydroxides are ionic solids that form strong bases in aqueous solution.
• The basicity increases with solubility as the group is descended.

Chemical Study of Sodium Hydroxide

• Sodium hydroxide is prepared electrolytically from a NaCl solution producing C₂ and 2Na⁺ at the anode, then 2Na⁺ + 2e⁻ is reduced to 2Na at the cathode. Lastly, 2Na + H₂O forms 2NaHO NaOH+ H₂.

Other features of Sodium Hydroxide

• To prevent C₂ from forming hypochlorite compounds, Isolate NaOH to the cathode by using a semi-permeable membrane.
• Causticization of sodium involves reaction with calcium hydroxide at a high temperature.
• Sodium (Na₂CO₃) is called soda.

Properties of Hydroxides

• They must be stored away from moisture, carbon dioxide, and an atmospheric environment because: (1) They are deliquescent/dissolving. (2) Will solidify as it continues to absorb atmospheric CO₂

Application of Hydroxides

• Excipients such as catalysts are mainly used with NaOH and KOH; helpful for adjusting pH.
• Therapeutic agents such as diluted KOH can be used for dermatoses and skin treatments.

Halogenides

• These are mainly separated or purified by crystal extraction.
• Can be combined with alkali halides.
• Or react MOH or M₂CO₃ with hydrohalogenic acids.
• Soluble in water, (Lithium is less soluble).
• Function as tonic agent with sodium chloride, or with KCl for production of isotonic solutions collyres and injections.

Carbonates and Hydrogen Carbonate Applications

• They act as alkaline agents.
• Carbonate and hydrogen carbonates react with acids by desintegrating.
• Also act as sodium sources to promote effective desintegration.
• Hydrogen carbonates react with sources such as gastric acid (active agents and excipients).

Biological and Pharmaceutical Properties

• The physiological distribution activity or toxicity is dependent on properties. The essential elements (sodium and potassium) are mainly based on mass. For sodium it's 0.2%, and for potassium it's 0.4%.

Sodium and Potassium Properties

• Charge passes through membranes in transmembrane pathways (ion channels). To achieve diffusion through active transportation that does not pass the apolar lipidic bilayer
• Difference in ionic radii between potassium (K⁺) and Sodium (Na⁺) initiates the selectivity of the canals. So sodium and potassium specifically ad