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The Group 1A and Group 2A Elements

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The Group 1A and Group 2A Elements Cotton chapter 10,11 Group 1A Group 1A Qualitative alkali metal analysis Alkali Metals y The group 1A elements with their ns1 valence electron configurations are very active metals. They lose their valence electrons very readily. They have low ionization energies and react with nonmetals to form ionic solids. 2Na(s) +Cl2(g) Æ 2NaCl(s) y The expected trend in reducing ability, Cs>Rb>K>Na>Li y Alkali metals all react vigorously with water to release hydrogen gas. + ‐ 2M(s) +2H2O(l) Æ 2M (aq) +2OH(aq) +H2(g) y Observed reducing abilities: Li>K>Na First ionization energy Soda production Properties and Trends in Group 1A y The Group 1A metals exhibit regular trends for a number of properties. y Irregular trends suggest that factors are working against each other in determining a property (such as the density “discrepancy” between sodium and potassium). y The alkali metals have two notable physical properties: they are all soft and have low melting points. y When freshly cut, the alkali metals are bright and shiny—typical metallic properties. The metals quickly tarnish, however, as they react with oxygen in the air. Alkali Metal Oxides In the presence of ample oxygen, 4Li + O2 → 2Li2O(regularoxide) 2Na + O2 → Na2O2 (peroxide) K+O2 → KO2 (superoxide) Rb + O2 → RbO2 (superoxide) Cs + O2 → CsO2 (superoxide) The oxides of Group 1A Direct reaction of the alkali metals with O2 gives : Li ‐> oxide, peroxide (trace) Na ‐> peroxide , oxide (trace) K,Rb,Cs ‐> superoxide Diagonal Relationships: The Special Case of Lithium In some of its properties, lithium and its compounds resemble magnesium and its compounds. y Lithium carbonate, fluoride, hydroxide, and phosphate are much less water soluble than those of other alkali metals. y Lithium is the only alkali metal that forms a nitride (Li3N). y When it burns in air, lithium forms a normal oxide (Li2O) rather than a peroxide or a superoxide. y Lithium carbonate and lithium hydroxide decompose to form the oxide on heating, while the carbonates and hydroxides of other Group 1A metals are thermally stable. Diagonal Relationships The elements in each encircled pair have several similar properties. Occurrence, Preparation, Use, and Reactions of the Alkali Metals y Sodium and potassium are isolated primarily from brines (solutions of NaCl and KCl). y Lithium is obtained mostly from the mineral spodumene, LiAl(SiO3)2. y To convert an alkali metal ion into an alkali metal atom, the ion must take on an electron—a process of reduction. y This is not easy with the alkali metals; they are excellent reducing agents. y Potassium was the first alkali metal to be prepared by electrolysis. The Hydroxides of Group 1A y The hydroxides are white crystalline solids y NaOH (318 ̊C) y KOH (360 ̊C) y Hydroxides are freely soluble exothermically in water and in alcohols y Strong alkali bases Ionic salts y Li has some special properties as compared to the other elements of group 1 y LiOH is less soluble then the other Hydroxides y Li3N is stable (the only stable Nitrite in this group) y Electrical conductive y Precipitation reactions: The larger the M‐ ion the more salts are available. y Na salts are very soluble, but the mixed salts with Zn and Uranium can be used in quantitative analysis. Complexation of Alakali Metals The binding constants for 18‐crown‐6 : Li<Na,Cs<Rb<K Biological transporters for Na and K Valinomycin and Nonactin: small cyclic polypeptides are the carriers of alkaline metals across membranes. Reaction summary Reaction summary Group 2A elements y Group 2A elements are very reactive y Valence electron configuration ns2 y Lose 2 valence electrons to form noble gas configuration y Form ionic compounds containing M2+ cations y Oxides are basic: 2+ ‐ MO(s) +H2O(l) Æ M (aq) +2OH(aq) y Ca, Sr, and Ba react vigorously with water: 2+ ‐ M(s) +2H2O(l) ÆM (aq) +2OH(aq) +H2(g) y Be form covalent bond because of small size and high electronegativity Be structure 2 e‐ 3 center bond Mg y Mg is intermediate in its behaviour between Be and the other elements in this group. y Its chemistry is entirely ionic in nature. y Mg: high polarization ability y The Hydroxides of Be and Mg are only slightly soluble in water. y The other hydroxides of this group are water soluble and highly basic. Ca, Sr, Ba, Ra They form a closely related group with the change in chemical and physical parameters follow a systematic change. e.g. Electropositive nature increases from Ca‐>Ra. Hydration energy of salts increase. Insolubility of most salts, especially sulfates increases in the same way. Be and its compounds y Be is obtained by Ca or Mg reduction of the BeCl2 or BeF2 salts. y Be metal is relatively unreactive. It does not react with water at red heat, and does not react with air below 600 deg C. y Reaction with air only as fine powder ‐> BeO, Be3N2. y Be salts do not lose their water molecules over strong desiccants (P2O5). y Aqueous solutions of Be salts are acidic Mg production At 2000 degrees Ca, Sr, Ba are made on a small scale from the Cl salts by reduction with Na. Ion exchange y Ca2+ and Mg2+ ions cause the hardness of water. These ions of hard water interfere with the action of detergents and form precipitate with soap. Those ions are removed by ion exchange with ion exchange resins. y Ca2+ and Mg2+ are often removed during ion exchange, releasing Na+ into solution. y Ion exchange resin: large molecules that have many ionic sites. Group trends Group trends.
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