Reacting acetylide salts with water produces acetylene and MOH aq. The heavier alkali metals K, Rb, and Cs also react with carbon in the form of graphite. Alkali metal cations are found in a wide variety of ionic compounds. In general, any alkali metal salt can be prepared by reacting the alkali metal hydroxide with an acid and then evaporating the water:. Hydroxides of alkali metals also can react with organic compounds that contain an acidic hydrogen to produce a salt. An example is the preparation of sodium acetate CH 3 CO 2 Na by reacting sodium hydroxide and acetic acid:.
Complex formation is primarily due to the electrostatic interaction of the metal cation with polar water molecules. These compounds absorb trace amounts of water from nonaqueous solutions to form hydrated salts, which are then easily removed from the solution by filtration.
Electrostatic interactions also allow alkali metal ions to form complexes with certain cyclic polyethers and related compounds, such as crown ethers and cryptands. As discussed in Chapter 13, crown ethers are cyclic polyethers that contain four or more oxygen atoms separated by two or three carbon atoms.
All crown ethers have a central cavity that can accommodate a metal ion coordinated to the ring of oxygen atoms, and crown ethers with rings of different sizes prefer to bind metal ions that fit into the cavity. Cryptands are more nearly spherical analogues of crown ethers and are even more powerful and selective complexing agents. Cryptands consist of three chains containing oxygen that are connected by two nitrogen atoms part b in Figure They can completely surround encapsulate a metal ion of the appropriate size, coordinating to the metal by a lone pair of electrons on each O atom and the two N atoms.
Like crown ethers, cryptands with different cavity sizes are highly selective for metal ions of particular sizes. Crown ethers and cryptands are often used to dissolve simple inorganic salts such as KMnO 4 in nonpolar organic solvents. A remarkable feature of the alkali metals is their ability to dissolve reversibly in liquid ammonia. Without a catalyst, the reaction in Equation In many cases, the alkali metal amide salt MNH 2 is not very soluble in liquid ammonia and precipitates, but when dissolved, very concentrated solutions of the alkali metal are produced.
One mole of Cs metal, for example, will dissolve in as little as 53 mL 40 g of liquid ammonia. The pure metal is easily recovered when the ammonia evaporates. A solvated electron is loosely associated with a cavity in the ammonia solvent that is stabilized by hydrogen bonds.
As the concentration of the metal increases above 3 M, the color changes to metallic bronze or gold, and the conductivity increases to a value comparable with that of the pure liquid metals. The anion is formed by adding an electron to the singly occupied ns valence orbital of the metal atom. Nonetheless, the solvated electron is a potent reductant that is often used in synthetic chemistry. Compounds that contain a metal covalently bonded to a carbon atom of an organic species are called organometallic compounds.
The properties and reactivities of organometallic compounds differ greatly from those of either the metallic or organic components. Because of its small size, lithium, for example, forms an extensive series of covalent organolithium compounds, such as methyllithium LiCH 3 , which are by far the most stable and best-known group 1 organometallic compounds.
These volatile, low-melting-point solids or liquids can be sublimed or distilled at relatively low temperatures and are soluble in nonpolar solvents.
Like organic compounds, the molten solids do not conduct electricity to any significant degree. Organolithium compounds have a tendency to form oligomers with the formula RLi n , where R represents the organic component. Effectively, the carbon atom of each CH 3 group is using a single pair of electrons in an sp 3 hybrid lobe to bridge three lithium atoms, making this an example of two-electron, four-center bonding.
Clearly, such a structure, in which each carbon atom is apparently bonded to six other atoms, cannot be explained using any of the electron-pair bonding schemes. Molecular orbital theory can explain the bonding in methyllithium, but the description is beyond the scope of this text. Because sodium remains liquid over a wide temperature range Cesium, because of its low ionization energy, is used in photosensors in automatic doors, toilets, burglar alarms, and other electronic devices.
In these devices, cesium is ionized by a beam of visible light, thereby producing a small electric current; blocking the light interrupts the electric current and triggers a response. Compounds of sodium and potassium are produced on a huge scale in industry. Each year, the top 50 industrial compounds include NaOH, used in a wide variety of industrial processes; Na 2 CO 3 , used in the manufacture of glass; K 2 O, used in porcelain glazes; and Na 4 SiO 4 , used in detergents.
Several other alkali metal compounds are also important. For example, Li 2 CO 3 is one of the most effective treatments available for manic depression or bipolar disorder. It appears to modulate or dampen the effect on the brain of changes in the level of neurotransmitters, which are biochemical substances responsible for transmitting nerve impulses between neurons. For each application, choose the more appropriate substance based on the properties and reactivities of the alkali metals and their compounds.
Explain your choice in each case. Use the properties and reactivities discussed in this section to determine which alkali metal is most suitable for the indicated application. Indicate which of the alternative alkali metals or their compounds given is more appropriate for each application. A Determine whether one of the reactants is an oxidant or a reductant or a strong acid or a strong base.
If so, a redox reaction or an acid—base reaction is likely to occur. Identify the products of the reaction.
Alkali metals have only a weak tendency to form complexes with simple Lewis bases. The first alkali metals to be isolated Na and K were obtained by passing an electric current through molten potassium and sodium carbonates. The alkali metals are among the most potent reductants known; most can be isolated by electrolysis of their molten salts or, in the case of rubidium and cesium, by reacting their hydroxide salts with a reductant. They can also be recovered from their silicate ores using a multistep process.
Lithium, the strongest reductant, and sodium, the weakest, are examples of the physical and chemical effects of opposing periodic trends. Chemical Bonding and Molecular Structure. Which statement is not correct? Questions from The s-Block Elements. Which alkali metal during flame test will show colour corresponding to maximum wavelength? AIIMS Which one of the following has maximum lattice energy?
AMU The halide of which alkaline earth metal is covalent and soluble in organic solvents? KEAM Which of the following is the weakest base? In which of the following the hydration energy is higher than the lattice energy? The decomposition temperature is the lowest for AMU They also have low boiling and melting points and are less dense than most elements.
All these characteristics can be attributed to these elements' large atomic radii and weak metallic bonding. It is all around us. This module will explore several aspects of the element and how they apply to the world. It is understood to be non-vital in human biological processes, although it is used in many drug treatments due to its positive effects on the human brain. Because of its reactive properties, humans have utilized lithium in batteries, nuclear fusion reactions, and thermonuclear weapons.
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