Chemistry General characteristics of the compounds of the alkali metals

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=> General characteristics of the compounds of the alkali metals

### GENERAL CHARACTERISTICS OF THE COMPOUNDS OF THE ALKALI METALS

All the common compounds of the alkali metals are generally ionic in nature. General characteristics of some of their compounds are discussed here.

### Oxides and Hydroxides

color{green}(★) On combustion in excess of air, lithium forms mainly the oxide, color{red}(Li_2O) (plus some peroxide color{red}(Li_2O_2) ) sodium forms the peroxide color{red}(Na_2O_2) (and some superoxide color{red}(Na O_2) ) whilst potassium, rubidium and caesium form the superoxides, color{red}(MO_2) .

color{green}(★) Under appropriate conditions pure compounds color{red}(M_2O , M_2O_2) and color{red}(MO_2) may be prepared.

color{green}(★) The increasing stability of the peroxide or superoxide, as the size of the metal ion increases, is due to the stabilisation of large anions by larger cations through lattice energy effects. These oxides are easily hydrolysed by water to form the hydroxides according to the following reactions :

color{red}(M_2O + H_2O → 2M^(+) + 2OH^(-))

color{red}(M_2O_2 + 2H_2O → 2M^(+) + 2OH^(-) + H_2O_2)

color{red}(2MO_2+2H_2O → 2M^(+) + 2OH^(-) + H_2O_2 + O_2)

color{green}(★) The oxides and the peroxides are colourless when pure, but the superoxides are yellow or orange in colour.

color{green}(★) The superoxides are also paramagnetic. Sodium peroxide is widely used as an oxidising agent in inorganic chemistry.

color{green}(★)The hydroxides which are obtained by the reaction of the oxides with water are all white crystalline solids.

color{green}(★) The alkali metal hydroxides are the strongest of all bases and dissolve freely in water with evolution of much heat on account of intense hydration.
Q 3242123933

Why is KO_2 paramagnetic ?
Class 11 Chapter ex.10 3
Solution:

The superoxide O_2^(-) is paramagnetic because of one unpaired electron in pi^(**)2p molecular orbital.

### Halides

color{green}(★) The alkali metal halides , color{red}(MX , ( X =F , Cl , Br , I)) are all high melting, colourless crystalline solids.

color{green}(★) They can be prepared by the reaction of the appropriate oxide, hydroxide or carbonate with aqueous hydrohalic acid (color{red}(HX)).

color{green}(★) All of these halides have high negative enthalpies of formation; the color{red}(△_f H^(⊖)) values for fluorides become less negative as we go down the group whilst the reverse is true for color{red}(△_fH^(⊖)) for chlorides bromides and iodides. For a given metal color{red}(△_fH^(⊖)) always becomes less negative from fluoride to iodide.

color{green}(★) The melting and boiling points always follow the trend : fluoride > chloride > bromide > iodide. All these halides are soluble in water.

color{green}(•) The low solubility of color{red}(LiF) in water is due to its high lattice enthalpy whereas the low solubility of color{red}(CsI) is due to smaller hydration enthalpy of its two ions.

color{green}(•) Other halides of lithium are soluble in ethanol, acetone and ethylacetate; color{red}(LiCl) is soluble in pyridine also.

### Salts of Oxo-Acids

color{green}(★) Oxo-acids are those in which the acidic proton is on a hydroxyl group with an oxo group attached to the same atom e.g., carbonic acid, color{red}(H_2CO_3 \ \ \ ( OC (OH)_2)) ; sulphuric acid, color{red}(H_2SO_4 (O_2S(OH)_2 )).

color{green}(★) The alkali metals form salts with all the oxo-acids.

color{green}(★) They are generally soluble in water and thermally stable.

color{green}(★) Their carbonates color{red}((M_2CO_3)) and in most cases the hydrogencarbonates color{red}((MHCO_3)) also are highly stable to heat.

color{green}(★) As the electropositive character increases down the group, the stability of the carbonates and hydorgencarbonates increases.

color{green}(•) Lithium carbonate is not so stable to heat; lithium being very small in size polarises a large color{red}(CO_3^(2-)) ion leading to the formation of more stable color{red}(Li_2O) and color{red}(CO_2) .

color{green}(•) Its hydrogen carbonate does not exist as a solid.