General behavior of Alkali metals

 GENERAL BEHAVIOUR OF ALKALI METALS

The reducing property of an element depends on the magnitude of its ionization energy. Reducing agent is a substance which can lose electrons. Since alkali metals have got low Ionization energies, so they are strong reducing agents. They are highly electropositive. They React readily with halogens giving alkali metal halides.

Trends in Chemical Properties of Alkali Metals

Low ionization energies make the alkali metals, the most reactive family of metals. Very high second ionization energies indicate that oxidation number higher than +1, are Ruled out for the alkali metals.

1. Beryllium in particular is quite resistant towards complete oxidation, even by acids, Because of its BeO coating.

2. Beryllium is the only member of its group which reacts with alkalies to give hydrogen. The other members do not react with alkalies.

3. The cations of alkali metals have low charge and large radii than the radius of any cation From the same period, so the lattice energies of their salts are relatively low. Consequently, most of the simple salts of the alkali metals are water soluble. Most of the Salts are dissociated completely in aqueous solution and the hydroxides are among the Strongest bases available.

4. They react with oxygen and the surface is tarnished due to the oxides formed. Only Lithium burns in air to form the normal oxide, Li,O.The exposed metals are oxidized almost immediately by oxygen in air, and in the Presence of moisture.  The oxides formed react with CO, in the atmosphere to form Carbonates. Sodium will undergo a similar reaction, but only if the supply of oxygen is limited. In the Presence of excess of oxygen, sodium forms the pale yellow peroxides. Potassium, rubidium and caesium react with oxygen to form superoxides (orange Yellow). Caesium explodes spontaneously when it is in contact with air or oxygen. 

5. Very rapid reactions occur when alkali metals react with water. A small piece of sodium (potassium or lithium) floated on water reacts vigorously to liberate hydrogen and Produce metal hydroxide. The reaction is highly exothermic. The energy produced by The reaction may even ignite the hydrogen.

6. The reaction becomes increasingly vigorous from lithium to caesium. Potassium, Rubidium and caesium are so highly reactive that they react with ice even at -100°C. 

7. Alkali metals form ionic hydrides with hydrogen Rubidium and caesium react violently with hydrogen at room temperature. The other Three metals require elevated temperature in order to form the hydride. Lithium and Sodium hydrides are useful sources of hydrogen when treated with water. Due to the presence of hydride ion (H), the ionic hydrides are used as powerful reducing Agents.

8. Lithium is the only Group lA metal that combines with nitrogen and carbon to e Nitride and carbide, respectively. Alkali metals react casily with halogen to give halides. Ithium and sodium, for Example, react slowly with chlorine at room temperature. Molten sodium burns with Brilliant yellow fiame in a chlorine atmosphere to form sodium chloride. Potassnunm, rubidium and caesium react vigorously with all the halogens, torming metal Halides All alkali metals form their sulphides when treated with molten sulpur

Trends in Chemical Properties ofAlkaline Earth Metals

1. The alkaline earth metals burn in oxygen to form oxides or in the case of barium, the Peroxide. Beryllium is the least reactive metal in the group. It is resistant to complete Oxidation and stable in air at ordinary temperature but oxidizes rapidly at about 800°C Therefore beryllium is not tarnished by atmospheric attack but the other metal soon loses The silvery appearance. When exposed to air magnesium quickly becomes coated with the layer of MgO. This Layer protects the surface from further corrosion at ordinary temperature. When magnesium is burnt in air a small amount of nitride is also formed along with Magnesium oxide: When barium is heated in air or oxygen at S00 – 600°C, its peroxide is formed.

2. Hydrides are produced by treating the olten alkaline earth metals with hydrogen, Magnesium reacts with hydrogen at high pressure and in the presence of a catalyst Usually under high pressures. Catalyst (Mgl,) forming magnesium hydrides.

3. Similarly All Group Il-A elements react with nitrogen on heating giving nitrides. Magnesium reacts with nitrogen to give magnesium nitride.The nitrides hydrolyse vigorously when treated with water, giving ammonia and the Respective hydroxides.

4. With sulphur, magnesium gives magnesium sulphide, MgS. The other Group II-A Metals also react similarly.

5. All group II-A elements react directly with halogens giving halides of the type MX, e.g.

6. Magnesium is more reactive than beryllium, even though it is not attacked by cold water. Magnesium reacts slowly with boiling water and quite rapidly with steam to liberate Hydrogen. Beryllium does not react with water even at red hot temperature but remaining alkaline Earth metals produce hydroxides with water.

General Trends in Properties of Compounds of Alkali and Alkaline Earth Metals

Oxides

Alkali metal oxides dissolve in water to give strong alkaline solutions. The reaction of an alkali metal oxide with water is an acid-base reaction and not an Oxidation reduction reaction since no element undergoes a change in its oxidation number. 

The Reaction simply involves the decomposition of water molecule by an oxide ion. The basic character of alkali metal oxides increases down the group. Potassium Superoxide (KO,) has a very interesting use in breathing equipments for mountaineers and in Space craft. It has the ability to absorb carbon dioxide while giving out oxygen at the same time.

The solubility of alkaline-earth metal oxides in water increases down the group. BeO And Mg are insoluble but CaO, SrO and BaO are soluble and react with water to form the Corresponding hydroxides. The basic character of the oxides of alkaline earth metals increases down the group. The

Hydroxides

The alkali metal hydroxides are all crystalline solids, very soluble in water except IliOH, Which is slightly soluble. They are generally hygrsocopic and are very strong bases, execpt LiOH. The solubility of alkaline earth metal hydroxides in water increases down the group.

Be(OH), is quite insoluble. Mg(OH), is sparingly soluble while Ba(OH), is more soluble. This Increase in solubility is due to low lattice energy of hydroxides which is, in turn, due to higher Ionic size Alkali metal hydroxides are stable to heat except LiOH, while alkaline earth metalHydroxides like Mg(OH), and Ca(OH), decompose on hcating.

A saturated solution of Ca(OH), in water is called lime water and is used as a test for CO A suspension of Mg(OH), in water is called milk of magnesia and it is used for treatment of Acidity in stomach.