S-block elements of the modern periodic table consisting of alkali metals and alkali earth metals. These metals are highly reactive in nature mainly due to their low Ionization enthalpy and large size of the molecules. As we go down the group, the reactivity of the metals increases. Due to the high reactivity with air and water-alkali metals are usually stored in a kerosene jar. Chemical properties of s-block elements are as explained below:
Reactivity of metals facing air: When the alkali metals come in contact with dry air they form their respective oxides and in turn form hydroxides in the presence of moisture. In the presence of oxygen, they burn actively to form oxides. For example, lithium forms monoxides when it comes in contact with oxygen. It is observed the oxidation state of the oxides formed is +1.
When the alkali metals come in contact with dry air they form their respective oxides and in turn form hydroxides in the presence of moisture. In the presence of oxygen, they burn actively to form oxides. For example, lithium forms monoxides when it comes in contact with oxygen. It is observed the oxidation state of the oxides formed is +1.
4Li+ O2 → 2Li2O (oxide)
In the presence of dihydrogen: The alkali metals of the s-block react at around 673K with dihydrogen to form their respective hydrides. Lithium is an exception and forms its hydride at 1073K.The metal hydrides formed are solids with high melting points.
2M + H2 → 2M+H–
Reaction with water: Respective hydroxides and dihydrogen is formed when an alkali metal reacts with water. Lithium acts as an exception while other metal reacts explosively with water.
2M +2H2O → 2M+ + 2OH– +H2
Reaction with halogen: The alkali metals react with halogens vigorously to form ionic halides. But lithium is as an exception and it forms a partial covalent bond due to the high polarization property, small size and the capability to break electron cloud which surrounds the halide ion.
Reducing activity: These elements act as strong reducing agents due to high hydration enthalpy. Due to this, the electropositivity is high and hence the reducing property is high.
The alkali metals diffuse with aqueous ammonia which results in the formation of a deep blue color solution. This solution is blue in color due to the electrons under the influence of ammonia. These electrons take in the energy from the visible region of the light and thus are responsible for the blue coloration of the solution. The solution is paramagnetic in nature and they conduct electricity.
M + (x + y) NH3 → [M (NH3) X] ++ [e(NH3)y]–
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