Chemical Properties of s-Block Elements
- Reaction with Oxygen
When moderately heated in a limited supply of oxygen, the alkali metals form their normal oxides(M2O). They form peroxides(M2O2) when they freely burn in air or oxygen, e.g.
K,Rb and Cs form super-oxides of the type MO2, e.g. K + O2KO2
- Reaction with Hydrogen
When dry hydrogen is passed over any gently heated alkali metal, a colourless crystalline hydride results which is an ionic compound, e.g.
The tendency to form alkali hydrides decreases from Li to Cs with increase in atomic number.
- Action with Halogens
Alkali metals readily combine with halogens to form metal halides having the formula MX where M is an alkali metal and X is the halogen, e.g.,
2Na + Cl22NaCl
The case with which alkali metals form their halides increase in moving down the group from Li to Cs.
Except Lithium fluoride, all other alkali metal halides are soluble.
Halides of alkali metals are crystalline ionic solids. These are good conductors in aqueous solution and also in the fused state. Alkali metal halides have high melting and boiling points.
Reaction with Water
The alkali metals react violently with water to form their hydroxides. The reaction is exothermic and in certain cases, the hydrogen evolved catches fire.
2Na + H2O 2NaOH + H2
The reactivity with water increases as we move down the group from Li to Cs.
The alkali metal hydroxides formed during the reaction are strong bases. The basic nature of the hydroxides goes on increasing on moving form Li to Cs. These hydroxides are highly soluble in water and stable towards heat. However, LiOH is a weak base, insoluble in water and decomposes on heating.
2LiOH Li2O + H2O