Why do the alkali metals give dihydrogen gas upon treatment with water?

1 Answer
Feb 4, 2016

Because the alkali metal is strongly reducing, and supplies electron to break the the H-OHO bond to form hydrogen gas and hydroxide ion.

Explanation:

For a Group I metal we can represent the equation this way:

M(s) + H_2O(aq) rarr 1/2H_2(g)uarr + MOH(aq)M(s)+H2O(aq)12H2(g)⏐ ⏐+MOH(aq)

This is an oxidation reduction reaction in which M^0M0 has been OXIDIZED to M^IMI (to Ca^(II)CaII for the Group II metal), and hydrogen has been REDUCED to the zerovalent gas, H^IHI to H^(0)H0 in dihydrogen gas H_2H2.

The alkali and alkali earth metals are sufficiently active to enable oxidation by water - they are electron rich metals. Now, of course, since the water molecule has been chemically modified, the other half of the molecule acquires a negative charge, to give formal HO^-HO, where the charge is conceived to be on the oxygen atom.

Can you represent the oxidation of calcium metal by water to give Ca(OH)_2Ca(OH)2 and dihydrogen by means of a chemical equation?