Question

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

Answer 1

Because the alkali metal is strongly reducing, and supplies electron to break the the `H-O` 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)`

This is an oxidation reduction reaction in which `M^0` has been OXIDIZED to `M^I` (to `Ca^(II)` for the Group II metal), and hydrogen has been REDUCED to the zerovalent gas, `H^I` to `H^(0)` in dihydrogen gas `H_2`.

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^-`, 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)_2` and dihydrogen by means of a chemical equation?