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

1 Answer
Feb 4, 2016

Answer:

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?