Question

Discuss the chemistry of chromate(VI), molybdate(VI), and tungstate(VI) anions when such anions are dissolved in NaOH, at pH 7.0, and at weakly acidic conditions.. You should look at the redox properties and what happens to the coordination numbers of the metal centers here.

Answer 1

a) the chromate ion is in equilibrium with dichromate ion

CrO₄^-2 and Cr₂O₇^-2

The oxidation state of chromium in dichromate salt is +6

so the equilibrium is like

Cr₂O₇^-2 + H2O ---> CrO₄^-2 + 2H⁺

a) on adding base (strong base like NaOH) the equilibrium will shift towards, forward direction and there will be more of chromate ion (Cr(IV)) [yellow coloured solution]. The OH- added will react with hydrogen ions forming water,

b) At pH = 7 , neutral condition the equilibrium will be towards CrO4-2 ions

c) in acidic medium the equilibrium will be towards backward direction and there will be more formation of dichromate ion [Cr(VI)] orange coloured solution

2)