Chemistry - Complete d-block Question with Solution | TestHub

When CO₂ gas is passed through a solution of sodium chromate (Na₂CrO₄), the solution turns orange due to the formation of sodium dichromate (Na₂Cr₂O₇). CO₂ is an acidic oxide, and it reacts with water to form carbonic acid (H₂CO₃), which then protonates the chromate ions.

The reaction is:

$$2\mathrm{N}{\mathrm{a}}_2\mathrm{Cr}{\mathrm{O}}_4+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7+\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$$

Or, more simply, in terms of the chromate-dichromate equilibrium:

$$2\mathrm{Cr}{\mathrm{O}}_4^{2-}(yellow)+2{\mathrm{H}}^{+}⟶\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}(orange)+{\mathrm{H}}_2\mathrm{O}$$

The carbonic acid formed from CO₂ provides the H⁺ ions, shifting the equilibrium from yellow chromate to orange dichromate.