Question

Chromium (VI) forms two different oxyanions, the orange dichromate ion (${\mathbf{Cr}}_2{{\mathbf{O}}_7}^{\mathbf{2}-}$ ), and the yellow chromate ion (${{\mathbf{CrO}}_4}^{\mathbf{2}-}$ ). (See the photos below.) The equilibrium reaction between the two ions is

${\mathbf{Cr}}_{\mathbf{2}}{{\mathbf{O}}_{\mathbf{7}}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(l\right)\mathbf{}\stackrel{}{\mathbf{\rightleftharpoons }}\mathbf{2}{{\mathbf{CrO}}_{\mathbf{4}}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}{\mathbf{H}}^{\mathbf{+}}\left(\mathrm{aq}\right)$

Explain why orange dichromate solutions turn yellow when sodium hydroxide is added.

Short answer

Because of the addition of a strong base, sodium hydroxide in the equilibrium reaction will react with the proton,${\mathrm{H}}^{+}$ to form water and sodium cation. This will shift the reaction to the right side in order to produce more chromate ions. Therefore, the orange dichromate solutions will change to yellow.

Step-by-step solution

The given chemical reaction

The given equilibrium reaction between dichromate ion and water is shown below.

${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{}{\rightleftharpoons }2{{\mathrm{CrO}}_4}^{2-}\left(\mathrm{aq}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)$

The color of dichromate ion solution is orange and the color of chromate ion solution isyellow.

Sodium hydroxide is a strong base and it dissociates into sodium cation and hydroxide ion as shown below:

$\mathrm{NaOH}\left(\mathrm{aq}\right)\stackrel{}{\to }{\mathrm{Na}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$

Explanation

The reaction between the proton and the hydroxide ion is represented as follows:

${\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\stackrel{}{\to }{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

The strong base, sodium hydroxide in the equilibrium reaction will react with the proton, to form water and sodium cation. This will shift the reaction to the right side in order to produce more chromate ions. Therefore, the orange dichromate solutions will change to yellow.