Chromium(VI) forms two different oxyanions, the orange...

Chromium(VI) forms two different oxyanions, the orange dichromate ion, $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$, and the yellow chromate ion, $\mathrm{CrO}_{4}{ }^{2-}$. The equilibrium reaction between the two ions is $$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons 2 \mathrm{CrO}_{4}{ }^{2-}(a q)+2 \mathrm{H}^{+}(a q)$$ The following pictures show what happens when sodium hydroxide is added to a dichromate solution. Explain what happened.

Chromium(VI) forms two different oxyanions, the orange dichromate ion, $\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}$, and the yellow chromate ion, $\mathrm{CrO}_{4}{ }^{2-}$. The equilibrium reaction between the two ions is
$$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons 2 \mathrm{CrO}_{4}{ }^{2-}(a q)+2 \mathrm{H}^{+}(a q)$$
The following pictures show what happens when sodium hydroxide is added to a dichromate solution.
Explain what happened.

Key Concepts

Chemical Equilibrium

Chemical equilibrium refers to a state in a reversible chemical reaction where the rates of the forward and reverse reactions are equal, so that the concentrations of reactants and products remain constant. In the context of the chromium oxyanions, the reaction between the dichromate and chromate ions is an example of such an equilibrium, which can shift when external conditions change.

Acid?Base Chemistry

Acid-base chemistry involves the interplay between acidic (proton-donating) and basic (proton-accepting) species in a solution. When a base like sodium hydroxide is added, it neutralizes hydrogen ions present in the solution, thereby affecting the acid-base balance. This change directly impacts equilibria that depend on the concentration of hydrogen ions.

Le Chatelier’s Principle

Le Chatelier’s Principle states that if an external change is imposed on a system at equilibrium, the system adjusts in a way that counteracts the change. In this case, adding sodium hydroxide lowers the concentration of hydrogen ions, prompting the equilibrium to shift in the direction that produces more H+—resulting in the transformation of dichromate ions into chromate ions.

pH-Dependent Equilibria

Certain chemical equilibria are highly sensitive to the pH of the environment. The equilibrium between dichromate and chromate ions is pH-dependent, with acidic conditions favoring the formation of the orange dichromate ion and basic conditions favoring the yellow chromate ion. The addition of sodium hydroxide increases the pH, shifting the color of the solution as the equilibrium shifts toward the chromate form.

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