Write the equilibrium that interconverts the chromate ion and the dichromate ion, and explain how the pH of th

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Q. Write the equilibrium that interconverts the chromate ion and the dichromate ion, and explain how the pH of the solution controls which species predominates.

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Chromium(VI) in aqueous solution exists as an equilibrium mixture of two interconvertible species, the chromate ion, $\text{CrO}_4^{2-}$ (yellow), and the dichromate ion, $\text{Cr}_2\text{O}_7^{2-}$ (orange), related by the equilibrium

2\text{CrO}_4^{2-} + 2\text{H}^+ \rightleftharpoons \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O}

Structurally, the dichromate ion is formed by two $\text{CrO}_4$ tetrahedra condensing together (sharing one corner oxygen, with loss of a water molecule), a reaction that consumes $\text{H}^+$ ions and is therefore directly sensitive to the solution's pH.

By Le Chatelier's principle, since $\text{H}^+$ appears as a reactant on the left-hand side of the equilibrium, increasing the concentration of $\text{H}^+$ (i.e. making the solution more acidic, by adding an acid) shifts the equilibrium to the right, favouring the orange dichromate ion; this is exactly why the manufacture of potassium dichromate, described earlier in this chapter, requires an explicit acidification step to convert the initially formed sodium chromate into the dichromate salt.

Conversely, decreasing the concentration of $\text{H}^+$ (i.e. making the solution more alkaline, by adding a base, which removes $\text{H}^+$ ions from solution) shifts the equilibrium to the left, favouring the yellow chromate ion -- so dissolving potassium dichromate in an alkaline solution turns the colour from orange back to yellow, a classic and easily demonstrated laboratory illustration of this equilibrium. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set 56/5/11 markMCQ
$Q. While doing qualitative analysis in chemistry lab, Abhishek added yellow coloured potassium chromate solution into a test tube. He was surprised to see the colour of the solution changing immediately to orange. He realised that the test tube was not clean and contained a few drops of some liquid. Which of the following substances will be the most likely liquid to be present in the test tube before adding potassium chromate solution ? (A) Sodium hydrogen carbonate solution (B) Methyl orange solution (C) Sodium hydroxide solution (D) HCl solution$
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The colour change from yellow (chromate, $\ce{CrO4^{2-}}$ ) to orange (dichromate, $\ce{Cr2O7^{2-}}$ ) is caused by adding an acid. The only acidic substance among the options is HCl, so the correct answer is (D).

The key here is the chromate–dichromate equilibrium. In aqueous solution, the yellow chromate ion ( $\ce{CrO4^{2-}}$ ) and the orange dichromate ion ( $\ce{Cr2O7^{2-}}$ ) exist in a pH-dependent equilibrium:
$2\ce{CrO4^{2-}} + 2\ce{H+} \rightleftharpoons \ce{Cr2O7^{2-}} + \ce{H2O}$
When you add an acid (increase $\ce{H+}$ concentration), the equilibrium shifts to the right, producing more orange dichromate. When you add a base (decrease $\ce{H+}$ ), it shifts left, giving yellow chromate. So the liquid left in the test tube must have been acidic to cause the immediate colour change.

Let’s check each option:

Sodium hydrogen carbonate solution — This is mildly basic (it hydrolyses to give $\ce{OH-}$ ). It would shift the equilibrium left, keeping the solution yellow, not orange. So not this.

Methyl orange solution — This is an indicator, not an acid or base. It would not change the pH significantly. The colour change from yellow to orange is not due to methyl orange itself (which is orange in acid, yellow in base — but here the chromate colour dominates). So not this.

Sodium hydroxide solution — This is a strong base. It would consume $\ce{H+}$ , shifting the equilibrium left, making the solution stay yellow. So not this. …
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Q.Write the equilibrium that interconverts the chromate ion and the dichromate ion, and explain how the pH of the solution controls which species predominates.

Concept understanding — Preparation and Properties of Potassium Dichromate

Q.Write the equilibrium that interconverts the chromate ion and the dichromate ion, and explain how the pH of the solution controls which species predominates.

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