Cr2O7^2-(aq) + 3 H2O(l) <=> 2 CrO4^2-(aq) + 2 H3O+(aq) The equilibrium system represented by the equ — Equilibrium Chemistry Question

STEPS:

1. Analyze the chemical equation and the species involved:
* Reactants side: $\text{Cr}_2\text{O}_7^{2-}(aq)$ is orange.
* Products side: $\text{CrO}_4^{2-}(aq)$ is yellow, and $\text{H}_3\text{O}^+(aq)$ represents the hydronium ions responsible for the acidity of the solution.
2. Identify the chemical effect of adding a strong base ($\text{NaOH}$):
* Sodium hydroxide ($\text{NaOH}$) is a strong base that dissociates completely in water to produce hydroxide ions ($\text{OH}^-$).
* Hydroxide ions react in a highly favorable neutralization reaction with hydronium ions to form water:
$$\text{OH}^-(aq) + \text{H}_3\text{O}^+(aq) \rightarrow 2\ \text{H}_2\text{O}(l)$$
* Consequently, adding $\text{NaOH}$ consumes the $\text{H}_3\text{O}^+$ ions, dramatically lowering their concentration in the solution.
3. Apply Le Chatelier's Principle:
* Le Chatelier's Principle states that when a system at equilibrium is subjected to a stress (such as the removal of a reactant or product), the equilibrium will shift in the direction that counteracts the stress.
* Since a product of the reaction ($\text{H}_3\text{O}^+$) is being removed, the system will shift to the right (toward the products) to replace the consumed hydronium ions.
4. Determine the final color change:
* Shifting the equilibrium to the right increases the concentration of the yellow $\text{CrO}_4^{2-}(aq)$ ions while decreasing the concentration of the orange $\text{Cr}_2\text{O}_7^{2-}(aq)$ ions.
* Thus, the mixture will become more yellow, making Option C the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect: This option incorrectly predicts that the mixture will become more orange (shifting left) and proposes a redox explanation. The oxidation state of chromium in both dinitrogen-like dichromate ($\text{Cr}_2\text{O}_7^{2-}$) and chromate ($\text{CrO}_4^{2-}$) is $+6$. No oxidation or reduction takes place in this acid-base equilibrium.
• Option B is incorrect: While this option correctly predicts that the mixture will become more yellow, its explanation is chemically incorrect. It claims that a reduction of chromium occurs, but as noted, the oxidation state of chromium remains $+6$ throughout this process. The shift is due to an acid-base neutralization, not redox.
• Option D is incorrect: While it is true that $\text{OH}^-(aq)$ is not explicitly written in the chemical equation, it reacts directly with $\text{H}_3\text{O}^+$, which *is* part of the equilibrium expression. The decrease in product concentration forces an equilibrium shift and a visible color change.