Question

The equilibrium $\mathrm{Cr_2O_7^{2-}} \rightleftharpoons 2\mathrm{CrO_4^{2-}}$ is shifted to the right in:

• A. an acidic medium
• B. a basic medium
• C. a weakly acidic medium
• D. a neutral medium

Answer 1

The Correct Option is B

The given equilibrium is:

\(\text{Cr}_2\text{O}_7^{2-} \rightleftharpoons 2\text{CrO}_4^{2-}\)

This is a classic example of a chemical equilibrium that is sensitive to the pH of the solution. Let's analyze the situation to understand why this equilibrium shifts in different pH conditions:

1. Equilibrium Shifts and pH: The equilibrium between dichromate ions \(\text{Cr}_2\text{O}_7^{2-}\) and chromate ions \(\text{CrO}_4^{2-}\) is affected by the concentration of hydrogen ions (H⁺) in the solution.
2. Effect of Basic Medium: In a basic medium, the concentration of hydroxide ions (OH^-) is high. These OH^- ions can react with H⁺, reducing their concentration. This removal of H⁺ ions causes the equilibrium to shift to the right to form more chromate ions (\(2\text{CrO}_4^{2-}\)), which consume more H⁺ to maintain balance.
3. Effect of Acidic Medium: In an acidic medium, there is an abundance of H⁺ ions. According to Le Chatelier's principle, increasing the concentration of one side of the equilibrium forces it to shift in the direction that consumes the excess H⁺, which in this case is towards \(\text{Cr}_2\text{O}_7^{2-}\). Thus, the equilibrium shifts to the left.
4. Effect in Neutral and Weakly Acidic Media: In a neutral medium, the concentration of H⁺ and OH^- ions is balanced, so there is no significant shift in equilibrium. In a weakly acidic medium, the reaction may still slightly favor the formation of \(\text{Cr}_2\text{O}_7^{2-}\), but not as strongly as in a strongly acidic medium.

Conclusion: Since the equilibrium shifts to the right in a basic medium (as OH^- ions neutralize H⁺ ions), the correct answer is a basic medium.

Answer 2

The given equilibrium reaction is:

\(\mathrm{Cr_2O_7^{2-}} \rightleftharpoons 2\mathrm{CrO_4^{2-}}\)

This equilibrium can be affected by the pH of the solution. Specifically, this reaction is an example of how pH influences equilibrium according to Le Chatelier's principle.

Explanation:

• The dichromate ion, \(\mathrm{Cr_2O_7^{2-}}\), is predominant in acidic conditions, while the chromate ion, \(\mathrm{CrO_4^{2-}}\), is predominant in basic conditions.
• When the medium is acidic, the equilibrium shifts to the left to form more dichromate ions as protons from the acidic solution react with chromate ions.
• In a basic medium, hydroxide ions react with hydronium ions to reduce the overall acidity, thereby shifting the equilibrium to the right to produce more chromate ions.

Justification of the Correct Answer (Basic Medium):

In a basic medium, the equilibrium will shift towards the formation of chromate ions, \(\mathrm{CrO_4^{2-}}\), moving the equilibrium to the right. This is in accordance with Le Chatelier's principle, as the system adjusts to reduce the effect of the added base (hydroxide ions).

Conclusion:

Thus, the equilibrium \(\mathrm{Cr_2O_7^{2-}} \rightleftharpoons 2\mathrm{CrO_4^{2-}}\) is shifted to the right in a basic medium.