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 equilibrium is represented as:

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

This equilibrium is pH-dependent. The influence of pH on this equilibrium is as follows:

1. pH and Equilibrium Shift: The balance between dichromate ions (\(\text{Cr}_2\text{O}_7^{2-}\)) and chromate ions (\(\text{CrO}_4^{2-}\)) is determined by the hydrogen ion (H⁺) concentration.
2. Basic Medium Effects: A basic environment has a high concentration of hydroxide ions (OH^-). These ions react with H⁺, lowering the H⁺ concentration. Consequently, the equilibrium shifts to the right, favoring the formation of more chromate ions (\(2\text{CrO}_4^{2-}\)) as they consume H⁺.
3. Acidic Medium Effects: In an acidic environment, H⁺ ions are abundant. According to Le Chatelier's principle, an excess of H⁺ drives the equilibrium to consume these ions, causing a shift to the left, towards the formation of dichromate ions (\(\text{Cr}_2\text{O}_7^{2-}\)).
4. Neutral and Weakly Acidic Conditions: In neutral solutions, H⁺ and OH^- concentrations are equal, resulting in no significant equilibrium shift. Weakly acidic conditions may slightly favor dichromate formation, but to a lesser extent than strongly acidic conditions.

Conclusion: Since the equilibrium shifts to the right in a basic medium (due to OH^- neutralizing H⁺), the correct condition is a basic medium.