AP Chemistry 7.8 Representations of Equilibrium - MCQs - Exam Style Questions

1. Analyze the Stress:
\(NaOH(aq)\) is a strong base. Adding it to the solution introduces a high concentration of \(OH^{-}(aq)\) ions.

2. Identify the Interaction:
The \(OH^{-}(aq)\) ions are a base and will react with the acid present in the equilibrium. In this case, the acid is the product \(H_{3}O^{+}(aq)\).
The neutralization reaction is: \(OH^{-}(aq) + H_{3}O^{+}(aq) \rightarrow 2~H_{2}O(l)\)

3. Apply Le Chatelier’s Principle:
This neutralization reaction consumes a product (\(H_{3}O^{+}\)). According to Le Chatelier’s principle, the system will shift to counteract this change. To replace the lost \(H_{3}O^{+}\), the equilibrium will shift to the right (toward the product side).

4. Observe the Result:
A shift to the right consumes the orange \(Cr_{2}O_{7}^{2-}\) reactant and produces more of the yellow \(CrO_{4}^{2-}\) product. Therefore, the mixture will become more yellow.

5. Evaluate Options:
Option (C) correctly states the mixture will become more yellow and provides the correct explanation (the \(OH^{-}\) ions cause the shift by reacting with \(H_{3}O^{+}\), which shifts the equilibrium toward products).
✅ Answer: (C)