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For Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq), adding more KSCN causes the Fe³⁺ concentration to:
Aincrease, as the reverse reaction speeds up
Bdecrease, as more is consumed forming the complex
Cstay constant, since Fe³⁺ is not added
Dfall to zero, as the reaction goes to completion
Answer & Solution
Correct answer: B. decrease, as more is consumed forming the complex
1. Adding KSCN raises the concentration of SCN⁻ in the mixture.
2. By Le Chatelier's principle the system shifts forward to reduce that added SCN⁻.
3. The forward shift consumes Fe³⁺ along with the SCN⁻ to form the red complex.
4. So the Fe³⁺ concentration falls and the red colour deepens.
5. It does not reach zero: the reaction is reversible, so some Fe³⁺ always remains at equilibrium.
_Source: NECTA ACSEE 2023 Chemistry 132/1, Question 8: Chemical Equilibrium_
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