A Red Equilibrium Under Stress: FeSCN²⁺ and Le Chatelier
Students study the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), which is red. They prepare four tubes with the same initial volumes. Tube A contains 0.0020 M Fe³⁺ and 0.0020 M SCN⁻ and has absorbance 0.30 at 450 nm. Tube B has extra Fe³⁺ and absorbance 0.52. Tube C has extra SCN⁻ and absorbance 0.47. Tube D has NaOH added; a brown precipitate forms and the red color fades to absorbance 0.08. In a fifth tube E, prepared like A, heating to 60 °C gives absorbance 0.18; cooling an identical tube to 5 °C gives 0.41. Absorbance is proportional to [FeSCN²⁺]. A student says tube D faded only because the solution was diluted by the NaOH solution and that heating shifted the equilibrium to the right because collisions are faster.
1. For tubes B, C, and D, state the direction of the equilibrium shift and identify the species added or removed.
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Adding Fe³⁺ in B increases a reactant; the system shifts right, making more FeSCN²⁺, so absorbance rises from 0.30 to 0.52. Adding SCN⁻ in C does the same, shifting right; the smaller rise to 0.47 reflects the amount added and the equilibrium position, not a different direction. In D, OH⁻ precipitates Fe³⁺ as brown Fe(OH)₃, removing a reactant. The equilibrium shifts left, FeSCN²⁺ decomposes, and the red color fades to 0.08.
2. Use tube E to decide whether the forward reaction is exothermic or endothermic. Predict how K compares at 60 °C and at 5 °C, and critique the 'faster collisions' explanation.
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Heating E lowers absorbance from 0.30 to 0.18, so [FeSCN²⁺] decreases. Heating shifts an equilibrium in the endothermic direction; therefore the reverse reaction is endothermic and the forward reaction is exothermic (ΔH < 0). Cooling increases absorbance to 0.41, consistent with shifting right. K is therefore larger at 5 °C and smaller at 60 °C. The 'faster collisions' statement explains rates, not the equilibrium position; both forward and reverse rates rise, but the temperature dependence of K is set by ΔH. A kinetic argument alone cannot predict the direction of the shift.
3. Evaluate the dilution claim for tube D and propose a control experiment that would test it.
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Dilution can lower absorbance by lowering all concentrations, but the effect here is too large and is accompanied by a brown precipitate. A small volume of NaOH would not by itself reduce [FeSCN²⁺] by 73%; the precipitation of Fe³⁺ removes the reactant and drives the large shift. Control: add the same volume of water or NaCl solution to an identical tube A and compare the absorbance change. If dilution alone were responsible, the control would show a similar decrease; it will not. Adding NaOH to a solution containing SCN⁻ but no Fe³⁺ should not produce the red complex, confirming Fe³⁺ removal.