At 298 K the aqueous equilibrium Fe^ 3+ (aq)+SCN^-(aq) FeSCN^ 2+ (aq) has
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
At 298 K the aqueous equilibrium
Fe3+(aq)+SCN−(aq)⇌FeSCN2+(aq)
has Kc=1.70×103. A student prepares 100.0 mL of a solution in which the equilibrium concentrations are
[Fe³⁺] = 1.00×10⁻³ M [SCN⁻] = 1.00×10⁻³ M [FeSCN²⁺] = 1.70×10⁻³ M
The student then quickly adds 100.0 mL of distilled water at the same temperature, mixing thoroughly so that the total volume becomes 200.0 mL.
(1) Immediately after the water is added but before the system shifts, calculate the reaction quotient Q for the diluted mixture.
(2) Compare the value of Q you obtained in part (a) with Kc and predict the direction in which the equilibrium will shift. Explain your reasoning.
(3) Let x be the change in molar concentration (in the 200.0 mL volume) of FeSCN²⁺ as the system moves to its new equilibrium. Set up, but do not solve, the algebraic expression involving x that enforces the equilibrium-constant condition.
(4) Solve the expression from part (c) to determine the equilibrium concentration of SCN⁻ in the diluted solution.