▶️ Answer/Explanation
(a) Le Chatelier’s principle states that if a system at equilibrium is subjected to a change in conditions, the equilibrium will shift to counteract the change and restore equilibrium.
Explanation: This principle helps predict the direction of shift in equilibrium when factors like concentration, temperature, or pressure are altered.
(b)
• Change in appearance: The mixture becomes paler red/more yellow as the equilibrium shifts left (endothermic direction).
• Change in [FeSCN²⁺(aq)]: The concentration of FeSCN²⁺ decreases because the reverse reaction is favored at higher temperatures.
• Change in \(K_c\): The equilibrium constant decreases because the reaction is exothermic (\(\Delta H = -x\)), and increasing temperature favors the reactants.
Explanation: Since the reaction is exothermic, increasing the temperature shifts the equilibrium to the left, reducing FeSCN²⁺ concentration and \(K_c\).
(c)(i) Initial amount of Fe³⁺(aq): \(1.60 \times 10^{-5} \, \text{mol}\).
Explanation: At equilibrium, [SCN⁻] = \(1.30 \times 10^{-3} \, \text{moldm}^{-3}\) and [FeSCN²⁺] = \(0.300 \times 10^{-3} \, \text{moldm}^{-3}\). The initial [SCN⁻] = equilibrium [SCN⁻] + [FeSCN²⁺] = \(1.60 \times 10^{-3} \, \text{moldm}^{-3}\). For 25.0 cm³, the amount is \(1.60 \times 10^{-3} \times 0.025 = 1.60 \times 10^{-5} \, \text{mol}\).
(c)(ii) \(K_c = 178 \, \text{mol}^{-1} \, \text{dm}^3\).
Explanation: Using \(K_c = \frac{[\text{FeSCN}^{2+}]}{[\text{Fe}^{3+}][\text{SCN}^-]}\), and substituting equilibrium concentrations: \(K_c = \frac{0.300 \times 10^{-3}}{(1.30 \times 10^{-3})(1.30 \times 10^{-3})} = 178 \, \text{mol}^{-1} \, \text{dm}^3\).
