▶️ Answer/Explanation
(a) Dynamic equilibrium occurs when the forward and reverse reaction rates are equal, and the concentrations of reactants and products remain constant over time. This is a state where no macroscopic changes are observed, but molecules continue to react at the molecular level.
Explanation: In dynamic equilibrium, the system is not static—reactants and products are continuously interconverting, but their concentrations do not change because the rates of formation and decomposition are balanced.
(b)(i)
- Change in appearance: The mixture becomes darker red due to increased formation of FeSCN²⁺.
- Change in [Fe³⁺(aq)]: The concentration of Fe³⁺ decreases as more reacts with the added SCN⁻.
- Change in \(K_c\): No change, as \(K_c\) is temperature-dependent and the temperature remains constant.
Explanation: Adding KSCN increases [SCN⁻], shifting the equilibrium to the right (Le Chatelier’s Principle), producing more FeSCN²⁺ and consuming Fe³⁺. \(K_c\) remains unchanged because temperature is constant.
(b)(ii) \(K_c = 1380 \, \text{dm}^3 \text{mol}^{-1}\)
Explanation: First, calculate initial concentrations: \([Fe³⁺]_0 = [SCN⁻]_0 = \frac{5.00 \times 10^{-5} \text{mol}}{0.025 \text{dm}^3} = 2.00 \times 10^{-3} \text{mol dm}^{-3}\). At equilibrium, \([FeSCN²⁺] = 4.23 \times 10^{-4} \text{mol dm}^{-3}\). Using the ICE table, equilibrium concentrations are \([Fe³⁺] = [SCN⁻] = 1.577 \times 10^{-3} \text{mol dm}^{-3}\). Substituting into \(K_c = \frac{[FeSCN²⁺]}{[Fe³⁺][SCN⁻]}\), we get \(K_c = 1380 \, \text{dm}^3 \text{mol}^{-1}\).
(c) \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5\)
Explanation: Fe has atomic number 26. Fe³⁺ loses 3 electrons (2 from 4s and 1 from 3d), resulting in the configuration \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5\).
(d)
Explanation: The SCN⁻ ion has a lone pair on the sulfur (S), carbon (C), and nitrogen (N) atoms, with covalent bonds between S-C and C-N. The negative charge is delocalized across the ion.