The halogens ,

CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry

Problem

The halogens Cl2 (g)Cl_2\,(g), Br2 (l)Br_2\,(l), and I2 (s)I_2\,(s) can oxidize iodide ions in aqueous solution according to the half-reaction below, with the halogen acting as the oxidizing agent. 12X2+e−→X−(aq)\frac{1}{2}X_2 + e^{-} \to X^{-}(aq) The following data are provided. Electron affinity of X (g)X\,(g): Cl:−349 kJ mol−1Cl: -349\,\mathrm{kJ\,mol^{-1}}, Br:−328 kJ mol−1Br: -328\,\mathrm{kJ\,mol^{-1}}, I:−295 kJ mol−1I: -295\,\mathrm{kJ\,mol^{-1}} Average XX–XX bond-dissociation energy in X2 (g)X_2\,(g): Cl2:243 kJ mol−1Cl_2: 243\,\mathrm{kJ\,mol^{-1}}, Br2:193 kJ mol−1Br_2: 193\,\mathrm{kJ\,mol^{-1}}, I2:151 kJ mol−1I_2: 151\,\mathrm{kJ\,mol^{-1}} Hydration enthalpy of X− (aq)X^{-}\,(aq): Cl−:−360 kJ mol−1Cl^{-}: -360\,\mathrm{kJ\,mol^{-1}}, Br−:−335 kJ mol−1Br^{-}: -335\,\mathrm{kJ\,mol^{-1}}, I−:−305 kJ mol−1I^{-}: -305\,\mathrm{kJ\,mol^{-1}} (a) Using the data above, calculate the enthalpy change per mole of electrons, ΔH∘\Delta H^\circ, for the half-reaction 12X2+e−→X−(aq)\frac{1}{2}X_2 + e^{-} \to X^{-}(aq) for Cl2Cl_2, Br2Br_2, and I2I_2. Clearly indicate the sign of each term you include in your calculation. (b) Rank the three halogens from strongest to weakest oxidizing agent in aqueous solution on the basis of your calculated ΔH∘\Delta H^\circ values. Justify your ranking. (c) The experimentally measured standard reduction potentials for these half-reactions follow the same order that you obtained in part (b) but differ in magnitude from the ΔH∘\Delta H^\circ values you calculated. Propose two factors, other than enthalpy, that contribute to the difference between ΔH∘\Delta H^\circ and the observed electrochemical potential.

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