A petroleum refinery stores large quantities of three pure elemental gases—lithium vapor, sodium vapor, and potassium vapor—and three pure elemental halogen gases—fluorine, chlorine, and bromine—in separate, sealed tanks. Emergency planners want to know which specific metal–halogen combination would release the greatest amount of energy per mole if an accidental leak caused direct contact between the two gases under standard conditions.
For all six elements, the refinery’s safety manual provides the following information derived from reliable sources:
• Each metal atom contains a single valence electron residing in an s orbital of its outermost shell.
• Each halogen atom contains seven valence electrons, the highest-energy one residing in a p orbital of its outermost shell.
• Successive elements down a group possess progressively larger atomic radii.
Use these data and periodic trends to predict which single metal–halogen pair would form an ionic compound most exothermically if accidentally mixed at 298 K and 1 atm. Justify your selection in detail by referencing the energetic favorability of electron loss and gain, the magnitude of electrostatic attraction in the resulting ionic lattice, and any relevant shielding or effective nuclear charge considerations.
(a) Identify the specific cation–anion pair you expect to release the most energy per mole of compound formed.
(b) Explain qualitatively, in terms of trends in first ionization energy and electron affinity, why the chosen metal and halogen favor electron transfer more strongly than any other possible pairing in the set.
(c) Using Coulombic reasoning, describe how ionic radius affects the lattice enthalpy for each possible metal–halogen combination, and show explicitly why the pair you selected is predicted to have the most negative (most exothermic) lattice enthalpy.
(d) Combine your discussions from parts (b) and (c) to construct a coherent argument showing that the overall enthalpy of formation for your selected compound is more negative than that of any alternative compound that could form from the available elements.