An exoplanetary research team predicts that the crust of a newly discovered rocky planet is enriched in the elements rubidium (Rb), calcium (Ca), oxygen (O), selenium (Se), and chlorine (Cl). Surface analyses indicate daytime temperatures of 800 K and pressures of 0.7 atm. The team seeks to predict which ionic minerals are likely to dominate the outermost centimeter of the crust.
Your task is to construct an evidence-based argument comparing the relative thermodynamic favorability of forming the three binary compounds Rb₂Se, CaCl₂, and RbCl under the stated environmental conditions.
(a) For each compound, write the electron-transfer process that converts the elemental reactants into the gaseous ions that subsequently assemble into the solid lattice. Include balanced symbols for the elemental reactants and their corresponding ions.
(b) Discuss, in qualitative terms, the trends in first and (where relevant) second ionization energies for Rb and Ca, and the trends in electron affinity for Cl and Se, that influence the overall enthalpy change for the electron-transfer step of each compound.
(c) Evaluate how ionic charge magnitude and ionic radius together determine the lattice energy for each compound’s solid phase. Reference periodic positioning to support your conclusions.
(d) Synthesize your analyses from parts (b) and (c) into a coherent comparison that ranks the three compounds in order of increasing thermodynamic favorability of formation at 800 K and 0.7 atm.
(e) Given that elevated temperature can decrease lattice stability, explain whether and how your ranking in part (d) might differ if the same compounds formed at 298 K instead of 800 K. Provide a rationale rooted in solid-state energetics.