The lattice energy of MgCl_2 is the energy change associated with which one of the following processes?

• A. The dissolution of solid MgCl_2 in water to form aqueous ions.
• B. The reaction of magnesium metal with chlorine gas to form solid MgCl_2.
• C. The conversion of solid MgCl_2 to gaseous MgCl_2 molecules.
• D. The formation of solid MgCl_2 from gaseous Mg^{2+} and Cl^- ions.

Answer: (D)

Verified step by step guidance

1. Understand that lattice energy is defined as the energy change when one mole of an ionic solid forms from its gaseous ions. It is a measure of the strength of the ionic bonds in the solid lattice.
2. Identify the process that involves gaseous ions combining to form a solid ionic compound. This corresponds to the formation of solid MgCl_2 from gaseous Mg^{2+} and Cl^- ions.
3. Recognize that the dissolution of solid MgCl_2 in water involves breaking the lattice and hydrating ions, which is related to hydration energy, not lattice energy.
4. Note that the reaction of magnesium metal with chlorine gas to form solid MgCl_2 is a synthesis reaction, not directly related to lattice energy.
5. Understand that converting solid MgCl_2 to gaseous MgCl_2 molecules involves breaking the ionic lattice into neutral molecules, which is different from lattice energy that involves ions.