Which of the following represents the standard heat of formation reaction for sodium chloride (NaCl) in its solid form?

• A. Na(g) + Cl(g) ightarrow NaCl(s)
• B. Na^+(aq) + Cl^-(aq) ightarrow NaCl(s)
• C. NaCl(s) ightarrow Na^+(aq) + Cl^-(aq)
• D. Na(s) + rac{1}{2}Cl_2(g) ightarrow NaCl(s)

Answer: (D)

Verified step by step guidance

1. Understand that the standard heat of formation (\( \Delta H_f^\circ \)) of a compound is defined as the enthalpy change when one mole of the compound is formed from its elements in their standard states under standard conditions (usually 1 atm and 25°C).
2. Identify the standard states of the elements involved: for sodium (Na), the standard state is solid sodium (\( \text{Na}(s) \)); for chlorine (Cl), the standard state is diatomic chlorine gas (\( \text{Cl}_2(g) \)).
3. Write the formation reaction such that one mole of sodium chloride (\( \text{NaCl}(s) \)) is produced from these elements in their standard states. Since chlorine is diatomic, only half a mole of \( \text{Cl}_2(g) \) is needed to provide one mole of Cl atoms.
4. Express the balanced formation reaction as: \(\$\text{Na}(s) + \frac{1}{2} \text{Cl}_2(g) \rightarrow \text{NaCl}(s)\)
5. Recognize that other given reactions involve ions in aqueous solution or gaseous atoms, which are not the standard elemental forms; therefore, they do not represent the standard heat of formation reaction.