Which of the following compounds is formed from its elements by an exothermic reaction at 298 K and 101.3 kPa?

• A. N2O (g)
• B. NaCl (s)
• C. H2O2 (l)
• D. NH3 (g)

Answer: (B)

Verified step by step guidance

1. Understand that the question asks which compound is formed from its elements by an exothermic reaction at standard conditions (298 K and 101.3 kPa). This relates to the compound's standard enthalpy of formation (\$\Delta H_f^\circ\$).
2. Recall that the standard enthalpy of formation (\$\Delta H_f^\circ\$) is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
3. Recognize that if \$\Delta H_f^\circ < 0\$, the formation reaction releases heat and is exothermic; if \$\Delta H_f^\circ > 0\$, the reaction absorbs heat and is endothermic.
4. Look up or recall the approximate standard enthalpies of formation for each compound: NaCl (s), N2O (g), H2O2 (l), and NH3 (g). Note that NaCl (s) has a strongly negative \$\Delta H_f^\circ\$, indicating an exothermic formation.
5. Conclude that the compound with a negative standard enthalpy of formation (exothermic formation) at 298 K and 101.3 kPa is NaCl (s).