Which statement best describes the concept used for standard enthalpy of formation?

• A. The enthalpy change when a compound dissolves in water
• B. The enthalpy change when 1 mole of a compound forms from its elements in their standard states at 298 K and 1 atm
• C. The enthalpy change when a compound is formed from reactants in the gas phase at any pressure
• D. The enthalpy change when a compound is heated to 100°C

Answer: (B)

The main idea here is that standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their most stable forms at standard conditions (298 K and 1 atm). This definition uses the elements in their standard states and specifies forming exactly one mole of the compound, all at the standard temperature and pressure. That’s why the wording describing formation from the elements in their standard states at 298 K and 1 atm is the right fit.

Dissolving a compound in water describes a dissolution enthalpy, not formation from elements. Forming a compound from reactants in the gas phase at any pressure ignores the standard-state requirement and the fixed 298 K and 1 atm conditions. Heating a compound to 100°C is about changing its temperature, not about forming it from elements.