In the standard enthalpy change of formation, the elements are in which state?

• A. The elements in their standard states.
• B. The elements in their ionic state in solution.
• C. The elements as gaseous molecules only.
• D. The elements as powders.

Answer: (A)

The key idea is that standard enthalpy change of formation is defined by forming one mole of a compound from its elements in their standard states, all at 298 K and 1 atm. The standard state is the most stable form of each element under those conditions, which for different elements can be a gas, a solid, or a liquid (for example, H2 and O2 gases; carbon as graphite solid; sulfur as solid S8). Because the reference forms are the standard states, the reactants in the formation process are these elemental forms, not ions, powders, or restricted to gaseous molecules. By convention, the enthalpy change for forming an element in its standard state from itself is zero, which provides a baseline for measuring formation enthalpies.