What is the standard enthalpy change of formation for a pure element in its standard state?

What is the standard enthalpy change of formation for a pure element in its standard state?

• A. Zero
• B. Positive
• C. Negative
• D. It depends on the element

Answer: (A)

The standard enthalpy of formation uses a zero reference for elements in their standard states. When you form 1 mole of a pure element from its elements in their standard states, nothing chemically changes—the element is already in its standard state at 1 atm and 298 K—so no heat is absorbed or released. That’s why this enthalpy change is defined as zero for any element in its standard state, providing a consistent baseline for comparing formation enthalpies. Examples like H2, O2, N2, and carbon as graphite all have ΔHf° = 0.

The standard enthalpy of formation uses a zero reference for elements in their standard states. When you form 1 mole of a pure element from its elements in their standard states, nothing chemically changes—the element is already in its standard state at 1 atm and 298 K—so no heat is absorbed or released. That’s why this enthalpy change is defined as zero for any element in its standard state, providing a consistent baseline for comparing formation enthalpies. Examples like H2, O2, N2, and carbon as graphite all have ΔHf° = 0.