Which statement is true about enthalpy of formation for elements in their standard state?

Which statement is true about enthalpy of formation for elements in their standard state?

• A. It is always zero
• B. It is always negative
• C. It is equal to the standard heat of formation of the element's oxide
• D. It is unknown

Answer: (A)

The key idea is that standard enthalpy of formation is defined for forming 1 mole of a substance from its elements in their standard states. An element in its standard state is already in its most stable form at 1 atm and 25°C, so forming that element from its own elements requires no energy change. By convention, this enthalpy change is zero for any element in its standard state. For example, the standard enthalpy of formation of O2, N2, graphite carbon, and H2 is zero. This is not about the energy of other reactions or compounds; it’s a definitional zero used to anchor tables of formation enthalpies.

The key idea is that standard enthalpy of formation is defined for forming 1 mole of a substance from its elements in their standard states. An element in its standard state is already in its most stable form at 1 atm and 25°C, so forming that element from its own elements requires no energy change. By convention, this enthalpy change is zero for any element in its standard state. For example, the standard enthalpy of formation of O2, N2, graphite carbon, and H2 is zero. This is not about the energy of other reactions or compounds; it’s a definitional zero used to anchor tables of formation enthalpies.