The standard enthalpy of formation of O2(g) in its standard state is defined as what?

• A. 0 kJ/mol
• B. 1 kJ/mol
• C. -10 kJ/mol
• D. 100 kJ/mol

Answer: (A)

The main idea is the convention for standard enthalpy of formation. ΔHf° is defined as the enthalpy change when 1 mole of a substance is formed from its elements in their standard states. For an element in its standard state, this value is defined as zero. Oxygen in its standard state at 1 bar and the usual reference temperature is O2 gas, so forming O2 from its elements in their standard states requires no energy change. That’s why the standard enthalpy of formation of O2(g) is zero. This convention provides a consistent reference point for calculating enthalpies of reactions.