Question

What is the standard enthalpy change of formation of hydrogen gas (H2) when one mole of hydrogen gas is formed from its elements under standard conditions, given that the standard enthalpy change of formation for water (H2O) is -285.8 kJ/mol?

Answer 1

The standard enthalpy change of formation for an element in its standard state is always zero. Hydrogen gas  H2  is an element in its standard state, so its standard enthalpy change of formation is 0 kJ/mol.