Final answer:
The enthalpy of the reaction for H2(g) is the same as its enthalpy of formation because the standard enthalpy of formation for any element in its most stable form is defined as zero.
Step-by-step explanation:
The enthalpy of the reaction is the same as the enthalpy of formation for H2(g) because the standard enthalpy of formation for any element in its most stable form, like hydrogen gas (H2), is defined to be zero under standard conditions. This is because the formation of a substance from the same substance involves no change, thus no enthalpy change.
By definition, formation reactions for elemental substances in their standard states, such as H2(g), O2(g), or C(graphite), have an enthalpy of formation of zero. This means there is no energy change when one mole of an element in its standard state forms from the elements in their standard states since the substance is already in its most stable form.