This question already has an answer here:
A reasonable definition of standard enthalpy change of formation would be:
'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions'
Enthalpy change itself is understood as 'the exchange of energy between a chemical reaction and its surroundings at constant pressure.'
I am a little confused on what the units of enthalpy change should be. For example, for the enthalpy change of formation of water, this is shown: ΔHfo = -285.8 kJ mol-1
Now, if enthalpy change already contains 'one mole of a compound' in its definition, do we really need to include the per mole (mol-1) part in the enthalpy change? If enthalpy change is the change in just energy, how are the units kJ mol-1 and not just kJ ? I am sorry if I am overthinking here.