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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.


marked as duplicate by Gaurang Tandon, Community Apr 18 '18 at 5:12

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  • $\begingroup$ You grab a shovelful of your compound, it reacts and gives you some kJ, you measure those and write the number down for later use. Next time you have a whole dump truck of the same compound. Will the number of kJ be the same? $\endgroup$ – Ivan Neretin Apr 18 '18 at 5:12
  • $\begingroup$ I agree with your logic, but enthalpy change of formation itself describes me taking only a mole in my shovelful of compound (as far as I know). $\endgroup$ – Supernova Apr 18 '18 at 5:24

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