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Enthalpy change of formation means the heat change when 1 mol of the compound is formed from its elements under standard condition. For example, the enthalpy change for the formation of carbon dioxide gas is -393.5 KJ/mol

But, I'm confused as the enthalpy change of any element under the standard condition is zero, so shouldn't the enthalpy change of formation of carbon dioxide is also zero as well?

I am missing something but I just couldn't figure out what it is

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  • $\begingroup$ You know what's an element, right? $\endgroup$ – Ivan Neretin Dec 6 '16 at 11:51
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$$\ce{C(graphite) + O2(g) -> CO2(g)}$$

$$\Delta H^{\varnothing} = -393.5\ \mathrm{kJ}\,\mathrm{mol}^{-1}$$

That is, the reaction is exothermic.

Since the reactants are elements in standard state, the heat of formation for carbon dioxide is the same as the enthalpy change of the above reaction.

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