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Why are the standard reaction enthalpies...

Why are the standard reaction enthalpies of the following two reactions different?
(1) C(graphite, s)`+O_(2)(g)toCO_(2)(g),DeltaH^(0)=-393.5kJ`
(2) C(diamond, s)`+O_(2)(g)toCO_(2)(g),DeltaH^(0)=-395.4kJ`

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Graphite and diamond are two allotropes of solid carbon. Different allotropic forms have different enthalpies. As the given two reactions involve different allotropes, the standard reaction enthalpies of these reactions are different.
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At 25^(@)C and 1 atm, the standard reaction enthalpy for the reaction C(graphite, s) +O_(2)(g)toCO_(2)(g) is the _____ of CO_(2)(g) .

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