The gaseous C2 molecule has a bond energy of 602 kJ mol^-1. Why isn't the standard heat of form

step 1 When we look at carbon and we describe its standard heat of formation, we are referring to carbo

The gaseous C2 molecule has a bond energy of 602 kJ mol^-1....

Key Concepts

Reference States and Allotropy

Carbon exists in multiple allotropes; its standard reference form is graphite, which is much more stable than gaseous or atomic carbon. When comparing bond dissociation energy to standard enthalpies, one must account for the different reference states. The energy required to convert graphite to isolated atomic carbon is considerably higher than half the bond energy of C?, as it includes energy contributions from breaking the extended network structure of graphite.

Bond Energy and Its Measurement

Bond energy refers to the energy required to homolytically break a bond in a molecule, yielding atoms each in their ground state. In the case of C?, the bond energy of 602 kJ/mol represents the energy needed to break the carbon–carbon bond, but it is measured in the context of destroying a molecular bond rather than forming isolated atoms from the element's standard state.

Standard Enthalpy of Formation

The standard enthalpy (or heat) of formation is the energy change when one mole of a substance is formed from its elements in their most stable forms under standard conditions. For carbon, the standard state is graphite, not the diatomic molecule C?, so the formation of gaseous atomic carbon involves additional energy steps, such as sublimation of graphite, which means its enthalpy of formation is not simply half the bond dissociation energy of C?.

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