alleryStart learning →What is the standard enthalpy of formation of an element in its most stable form?
AIt equals plus 393.5 kJ per mole
BIt equals minus 393.5 kJ per mole
CIt equals the enthalpy of combustion
DIt equals zero by definition
Answer & Solution
Correct answer: D. It equals zero by definition
1. A formation reaction starts from elements in their most stable states.
2. For such an element, forming it from itself involves no change at all.
3. So its standard enthalpy of formation is defined as zero.
4. Graphite is the standard state of carbon, so graphite carries zero while diamond does not.
5. The value minus 393.5 kJ per mole belongs to CO2, a compound, not to an element.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_
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