The standard enthalpy of formation of an element in its most stable form is
- A. always negative
- B. always positive
- C. zero by definition
- D. equal to its atomic mass
Explanation
Forming an element from itself involves no change, so the value is defined as zero and this provides the reference point from which all other enthalpies of formation are measured. The convention applies only to the most stable allotrope, so graphite is zero while diamond is not. Choosing an arbitrary zero is necessary because absolute enthalpies cannot be measured.