Question

Which of the following substances has A H°= 0? (Select all answers that are correct.) а. СІ, (g) b. H,0 (1) с. Hg (1) d. O (g) е. С (5) ООООО

Answer 1

Enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its constituent elements, each existing in its standard state.

All compounds or elements which can exist in their standard states at standard pressure and temperature (STP) have a zero enthalpy of formation.

In the case of Halogens, the diatomic halogen compound is the stable one taken as reference having a zero enthalpy of formation. In the case of metals, they can exist independently, hence having a zero enthalpy of formation. Elements are taken as a reference while building up molecules out of them, hence, they also have a zero enthalpy of formation.

Therefore, Cl₂(g), Hg(l), O(g) and C(s) have a zero enthalpy of formation