Question

Choose all the molecules with zero standard-enthalpy-of-formation.

A) CO2
B) O2
C) N2
D) H2
E) CH4

Answer 1

O₂, H₂, and graphite have a zero standard-enthalpy-of-formation.

Step-by-step explanation:

The molecules with zero standard-enthalpy-of-formation are O₂, H₂, and graphite.

In chemical reactions, the standard enthalpy of formation represents the enthalpy change when one mole of a compound is formed from its elements in their standard states. By definition, the standard enthalpy of formation of an element in its most stable form is equal to zero.

For example, O₂ is the most stable form of oxygen at 1 atm pressure and 25°C, so its standard enthalpy of formation is zero. Similarly, H₂ is the most stable form of hydrogen, and graphite is the most stable form of carbon at those conditions.