For which substance is the standard heat of formation not equal to zero? Hydrogen (H2)

Aluminum oxide (Al2O3)

Oxygen (O2)

Potassium (K)

Aluminum oxide (Al₂O₃)

Explanation:

The standard heat (or enthalpy) of formation of a compound, is the amount of change in the enthalpy that takes place when 1 mole of the compound is produced from the elements it contains whereby the compounds and elements in the reaction are in their standard states

The standard state of an element, is the state in which the element is in the most stable form at a pressure of 1 bar

Therefore, the heat of formation of hydrogen, H₂, oxygen, O₂, and potassium, (K), will have a heat of formation of zero, while Al₂O₃ which is made of aluminum and oxygen in combining to form a compound in proportions which are different from their initial standard states will have a heat of formation not equal to zero