Final answer:
The substance for which ∆hf0 = 0 is hydrogen.
Step-by-step explanation:
The substance for which ∆hf0 = 0 is C) Hydrogen.
The standard enthalpy of formation (∆Hf) is the change in enthalpy that occurs when one mole of a compound is formed from its elements in their standard states. By definition, the standard enthalpy of formation of any element in its standard state is zero.
Therefore, ∆Hf0 = 0 for all elements in their standard states, including hydrogen (H₂).
Hydrogen is the first element on the periodic table and the most abundant element in the universe. It is a nonmetal and the lightest element, with an atomic number of 1 and an atomic mass of 1.0079.
At standard temperature and pressure, hydrogen exists as a colorless, odorless, tasteless, non-toxic, and highly combustible gas. It is the most abundant element in the universe, making up about 75% of the mass of all ordinary matter. Hydrogen is also the most abundant element in the Earth's crust, but it is found in a combined state with other elements, such as in water (H2O) and hydrocarbons.