Question

The species which by definition has zero standard molar enthalpy of formation at 298 K is:

Answer 1

Explanation:

The standard molar enthalpy of formation of the element in its standard state is taken as zero. Since the standard state of Cl2 is gas, it's ΔfH=0. Bromine and water exist in liquid state at 298 K. Methane is not an elemental specie