The species which by definition has zero standard molar enthalpy of formation at $298 \mathrm{~K}$ is
The species which by definition has zero standard molar enthalpy of formation at $298 \mathrm{~K}$ is
$\mathrm{Br}_{2}(\mathrm{~g})$
$\mathrm{Cl}_{2}(\mathrm{~g})$
$\mathrm{H}_{2} \mathrm{O}(\mathrm{g})$
$\mathrm{CH}_{4}(\mathrm{~g})$
Solution
The species in its elemental form has zero standard molar enthalpy of formation at $298 \mathrm{~K}$. At $298 \mathrm{~K}, \mathrm{Cl}_{2}$ is gas while $\mathrm{Br}_{2}$ is liquid.
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