Question

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

• A. $Br_2(g)$
• B. $Cl_2(g)$
• C. $H_2O (g )$
• D. $CH_4(g)$

Answer 1

Answer: (B)

$Cl_2(g)$

Answer 2

Elements in its standard state have zero enthalpy of formation. $Cl_2$ is gas at room temperature, therefore $\Delta H^\circ_f$ of $Cl_2(g)$ is zero