Q6 / 84 · JEE Advanced 2010 (Paper 1) · Chemistry

The species which by definition has zero standard molar enthalpy of formation at $298 \mathrm{~K}$ is

  • A
    $\mathrm{Br}_2(g)$
  • Correct answer
    B
    $\mathrm{Cl}_2(g)$
  • C
    $\mathrm{H}_2 \mathrm{O}(g)$
  • D
    $\mathrm{CH}_4(g)$

Solution

$\Delta H_f^{\circ}\left(\mathrm{Cl}_2, g\right)=0$ $\Delta H_f^{\circ}$ of elements in their standard state is taken to be zero. $\mathrm{Cl}_2$ is gas, $\mathrm{Br}_2$ is liquid and $\mathrm{I}_2$ is solid at room temperature. Energetics Conceptual, implication of rules/laws III

Hence, the correct answer is option: 'B'.

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