Question

Which of the following species by definition has ZERO standard Molar enthalpy of formation at 298 K?

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

Answer 1

The correct option is C

${\mathrm{Cl}}_2\left(\mathrm{g}\right)$

Explanation for correct option:

C. ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$

Enthalpy of formation:

• “Enthalpy change when 1 mole of a substance is formed from its elements when the elements are in their most stable states it is called standard enthalpy of formation.”
• All elements in their standard states have a standard enthalpy of formation of zero kilojoules per mole.
• Chlorine exists as a gas at room temperature.
• The standard enthalpy of formation of chlorine gas is ${∆}_{\mathrm{f}}{\mathrm{H}}^0=0\mathrm{kJ}{\mathrm{mol}}^{-1}$ as the element is already in its standard states.

Explanation for incorrect options:

A. ${\mathrm{Br}}_2\left(\mathrm{g}\right)$

• The standard enthalpy of formation of Bromine gas is ${∆}_{\mathrm{f}}{\mathrm{H}}^0=111.852\mathrm{kJ}{\mathrm{mol}}^{-1}$.

B. ${\mathrm{CH}}_4\left(\mathrm{g}\right)$

• The standard enthalpy of formation of Methane gas is ${∆}_{\mathrm{f}}{\mathrm{H}}^0=-74.87\mathrm{kJ}{\mathrm{mol}}^{-1}$.

D. ${\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$

• The standard enthalpy of formation of Methane gas is ${∆}_{\mathrm{f}}{\mathrm{H}}^0=243.0\mathrm{kJ}{\mathrm{mol}}^{-1}$.

Hence, the correct option is C i.e. ${\mathrm{Cl}}_2\left(\mathrm{g}\right)$.