Question

The species which by definition has zero standard molar enthalpy of formation at 298 K is:
A.${\rm{B}}{{\rm{r}}_{\rm{2}}}\left( g \right)$
B.${\rm{C}}{{\rm{l}}_{\rm{2}}}\left( g \right)$
C.${{\rm{H}}_{\rm{2}}}{\rm{O}}\left( g \right)$
D.${\rm{C}}{{\rm{H}}_{\rm{4}}}\left( g \right)$

Answer 1

Hint: We know that enthalpy of formation is the heat energy change that accompanies the formation of one mole of a compound from its constituting elements at a given temperature and pressure.

Complete step by step answer:
Let’s understand standard enthalpy of formation. The enthalpy of formation under standard conditions may be defined as the change in enthalpy that accompanies the formation of one mole of a substance under standard conditions of temperature (298 K) and pressure (1 atm).

Now, come to the question. We have to identify the species whose standard molar enthalpy of formation is zero at 298 K.

Option A is ${\rm{B}}{{\rm{r}}_{\rm{2}}}\left( g \right)$. We know that bromine exists in liquid state at room temperature (298 K). So, some amount of heat must be supplied to convert it into a gaseous state. So, standard molar enthalpy of formation is not zero.

Option B is ${\rm{C}}{{\rm{l}}_{\rm{2}}}\left( g \right)$. We know that chlorine exists in a gaseous state at room temperature. So, standard molar enthalpy of formation of ${\rm{C}}{{\rm{l}}_{\rm{2}}}\left( g \right)$ is zero.

Option C is ${{\rm{H}}_{\rm{2}}}{\rm{O}}\left( g \right)$. We know that water exists in a liquid state at room temperature. At $100^\circ {\rm{C}}$, water changes to vapour (gaseous state). So, heat must be supplied to convert water to gaseous state. So, standard molar enthalpy of formation of ${{\rm{H}}_{\rm{2}}}{\rm{O}}\left( g \right)$ is not zero.

Option D is ${\rm{C}}{{\rm{H}}_{\rm{4}}}\left( g \right)$. Methane gas exists in a gaseous state at room temperature but it is not a constituent element. It forms when carbon undergoes reaction with hydrogen. So, standard molar enthalpy of formation of ${\rm{C}}{{\rm{H}}_{\rm{4}}}\left( g \right)$ is not zero.

So, the correct answer is Option B.

Note: It is to be noted that enthalpies of formation of all free elements in their standard states are arbitrarily assumed to be zero. The standard state of an element refers to its purest and most stable state at 298 K and 1 atm pressure.