Which of the following reactions corresponds to the definition of enthalpy of formation at $298 \mathrm{~K}$ ?

Which of the following reactions corresponds to the definition of enthalpy of formation at $298 \mathrm{~K}$ ?
  1. $\mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) ightarrow \mathrm{H}_{2} \mathrm{O}(l)$
  2. $2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) ightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)$
  3. $\mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(l) ightarrow \mathrm{H}_{2} \mathrm{O}(l)$
  4. $\mathrm{H}_{2}(\mathrm{~g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{~g}) ightarrow \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$

Solution

In the enthalpy of formation, we have a stoichiometric number of the desired species equal to one and it appears as the product. Moreover, the reactants must be in a stable state of aggregation.

Asked in: JEE-TOPICTESTS-CHEMISTRY

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