For which one of the following equations is $\Delta_{\mathrm{r}} \mathrm{H}^{\circ}$ equal to…

For which one of the following equations is $\Delta_{\mathrm{r}} \mathrm{H}^{\circ}$ equal to $\Delta_{\mathrm{f}} \mathrm{H}^{\circ}$ ?
  1. $\mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{Cl}_{2}(\mathrm{~g}) ightarrow\mathrm{CH}_{2} \mathrm{Cl}_{2}(l)+2 \mathrm{HCI}(\mathrm{g})$
  2. $\mathrm{Xe}(\mathrm{g})+2 \mathrm{~F}_{2}(\mathrm{~g}) ightarrow \mathrm{XeF}_{4}(\mathrm{~g})$
  3. $2 \mathrm{CO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g}) ightarrow 2 \mathrm{CO}_{2}(\mathrm{~g})$
  4. $\mathrm{N}_{2}(\mathrm{~g})+\mathrm{O}_{3}(\mathrm{~g}) ightarrow \mathrm{N}_{2} \mathrm{O}_{3}(\mathrm{~g})$

Solution

The heat of formation is defined as the heat generated or absorbed when the compound is formed form its component elements in their standard state.
By this defination, $1$ and $3$ are clearly not correct as the starting materials are compounds.
$4$ is not correct, since ozone is not correct standard state of oxygen.
So, $2$ is the correct answer.

Asked in: JEE-TOPICTESTS-CHEMISTRY

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