In an exothermic reaction, heat is evolved, and system loses heat to the surrounding. For such system

Consider the following reaction between zinc and oxygen and choose the correct options out of the options given below : 

$2 \mathrm{Zn} \left(\mathrm{s}\right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)\to 2 \mathrm{ZnO} \left(\mathrm{s}\right) ; ∆\mathrm{H} = – 693.8 \mathrm{kJ} {\mathrm{mol}}^{-1}$.

Standard molar enthalpy of formation, ${∆}_{\mathrm{f}} {\mathrm{H}}^{\circ }$ is just a special case of enthalpy of reaction, ${∆}_{\mathrm{r}} {\mathrm{H}}^{\circ }$. Is the ${∆}_{\mathrm{r}} {\mathrm{H}}^{\circ }$ for the following reaction same as ${∆}_{\mathrm{f}} {\mathrm{H}}^{\circ }$? Give reason for your answer. 

$\mathrm{CaO}\left(\mathrm{s}\right) + {\mathrm{CO}}_2\left(\mathrm{g}\right) \to {\mathrm{CaCO}}_3\left(\mathrm{s}\right); {∆}_{\mathrm{f}} {\mathrm{H}}^{\circ } = –178.3 \mathrm{kJ} {\mathrm{mol}}^{-1}$. 

The value of ${△}_{\mathrm{f}}{\mathrm{H}}^{\circ }$ for ${\mathrm{NH}}_3$ is $– 91.8 \mathrm{kJ} {\mathrm{mol}}^{-1}$. Calculate the enthalpy change for the following reaction : 

$2{\mathrm{NH}}_3\left(\mathrm{g}\right) \to {\mathrm{N}}_2\left(\mathrm{g}\right) + 3{\mathrm{H}}_2\left(\mathrm{g}\right)$