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(A) The standard enthalpy of formation $(\Delta_fH^\circ)$ is defined as the enthalpy change when $1 	ext{ mole}$ of a compound is formed from its co

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$CaO_{(s)} + CO_{2_{(g)}} 	o CaCO_{3_{(s)}}$

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(A) The standard enthalpy of formation $(\Delta_fH^\circ)$ is defined as the enthalpy change when $1 	ext{ mole}$ of a compound is formed from its constituent elements in their most stable standard states.In option $A$,$CaCO_3$ is formed from $CaO$ and $CO_2$,which are compounds,not elements. Therefore,this reaction represents the enthalpy of reaction,not the enthalpy of formation.In options $B$,$C$,and $D$,the products are formed from their constituent elements in their standard states ($Ca_{(s)}$,$C_{(graphite)}$,$O_{2_{(g)}}$,$H_{2_{(g)}}$,$S_{(rhombic)}$),which correctly represents the enthalpy of formation.

Enthalpy of formation is a special case of enthalpy of reaction. Which of the following reactions does $NOT$ represent the enthalpy of formation of the product?

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