Question
For which one of the following equations $$\Delta H_r^ \circ $$ equal to $$\Delta H_f^ \circ $$ for the product?
A.
$$Xe\left( g \right) + 2{F_2}\left( g \right) \to Xe{F_4}\left( g \right)$$
B.
$$2CO\left( g \right) + {O_2}\left( g \right) \to 2C{O_2}\left( g \right)$$
C.
$${N_2}\left( g \right) + {O_3}\left( g \right) \to {N_2}{O_3}\left( g \right)$$
D.
$$C{H_4}\left( g \right) + 2C{l_2}\left( g \right) \to C{H_2}C{l_2}\left( l \right) + 2HCl\left( g \right)$$
Answer :
$$Xe\left( g \right) + 2{F_2}\left( g \right) \to Xe{F_4}\left( g \right)$$
Solution :
When one mole of a substance is directly formed from its constituent elements, then the enthalpy change is called heat of formation.
For the reaction,
$$\eqalign{
& Xe\left( g \right) + 2{F_2}\left( g \right) \to \mathop {Xe{F_4}\left( g \right)}\limits_{1\,\,mol} \cr
& \Delta {H^ \circ }_{{\text{react}}} = \Delta H_f^ \circ \cr} $$