Question

Write the reaction which has a heat of reaction equal to heat of formation for $HCl(g)$?

Answer 1

We know that the amount of heat that should be added or removed during a chemical reaction to keep all the substances at the same temperature is called the heat of reaction. The heat of formation of a chemical reaction is the amount of heat that is absorbed or evolved when a mole of a compound is formed from elements, each element being in its normal physical state.

Complete answer:
One molecule of hydrogen chloride has one atom of $H$and one atom of $Cl$. We know that hydrogen and chlorine exist as diatomic elements in their standard state. So, the balanced chemical reaction can be written as follows:
${H_2}(g) + C{l_2}(g) \to 2HCl(g)$
For the standard enthalpy change of formation to be equal to the enthalpy change of reaction, we have to divide the chemical reaction by $2$. Then the resulting equation is as follows:
$\dfrac{1}{2}{H_2}(g) + \dfrac{1}{2}C{l_2}(g) \to HCl(g)$
This chemical equation has $\Delta H_{{\text{rxn}}}^ \circ = \Delta H_f^ \circ$ as it shows the formation of one mole of hydrogen chloride from hydrogen and chlorine.
Therefore, the reaction having a heat of reaction equal to heat of formation for $HCl(g)$ is $\dfrac{1}{2}{H_2}(g) + \dfrac{1}{2}C{l_2}(g) \to HCl(g)$.

Note:
The standard heat of formation of a compound generally represents the change in enthalpy when one mole of that compound is formed from its constituent elements in their standard state. The change in enthalpy of a chemical reaction describing the formation of one mole of hydrogen chloride from its constituent elements in their standard state is equal to $\Delta H_f^ \circ$.