Question

Choose the reaction, for which the standard enthalpy of reaction is equal to the standard enthalpy of formation:

• A. $2{\mathrm{C}}_{}\left(\mathrm{g}\right)+3{\mathrm{H}}_2\left(\mathrm{g}\right)\to {\mathrm{C}}_2{\mathrm{H}}_6\left(\mathrm{g}\right)$
• B. $\frac{3}{2}{O}_2\left(\mathrm{g}\right)\to {\mathrm{O}}_3\left(\mathrm{g}\right)$
• C. $\frac{1}{8}{\mathrm{S}}_8\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{SO}}_2\left(\mathrm{g}\right)$
• D. $2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_{2_{}}\left(\mathrm{g}\right)\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$

Answer 1

Answer: (B), (C)

$\frac{1}{8}{\mathrm{S}}_8\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{SO}}_2\left(\mathrm{g}\right)$

The explanation for correct options:

(B) and (C):

Enthalpy change is the standard enthalpy of formation, which has been determined for a vast number of substances. In any general chemical reaction, the reactants undergo chemical changes and combine to give products. It can be represented by the following equation:

$\mathrm{Reactants}\to \mathrm{Products}$

The standard heat of formation or standard enthalpy of formation is the change in enthalpy that requires the formation of one mole of a compound from its constituents elements in their standard state.

(B) $\frac{3}{2}{O}_2\left(\mathrm{g}\right)\to {\mathrm{O}}_3\left(\mathrm{g}\right)$

Oxygen($O_2$) is in its standard state i.e., gaseous state.

(C) $\frac{1}{8}{\mathrm{S}}_8\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{SO}}_2\left(\mathrm{g}\right)$

Sulfur (${\mathrm{S}}_8$) and oxygen(${\mathrm{O}}_2$) are in their gaseous state i.e., solid and gaseous state respectively.

The explanation for incorrect options:

In the case of A and D, the product is formed from the constituent elements which are not present in their standard state.

Hence, B and C are the correct options.