Question

Choose the reaction(s) from the following options for which the standard enthalpy of reaction is equal to the standard enthalpy of formation:

• A. $2C\left(g\right)+3H_2\left(g\right)⟶C_2{H}_6\left(g\right)$
• B. $\frac{3}{2}{O}_2\left(g\right)⟶O_3\left(g\right)$
• C. $2H_2\left(g\right)+O_2\left(g\right)⟶2H_{2}O\left(\ell \right)$
• D. $\frac{1}{8}{S}_8\left(s\right)+O_2\left(g\right)⟶S{O}_2\left(g\right)$

Answer 1

Answer: (B), (D)

$\frac{1}{8}{S}_8\left(s\right)+O_2\left(g\right)⟶S{O}_2\left(g\right)$

Stanard enthalpy of formation is the change in enthalpy during the formation of 1 mole of a substance from its constituent element with all the substances in their standard states.
In option (a) carbon is not in its standard state and in option (c) two moles of $H_{2}O$ are being formed.
Thus only (b) and (d) are correct.