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

Options

A. `2H_(2)(g) +O_(2)(g) to 2H_(2)O(l)`
B. `2C(g)+3H_(2)(g) to C_(2)H_(6)(g)`
C. `3/2 O_(2)(g) to O_(3)(g)`
D. `1/8 S_(8) (s)+O_(2) (g) to SO_(2) (g)`

Answer

C, D

Explanation

Standard enthalpy of formation `(triangle_(f) H^@)` is the change in enthalpy for formation of 1 mol of the compound from its elements present naturally in their most stable form. Standard enthalpy of reaction is the enthalpy required to transform 1 mol of matter by a chemical reaction. It is given by `triangle_(r) H^@=triangle_(f) H^@ ("products")-triangle_(f) H^@ ("reactants")`
Hence the reaction `3/2 O_(2)(g) to O_(3) (g) and 1/8 S_(8) O_(2) (g) to SO_(2) (g)` are those ones where enthalpy of reaction is equal to enthalpy of formation.