Question

Choose the reaction(s) from the following options, for which the standard enthalpy of reaction is equal to the standard enthalpy of formation
This question has multiple correct options
(A) $\dfrac{1}{8}{{S}_{8}}(s)+{{O}_{2}}(g)\to S{{O}_{2}}(g)$
(B) $2{{H}_{2}}(g)+{{O}_{2}}(g)\to 2{{H}_{2}}{{O}_{(I)}}$
(C) $\dfrac{3}{2}{{O}_{2}}(g)\to {{O}_{3}}(g)$
(D) $2C(g)+3{{H}_{2}}(g)\to {{C}_{2}}{{H}_{6}}(g)$

Answer 1

The term standard enthalpy of formation of a compound is the change in the standard enthalpy when one mole of the compound is formed starting from requisite amount of constituent elements in standard state and standard enthalpy of reaction can be defined as the enthalpy change which occurs in a system when a chemical reaction transforms one mole of matter under specific standard conditions.

Complete step by step solution:
- As we know, the standard state of a material, be it a mixture, solution or pure substance, is a reference point used to calculate its properties under different conditions and the standard states for atomic elements are given in terms of the most stable allotrope for each element. Hence, they are used as reference points or standard states for calculations of various thermodynamic properties of these elements.
- The standard heat of formation, or standard enthalpy of formation, of a compound can be defined as the change in enthalpy which accompanies the formation of one mole of the compound from its elements in their standard states or their stable states of aggregation.
- The standard enthalpy of reaction can be defined as the enthalpy change which occurs in a system when a chemical reaction transforms one mole of matter under specific standard conditions. Among the given reactions the reactions $\dfrac{1}{8}{{S}_{8}}(s)+{{O}_{2}}(g)\to S{{O}_{2}}(g)$ and $\dfrac{3}{2}{{O}_{2}}(g)\to {{O}_{3}}(g)$ which the standard enthalpy of reaction is equal to the standard enthalpy of formation.

Therefore the answers are options (A) and (C).

Note: Keep in mind that the standard enthalpy of formation of any element in its most stable form is zero. Also, there should not be any confusion with the standard state and standard temperature and pressure (STP) for gases, or with the standard solutions used in analytical chemistry. Similarly, the standard enthalpies of formation are always measured in units of $KJmo{{l}^{-1}}$ of the compound formed.