Which one of the given reactions indicates the formation reaction of the compound produced in the reaction?
(i) S (moochlinic,s)`+O_(3)(g)toSO_(3)(g)`
(ii) C (graphite, s)`+2H_(2)(g)toCH_(4)(g)`
(iii) `N_(2)(g) +O_(2)(g) to 2NO(g)`.

Explanation

(i) In the formation reaction of a compound, 1 mol of the compound is from its constituent element.
S(s,monoclinic)`+O_(2)(g)toSO_(2)(g)`, at `25^(@)C` this reaction does not represent the formation of `SO_(2)(g)` because,t he stable form of sulphur is S(s,rhombic) at `25^(@)C`.
(ii) C(s, graphite)`+2H_(2)(g)toCH_(4)(g)`, at `25^(@)C` this reaction represents the formation reaction of `CH_(4)(g)` because 1 mol of `CH_(4)(g)` is formed from its stable constituent elements.
(iii) `N_(2)(g)+O_(2)(g)to2NO(g)`. at `25^(@)C` this reaction does not represent the formation reaction of NO(g) because 2 mol of NO(g) are formed in the reaction.