N₂+3H₂ ⇄2NH₃ is a thermochemical reaction whereas A+BC⇄AB is not.
A+BC⇄AB is a reaction of pure a element with a compound while N₂+3H₂ ⇄2NH₃ is a reaction between two pure elements.
Explanation:
Let A+BC⇄AB be equation i and N₂+3H₂ ⇄2NH₃ be equation ii.
The two reactions differ in that ii is a thermo-chemical reaction whereas i is not. This is because energy is included in reaction ii but not included in reaction i.
Also i is a reaction of pure a element with a compound while ii is a reaction between two pure elements. The compound is BC while the pure element is A.
Gold is actually very soft. If rings were made of pure gold they would bend and become disfigured. Other elements are added to give the ring its stability.