How does the reaction, a + bc ⇌ ab + c, differ from the reaction, n2 + 3h2 ⇌ 2nh3 + energy, by which ammonia is synthesized?

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.

A is a single displacement reaction; B is a combination (synthesis) reaction.

Explanation:

The first reaction

A + BC → AB + C

This is a single displacement reaction, in which an element A displaces another element C from its compound BC.

The second reaction

N2 + 3H2 → 2NH3

The coefficients don't matter in determining the type of reaction, so we can rewrite this equation as

A + B → AB

It then becomes more obvious that this is a combination (synthesis) reaction, in which two elements combine to form a compound.