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.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.