Chemistry: why equilibrium persists

The tutor explains why a chemical system will exist at equilibrium.

Over my last couple of posts I’ve defined reaction and equilibrium. I’ve also explained why a reaction will proceed spontaneously. (For background see here and here.)

Today, the focus is equilibrium: why a given chemical system may not decisively transform in one direction, but instead persist in two complementary forms. Consider, by example, the following equilibrium:

3H2+N2 ⇋ 2NH3, ΔH=-92kJ/mol

Recall, from my previous post, Nature’s two preferred trends:

  1. Entropy increasing.
  2. Enthalpy decreasing.

High entropy favours the left side of the equilibrium above, since it has 4 molecules, as opposed to only 2 on the right. However, low enthalpy favours the right side, denoted by negative ΔH.

With one of Nature’s trends driving to the left, while the other is driving to the right, the system remains undecided; the molecules constantly break down and reform from side to side. Left alone, a stable proportion of each will persist, while the individual molecules constantly change between forms. Such is an equilibrium.


Hebden, James A. Chemistry: Theory and Problems, book 2. Toronto: McGraw-Hill Ryerson, 1980.

Mortimer, Charles E. Chemistry, 6th ed. Belmont: Wadsworth, 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Tagged with: , , , , ,

Leave a Reply