7.8 Representations

Representations of Equilibrium

Two particulate boxes, Initial and At Equilibrium. Drag an extent slider to convert reactant dots into products. K_c updates live from the counts across 4 reactions

Particulate Model4 ReactionsK_c from CountsExtent Slider
Topic 7.8

Representations of Equilibrium

Represent a system undergoing a reversible reaction with a particulate model.

Particulate representations describe the relative numbers of reactant and product particles present prior to and at equilibrium, and can be used to determine the value of the equilibrium constant.

Reading a particulate diagram to find K. Count the particles of each species in the equilibrium box, convert to concentrations by dividing by the container volume, and substitute into the K expression. If the volume is not given, you can still get a relative sense of K's magnitude — mostly product particles means K > 1.

Drawing a particulate diagram from K:

  • K ≫ 1 → draw mostly product particles with only a few reactants.
  • K ≈ 1 → draw comparable numbers.
  • K ≪ 1 → draw mostly reactants with only a trace of product.

Rules that carry points:

  • Conserve atoms between the "before" and "at equilibrium" boxes.
  • Both species must be present at equilibrium. A diagram with zero reactant particles is not an equilibrium diagram — it depicts a completed reaction.
  • Respect the stoichiometry. For A₂ + B₂ ⇌ 2 AB, forming two AB molecules consumes one A₂ and one B₂.

Comparing two systems. A common exam task shows two boxes at equilibrium and asks which has the larger K. Count the ratio of product to reactant particles in each — no arithmetic required beyond that.

Key points

  • Count particles at equilibrium, convert to concentrations, and substitute to find K.
  • Both reactants and products must appear in an equilibrium diagram.
  • Atoms are conserved between the before and equilibrium boxes.
  • Relative particle counts are a direct visual proxy for the magnitude of K.

Common mistakes

  • Drawing zero reactant particles. That is completion, not equilibrium.
  • Losing or inventing atoms.
  • Forgetting to divide by volume when the question gives one.
  • Ignoring exponents in K when a coefficient is greater than 1.

Worked example

A 1.00 L container at equilibrium for A₂(g) + B₂(g) ⇌ 2 AB(g) contains 2 A₂ molecules, 2 B₂ molecules, and 6 AB molecules. Each particle drawn represents 0.10 mol. Calculate Kc.

Convert particle counts to concentrations (V = 1.00 L, each particle = 0.10 mol):
[A₂] = 2 × 0.10 = 0.20 M
[B₂] = 2 × 0.10 = 0.20 M
[AB] = 6 × 0.10 = 0.60 M

K expression: Kc = [AB]² / ([A₂][B₂])

Kc = (0.60)² / [(0.20)(0.20)] = 0.36 / 0.040 = 9.0

K > 1, consistent with the diagram showing more product particles than reactant particles.

Full notes for topic 7.8 →