AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
1 Atomic Structure and Properties 2 Compound Structure and Properties 3 Properties of Substances and Mixtures 4 Chemical Reactions 5 Kinetics 6 Thermochemistry 7 Equilibrium 8 Acids and Bases 9 Thermodynamics and Electrochemistry

Equilibrium

7.1 Introduction to Equilibrium 7.2 Direction of Reversible Reactions 7.3 Reaction Quotient and Equilibrium Constant 7.4 Calculating the Equilibrium Constant 7.5 Magnitude of the Equilibrium Constant 7.6 Properties of the Equilibrium Constant 7.7 Calculating Equilibrium Concentrations 7.8 Representations of Equilibrium 7.9 Introduction to Le Châtelier’s Principle 7.10 Reaction Quotient and Le Châtelier’s Principle 7.11 Introduction to Solubility Equilibria 7.12 Common-Ion Effect

Representations of Equilibrium

Learning Objective 7.8.A Represent a system undergoing a reversible reaction with a particulate model.

Quick Notes

  • Particulate diagrams visualize chemical systems at the particle level.
  • At equilibrium, both reactants and products are present; their relative amounts reflect the value of K.
  • K ≫ 1: mostly product particles at equilibrium.
  • K ≪ 1: mostly reactant particles at equilibrium.
  • K ≈ 1: similar amounts of reactants and products.

Full Notes

Particulate Models at Equilibrium

Particulate representations (dot diagrams or molecular illustrations) can be used to help show the proportions of reactant and product particles, the progress of a reversible reaction toward equilibrium, and the magnitude of the equilibrium constant K based on relative quantities of species.

Interpreting Equilibrium with Particles

Before equilibrium: typically more reactant particles than products (or vice versa, depending on the starting conditions). The reaction proceeds in the direction that reduces the imbalance between reactants and products until equilibrium is reached. See Q and K for more.

AP Chemistry Particulate model showing mostly reactant particles A and B before equilibrium is reached

At equilibrium: there is no net change in particle numbers. Both reactants and products are present, and their ratio matches the equilibrium expression for the reaction and the value of K (Kc or Kp).

AP Chemistry Particulate diagram with more products than reactants at equilibrium, indicating K greater than 1
AP Chemistry Particulate diagram with more reactants than products at equilibrium, indicating K less than 1
Particulate diagram with similar amounts of reactants and products at equilibrium, indicating K about 1

Value of K and Particle Ratios

Photo of Matt
Matt’s Exam Tip

When reading particulate diagrams, count species and relate them to the balanced equation. The ratio of products to reactants at equilibrium reflects the magnitude of K—not the drawing scale or particle size.

Summary