Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- A particle diagram is a snapshot of a small, fixed volume. The count of each species is proportional to its concentration or partial pressure.
- In a sequence of boxes, the counts change while Q moves towards K. When the counts stop changing, Q = K and the system is at equilibrium.
- Particles in later equilibrium boxes sit in different places because reactions continue in both directions at equal rates. Equilibrium is dynamic.
- The same equilibrium box can be reached from mostly reactants or from mostly products, if the atoms, volume and temperature are the same.
- At the lab scale, colour, pressure and concentration stay constant because the particle-level changes cancel out.
Key relationships
| Idea | Rule | Use it to |
|---|---|---|
| Concentration from a box | c = (count × mol per particle) ÷ box volume | turn a picture into numbers for Kc |
| Counts straight into K | allowed only when both sides have the same total number of particles | save a step in reactions such as A₂ + B₂ ⇌ 2 AB |
| Q from a box | same form as K, using the counts (or concentrations) shown | decide which way the box will change |
| Mostly products at equilibrium | K very large | link a picture to Topic 7.5 |
| Mostly reactants at equilibrium | K very small | link a picture to Topic 7.5 |
Assumptions behind a diagram
- Every box in a sequence has the same volume and the same temperature.
- A box is a representative sample: its ratio matches the whole container.
- Very small amounts (less than one particle’s worth) cannot be drawn, so they look like zero.
Mistakes to avoid
- Equal counts ≠ equilibrium. Equilibrium means counts that no longer change.
- Using counts directly when the particle numbers differ on each side. Convert first, or K comes out wrong by a factor.
- Counting a joined pair as two particles. One J₂ is one particle.
- Breaking atom conservation. Count each element in every box you draw.
- Saying reactions stop at equilibrium. They continue at equal rates.
- No key. Use shapes or shading, not colour alone, and say what each symbol is.
Quick self-check
- For X₂ + Y₂ ⇌ 2 XY, an equilibrium box holds 2 X₂, 1 Y₂ and 4 XY. What is K? (4² ÷ (2 × 1) = 8.0; counts can be used directly)
- For 2 N ⇌ N₂, a 1.0 L box at equilibrium holds 2 N and 2 N₂, and each particle is 0.050 mol. What is Kc? ([N] = [N₂] = 0.10 mol L⁻¹, so Kc = 0.10 ÷ 0.10² = 10; raw counts would give 0.5)
- What must stay the same in every box of a sequence? (The number of atoms of each element, and the box volume)
Next: practice questions.