Short on time? This page is the recap. For explanations, a concentration-time graph and worked examples, use the full study guide.
Recap
- K is found from experimental measurements taken at equilibrium, when the concentrations or partial pressures have stopped changing.
- Method: balanced equation → expression (no solids or pure liquids) → equilibrium values in the right units → substitute → K with no unit.
- If you know the starting amounts and one equilibrium value, an ICE table gives the rest. The change row follows the coefficients.
- At a fixed temperature, every experiment gives the same K, whatever the starting amounts.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| c = n / V | concentration in mol L⁻¹ from moles and volume in L | turn moles into what Kc needs |
| Change ratio = coefficient ratio | if C rises by x and its coefficient is 1, a species with coefficient 4 changes by 4x | fill the C row of an ICE table |
| P_X = (fraction of gas moles that are X) × P_total | partial pressure from a total pressure | turn a total pressure into what Kp needs |
| K = expression with equilibrium values | Kc from [X]ₑ, Kp from P_X at equilibrium | calculate K |
Assumptions behind the numbers
- The readings were taken at equilibrium (constant over time).
- The temperature is constant throughout and is the same in every experiment you compare.
- The species measured is the only source of the signal (for example, only one species absorbs light at the chosen wavelength).
Mistakes to avoid
- Putting starting values into K.
- Forgetting to divide moles by the volume.
- Ignoring coefficients in the change row (H₂ in CH₄ + 2H₂S ⇌ CS₂ + 4H₂ changes 4 times as much as CS₂).
- Wrong signs: reactants go down, products go up for a forward reaction.
- Splitting a total pressure equally between gases made in a 2 : 1 ratio.
- Including a solid or giving K a unit.
Quick self-check
- For A(g) + B(g) ⇌ C(g), the equilibrium concentrations are [A] = 0.20, [B] = 0.50 and [C] = 0.40 mol L⁻¹. Find Kc. (0.40 ÷ (0.20 × 0.50) = 4.0)
- A 3.00 L vessel holds 0.60 mol of a gas at equilibrium. What concentration goes into Kc? (0.20 mol L⁻¹)
- For 2SO₃(g) ⇌ 2SO₂(g) + O₂(g), [SO₃] starts at 0.100 mol L⁻¹ with no products. At equilibrium [O₂] = 0.020 mol L⁻¹. Find Kc. ([SO₂] = 0.040, [SO₃] = 0.060; Kc = (0.040)² × 0.020 ÷ (0.060)² = 8.9 × 10⁻³)
Next: practice questions.