AP Chemistry · Handsworth Secondary 2026–27
Unit 7 · Study Guide
Dr. Ras Mulinta
Equilibrium
Exam-focused review
A one-page map of what the Unit 7 test (and the AP exam) expects. This is a checklist, not a re-teach, if a line doesn't click, go back to that section of the notes package. A periodic table and Ksp values are provided on the test.
Must be able to do
- 7.1–7.2 Explain dynamic equilibrium (equal forward/reverse rates, constant amounts) and link reaction direction to which rate is larger; read concentration- and rate-vs-time graphs.
- 7.3 Write Kc and Kp from a balanced equation, products over reactants, each to its coefficient; omit pure solids and liquids.
- 7.4–7.5 Calculate K from equilibrium concentrations/pressures; read magnitude (large K → products favored, small K → reactants favored).
- 7.6 Manipulate K: reverse → 1/K, scale coefficients ×c → Kᶜ, add reactions → multiply K's.
- 7.7 Compute Q and compare to K (Q<K forward, Q>K reverse); solve ICE tables, using the small-x approximation when K is tiny and checking the 5% rule.
- 7.8–7.10 Apply Le Châtelier to concentration, pressure/volume, and temperature stresses; know that concentration/pressure changes shift Q only while temperature changes K itself.
- 7.11–7.12 Relate Ksp ↔ molar solubility; apply the common-ion effect (a shared ion lowers solubility).
The big idea that ties it together
Q chasing K is the engine of the whole unit. K is just the value of the mass-action ratio at equilibrium; Q is that same ratio at any moment. Every behaviour here, direction of reaction, ICE algebra, Le Châtelier shifts, even solubility and the common-ion effect, is the system moving Q until it equals K. The one thing that actually moves K is a change in temperature; everything else only moves Q.
Don't waste time on (excluded by the CED)
Converting between Kc and Kp (no K_p = K_c(RT)^Δn) · equilibria between a dissolved species and that same species as a gas · the effect of adding an inert gas at constant volume (no shift). Know whether a question is in concentrations or pressures, and reason qualitatively, skip these edge cases.
Quick self-check (answer in your head, then verify)
- For 2NOBr(g) ⇌ 2NO(g) + Br₂(g), equilibrium values are [NOBr] = 0.50 M, [NO] = 0.40 M, [Br₂] = 0.20 M. Find Kc.
- For A + B ⇌ C, K = 10. A mixture has [A] = [B] = [C] = 1.0 M. Find Q; which way does it shift?
- If K = 2.5×10³ for the forward reaction, what is K for the reverse reaction?
- For X ⇌ Y + Z, Kc = 2.5×10⁻⁵, [X]₀ = 0.40 M. Use the small-x approximation to find [Y]. Is the approximation valid?
- Find the molar solubility of Ag₂CrO₄, Ksp = 1.1×10⁻¹². (Set Ksp = 4s³.)
- Find the molar solubility of BaSO₄ (Ksp = 1.1×10⁻¹⁰) in 0.10 M Na₂SO₄.
- An exothermic reaction is at equilibrium. You cool it. Which way does it shift, and does K increase or decrease?
Check yourself: 1) Kc = (0.40)²(0.20)/(0.50)² = 0.128 2) Q = (1.0)/((1.0)(1.0)) = 1.0 < 10, so shifts forward 3) 1/(2.5×10³) = 4.0×10⁻⁴ 4) x²/0.40 ≈ 2.5×10⁻⁵ → x = √(1.0×10⁻⁵) = 3.2×10⁻³ M = [Y]; 3.2×10⁻³/0.40 = 0.79% < 5% ✓ valid 5) Ag₂CrO₄ ⇌ 2Ag⁺ + CrO₄²⁻, Ksp = (2s)²(s) = 4s³ = 1.1×10⁻¹² → s = ∛(2.75×10⁻¹³) = 6.5×10⁻⁵ M 6) [SO₄²⁻] ≈ 0.10, so s = Ksp/0.10 = 1.1×10⁻⁹ M 7) Heat is a product; removing heat shifts right (toward products) and K increases.