AP Chemistry › Unit 7
Equilibrium is dynamic: the forward and reverse reactions keep going at the same rate. Unit 7 covers writing K and Q expressions, comparing Q to K to predict direction, calculating equilibrium concentrations, and using Le Chatelier's principle. Solubility equilibria, Ksp, and the common-ion effect close the unit. Exam questions often pair calculations with particle diagrams or concentration graphs. Students often miss what a large K means: the equilibrium mixture is mostly products. Some also put pure solids or liquids into the K expression, where they do not belong.
These are taken from the Regular set, with the answer and explanation shown.
1. Water left in an open beaker evaporates completely over a few days. The same amount of water in a sealed flask at the same temperature does not. Which best explains the difference?
Answer: D. In the sealed flask, vapor collects, so more and more vapor molecules return to the liquid. Once condensation matches evaporation, the amounts stop changing. In the open beaker the vapor escapes, so condensation never catches up.
Common trap: Choosing "the rising pressure stops evaporation." Evaporation keeps going. It is condensation that rises to match it.
2. For H₂(g) + I₂(g) ⇌ 2 HI(g), Kc = 49 at a certain temperature. A flask starts with [HI] = 0.90 M and no H₂ or I₂. What is [H₂] at equilibrium?
Answer: A. The reaction runs in reverse. Let [H₂] = [I₂] = x, so [HI] = 0.90 − 2x. Then (0.90 − 2x)² / x² = 49, so (0.90 − 2x) / x = 7. That gives 0.90 = 9x and x = 0.10 M.
Common trap: Answering 0.42 M by putting HI on the bottom of K. The expression stays [HI]² / ([H₂][I₂]) no matter where you start.
| Salt | Dissolving equation | Ksp at 25 °C |
|---|---|---|
| AgCl | AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) | 1.8 × 10⁻¹⁰ |
| Ag₂CrO₄ | Ag₂CrO₄(s) ⇌ 2 Ag⁺(aq) + CrO₄²⁻(aq) | 1.1 × 10⁻¹² |
| CaF₂ | CaF₂(s) ⇌ Ca²⁺(aq) + 2 F⁻(aq) | 3.5 × 10⁻¹¹ |
Ksp values at 25 °C, rounded to two significant figures from the CRC Handbook values listed by The Engineering ToolBox, Solubility Product Constants.
3. Using the table, what is [Ag⁺] in a saturated solution of Ag₂CrO₄ at 25 °C?
Answer: D. If s dissolves, [CrO₄²⁻] = s and [Ag⁺] = 2s. Ksp = (2s)²(s) = 4s³ = 1.1 × 10⁻¹², so s = 6.5 × 10⁻⁵ M and [Ag⁺] = 2s = 1.3 × 10⁻⁴ M.
Common trap: Answering 6.5 × 10⁻⁵ M, which is s. Each Ag₂CrO₄ releases two Ag⁺ ions.