Solutions, Kinetics, and Equilibrium
What Domain V Tests
This domain covers solution concentration and behavior, reaction rates, and chemical equilibrium, including acid–base and solubility equilibria. Questions ask you to calculate molarity or pH, interpret rate laws, apply Le Chatelier's principle, and reason about K, Q, and saturation.
Core Rules and Concepts
- Molarity M = moles solute / liters solution. Dilution conserves moles: M₁V₁ = M₂V₂.
- Solubility rules predict precipitation; Ksp describes the equilibrium for a sparingly soluble salt. Q compared with Ksp predicts whether a precipitate forms.
- Colligative properties depend on the number of dissolved particles: vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure. Electrolytes produce more particles per formula unit.
- Kinetics: rate = k[A]^m[B]^n. Reaction order is determined experimentally, not from coefficients. Increasing temperature increases the fraction of collisions with enough energy; catalysts lower activation energy without changing ΔH or equilibrium position.
- Equilibrium: at equilibrium, forward and reverse rates are equal and concentrations are constant. K expressions omit pure solids and liquids. If Q < K, reaction shifts right; if Q > K, it shifts left.
- Le Chatelier: changes in concentration, pressure, or temperature shift equilibrium. Only temperature changes alter the value of K.
Worked Example
10.0 mL of 0.50 M NaOH is diluted to 500.0 mL. What is the pH? Reasoning: moles OH⁻ = 0.0100 L × 0.50 mol/L = 0.0050 mol. New volume = 0.5000 L, so [OH⁻] = 0.0050 mol / 0.5000 L = 0.010 M. pOH = −log(0.010) = 2.00. pH = 14.00 − 2.00 = 12.00. This assumes NaOH is a strong base and dissociates completely; if it were a weak base, an ICE table with Kb would be needed.
Traps That Cost Marks
- Using milliliters instead of liters in molarity or dilution calculations.
- Treating a weak acid as if it dissociates completely; pH then requires Ka and an equilibrium calculation.
- Including solids or pure liquids in K expressions.
- Thinking a catalyst shifts equilibrium; it only helps the system reach equilibrium faster.
- Assuming a temperature change alters K in the same direction for all reactions; for exothermic reactions, increasing T decreases K.