Unit 4: Chemical Equilibrium — Unit Test

Assessment OF Learning
Graded
Duration: 75 min | Total: /60
K/U
/15
Thinking
/15
Comm.
/15
Applic.
/15
Part A: Knowledge & Understanding [15]
Q1 [3]
Write Kc expressions for: (a) 2SO₂ + O₂ ⇌ 2SO₃; (b) CaCO₃(s) ⇌ CaO(s) + CO₂(g).
Q2 [3]
Define a buffer. List its two essential components.
Q3 [4]
Multiple choice — for HCN ⇌ H⁺ + CN⁻ with Ka = 6.2×10⁻¹⁰, the solution is:
Q4 [5]
Find pH of 0.020 M NaOH.
Part B: Thinking & Investigation [15]
Q5 [5]
ICE: 1.00 mol PCl₅ in 2.00 L → at eq, 0.50 mol PCl₃ formed. PCl₅ ⇌ PCl₃ + Cl₂. Find Kc.
Q6 [5]
Predict pH at equivalence for the titration of 25.0 mL of 0.100 M NH₃ (Kb = 1.8×10⁻⁵) with 0.100 M HCl.
Q7 [5]
Ksp(Ag₂CrO₄) = 1.1×10⁻¹². Find molar solubility.
Part C: Communication [15]
Q8 [5]
Sketch the titration curve of strong acid + strong base. Label initial pH, equivalence (pH and volume), and end of curve.
Q9 [5]
Explain how the Haber process applies Le Chatelier's principle in industrial NH₃ synthesis (T, P, catalyst).
Q10 [5]
Compare Brønsted-Lowry and Arrhenius acid-base theories. Give one example unique to Brønsted-Lowry.
Part D: Application [15]
Q11 [5]
Calculate pH of a buffer with 0.10 M CH₃COOH and 0.10 M CH₃COONa (Ka = 1.8×10⁻⁵). Use Henderson–Hasselbalch.
Q12 [5]
Ocean acidification: explain how rising CO₂ shifts CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻ and threatens calcite shells.
Q13 [5]
A blood buffer maintains pH ≈ 7.4. Identify the main buffer system and explain how it resists acid added during exercise.
Complete Answer Key

Q1. (a) [SO₃]²/([SO₂]²[O₂]); (b) [CO₂] (solids excluded).

Q2. Buffer = weak acid + its conjugate base (or weak base + conjugate acid).

Q3. Weakly acidic.

Q4. pOH = −log(0.020) = 1.70; pH = 12.30.

Q5. [PCl₅] = (1.00−0.50)/2 = 0.25; [PCl₃]=[Cl₂] = 0.25 M. Kc = (0.25)(0.25)/0.25 = 0.25.

Q6. At eq: 0.0500 M NH₄⁺ in 50 mL. Ka(NH₄⁺) = Kw/Kb = 5.6×10⁻¹⁰. x² = 5.6×10⁻¹⁰ × 0.050 = 2.8×10⁻¹¹ → x = 5.3×10⁻⁶ → pH ≈ 5.28.

Q7. Ag₂CrO₄ → 2Ag⁺ + CrO₄²⁻; Ksp = 4s³ = 1.1×10⁻¹² → s = (2.75×10⁻¹³)^(1/3) ≈ 6.5×10⁻⁵ M.

Q8. Curve: starts low pH (~1), gradual rise, sharp jump near eq (pH 7), levels at ~13. Eq vol = 25.0 mL.

Q9. N₂+3H₂⇌2NH₃, ΔH<0. ↑P shifts forward (fewer mol gas products); ↓T shifts forward (exo). Compromise ~450 °C, 200 atm + Fe catalyst (catalyst speeds equilibrium attainment, doesn't shift it).

Q10. Arrhenius: H⁺/OH⁻ in water. B-L: proton donor/acceptor (broader). Example: NH₃ + HCl → NH₄Cl (no water, but B-L).

Q11. pH = pKa + log([base]/[acid]) = 4.74 + log(1) = 4.74.

Q12. ↑CO₂ shifts forward → ↑[H⁺]; lowers pH. CO₃²⁻ decreases; CaCO₃ shells dissolve more readily, threatening corals/shellfish.

Q13. H₂CO₃/HCO₃⁻ buffer. Lactic acid added: H⁺ + HCO₃⁻ → H₂CO₃ → CO₂ exhaled, pH stays near 7.4.