Unit 7 Mini Quiz 1

1. 2019 AP Chemistry FRQ 2 – Decomposition of BrCl

Question: 2BrCl(g) ⇌ Br₂(g) + Cl₂(g)

• (a) Calculate the initial pressure of BrCl using PV = nRT.
• (b) Write the expression for K_p.
• (c) Calculate K_p using equilibrium pressures.
• (d) Estimate ΔH using bond energies.

🟠 Answer:

• (a) P = nRT/V
• (b) K_p = (P_Br₂ × P_Cl₂) / (P_BrCl)²
• (c) Plug values into K_p expression
• (d) ΔH = [2 × D(Br–Cl)] – [D(Br–Br) + D(Cl–Cl)]

✅ Explanation:

At equilibrium, the rate of the forward reaction (decomposition of BrCl) equals the rate of the reverse reaction (recombination of Br₂ and Cl₂). This means the amounts of all species remain constant over time.

K_p is used when dealing with partial pressures. It relates product and reactant gases. If the partial pressures are known at equilibrium, they can be substituted into the expression to calculate the equilibrium constant.

The enthalpy change of the reaction is found by comparing the total bond energy needed to break the bonds in the reactants with the energy released from forming bonds in the products.

2. 2014 AP Chemistry FRQ 1 – N₂O₄ ⇌ 2NO₂

Question: N₂O₄(g) ⇌ 2NO₂(g)

• (a) Calculate K_c using equilibrium concentrations.
• (b) Predict direction if volume decreases.
• (c) Predict shift when temperature increases.

🟠 Answer:

• (a) K_c = [NO₂]² / [N₂O₄]
• (b) Volume ↓ → pressure ↑ → equilibrium shifts to left (fewer moles)
• (c) If ΔH > 0 (endothermic), ↑T shifts right; if ΔH < 0, shifts left

✅ Explanation:

The equilibrium constant K_c relates the concentrations of products and reactants at equilibrium. If the volume is decreased, the system responds by shifting toward the side with fewer gas molecules to reduce pressure.

Temperature changes affect equilibrium depending on the reaction's enthalpy. If the reaction is endothermic, increasing temperature favors the forward reaction (more NO₂). If it's exothermic, equilibrium shifts to the left.

3. 2008 AP Chemistry FRQ 1 – H₂ + I₂ ⇌ 2HI

Question: H₂(g) + I₂(g) ⇌ 2HI(g)

• (a) Calculate K_c using given concentrations.
• (b) Calculate ΔG° using K.
• (c) Predict effect of changing pressure or temperature.

🟠 Answer:

• (a) K_c = [HI]² / ([H₂][I₂])
• (b) ΔG° = –RT ln K
• (c) Use Le Chatelier’s Principle to predict direction

✅ Explanation:

The equilibrium constant tells us how far the reaction proceeds. ΔG° is related to K and determines whether the reaction is spontaneous. A negative ΔG° indicates a spontaneous forward reaction.

Changes in pressure or temperature shift the position of equilibrium. For this reaction, since the number of moles of gas is the same on both sides, pressure changes have no effect. Temperature shift depends on whether the reaction is endothermic or exothermic.

4. 2002 AP Chemistry FRQ 2 – Solubility of Ca(OH)₂

Question: Ca(OH)₂(s) ⇌ Ca²⁺(aq) + 2OH⁻(aq)

• (a) Write the K_sp expression.
• (b) Let molar solubility = s and calculate K_sp.
• (c) Predict effect of adding NaOH to the system.

🟠 Answer:

• (a) K_sp = [Ca²⁺][OH⁻]²
• (b) [Ca²⁺] = s, [OH⁻] = 2s → K_sp = 4s³
• (c) Adding OH⁻ shifts equilibrium left → solubility decreases

✅ Explanation:

The K_sp expression represents the product of the concentrations of the ions in a saturated solution of a sparingly soluble salt.

If you let solubility = s, the ion concentrations can be expressed in terms of s and substituted into the K_sp formula.