Equilibrium: Le Chatelier's Principle and Q vs. K

• Exam consists of questions covering representative topics
• Exam is on chemistry
• Professor Bleeke is teaching
• Exam is worth 100 points

Exam Key Details

• The alotted time for the exam is 1.5 hours
• Exam includes 8 questions on 10 numbered pages
• Students are asked to write their student ID on each page of the exam
• The exam also includes a scratch page

Question 1a

• Considers the balanced chemical reaction of 2 ClO (g) = Cl (g) + ClO2 (g)
• K = 4.26 × 10−4 at 385 K
• 1.240 atm of ClO (g) and 0.330 atm of Cl (g) are sealed in a container at 385 K
• No ClO2 (g) is present initially
• Partial pressures of all three gases, ClO (g), Cl (g) and ClO2 (g) (in atm), after equilibrium is established
• Pclo = 1.236 atm
• Pc1= 0.332 atm
• Pc102= 0.00196 atm

Question 1b

• Considers the balanced chemical reaction: BrO (g) + ClO (g) = ClO2 (g) + Br (g)
• The equilibrium constant, K, for this reaction at 385 K is 0.040
• The reaction is endothermic
• Plot A depicts the reaction spontaneously proceeding in the forward direction to reach equilibrium at a temperature greater than 385 K
• Plot B depicts the reaction spontaneously proceeding in the forward direction to reach equilibrium at a temperature greater than 385 K
• Plot C depicts the reaction spontaneously proceeding in the reverse direction to reach equilibrium at a temperature equal to 385 K

Question 2

• Considers generic, balanced chemical reactions that are either endothermic or exothermic in the forward direction
• Each reaction is allowed to reach equilibrium
• Reactions are heated from T1, to T2
• Reaction shifts spontaneously in either the forward or the reverse direction to re-establish equilibrium.
• A) A(g) + 2 B(g) = 2 C(g) is endothermic proceeds in the forward direction
• B) A(g) + B(g) = 2 C(g) is exothermic proceeds in the reverse direction
• C) A(g) + 2 B(g) = C(g) + D(g) exothermic is ambiguous
• D) A(g) + B(g) = C(s) + 2 D(g) endothermic proceeds in the forward direction

Question 3

• The gas phase reaction of 4 NO (g) + 6 H2O (g) = 4 NH3 (g) + 5 O2 (g)
• K = 8.32 × 10-19 at temperature T
• 1.25 atm of NH3(g) and 1.25 atm of O2(g) are placed in a sealed container at temperature T
• No NO (g) or H2O (g) are initially present
• PNH3= 0.251 atm
• Po₂= 0.00119 atm

Question 4a

• The solid compound (NH4)2CO3 decomposes according to the reaction (NH4)2CO3 (s) = 2 NH3 (g) + CO2 (g) + H2O (g)
• Total pressure is 3.52 atm after equilibrium is established after the excess (NH4)2CO3 (s) is sealed in an evacuated container at 750 K
• K = 2.40

Question 4b

• The chemical reaction is at temperature T = 775 K
• The equilibrium constant K at 775 K is equal to 5.47
• When 1.40 atm of each gas are initially sealed in a container at 775 K, a calculation shows that the the Q (3.84) is less than K (5.47), which makes the reaction is spontaneous in the forward direction and no solid forms.
• Minimum of 0.27 atm of NH3 must be added

Question 5a

• A 0.40 M aqueous solution of HF is prepared at 25°C
• Ka (HF, 25°C) = 6.6 × 10-4
• pKa (HF, 25°C) = 3.18
• The percent dissociation is 3.98%

Question 5b

• Determine the number of moles of HClO4 that should be added to 600.0 mL of pure water at 25°C
• Solution should have same final pH as the 0.40 M aqueous solution of HF
• 0.00955 moles should be added

Question 5c

• Comparing a 600.0 mL sample of the 0.40 M aqueous solution of HF and the 600.0 mL sample of the HClO4 solution
• The [H3O⁺] is the same for both solutions after equilibrium is established for each solution
• The same number of moles of OH- would be required to completely neutralize each solution.
• Both solutions would have a pH of 7.00 after completely neutralizing the acids with OH

Question 6a

• Ka for the weak acid, hypochlorous acid, HClO, is 3.0 ×_10-8 (pKa=7.53) at 25°C
• The mass of solid potassium hypochlorite, K+ClO¯, that must be added to a 500 mL aqueous solution of 0.10 M HClO to prepare a buffer solution with pH = 8.00 is 13.4 grams

Question 6b

• The equilibrium chemical equation is ClO (aq) + H2O (l) = HClO (aq) + OH¯ (aq)

Question 6c

• 50 mL of a 0.20 M aqueous HCl solution is added to the buffer (from part a)
• The pH = 7.89

Question 6d

• The number of moles of KClO that must be added to the buffer solution in part (c) to bring it back to the original pH of 8.00 is 0.0391 mol

Question 7a

• 200.0 mL aqueous solution at 25°C contains a weak, monoprotic acid, HA
• Titrated with a 0.50 M aqueous solution of NaOH at 25°C
• 0.076 moles HA

Question 7b

• 0.076 moles of HA is converted to A- after adding of 152 mL NaOH
• Kb = 5.32 × 10-10

Question 7c

• The monoprotic weak acid, HA from the titration is HN3
• Ka = 1.9 × 10-5

Question 8a

• 500.0 mL of a 0.50 M aqueous solution of MgCl2 is added to 250.0 mL of a 0.50 M aqueous solution of NaF at 25° C
• A precipitate of the sparingly-soluble salt, MgF2, is formed
• MgCl2 and NaF are completely soluble in water to produce Mg2+, Cl-, Na+, and F− ions
• Ksp for MgF2 = 6.6 × 10-9 at 25°C
• MW (MgF2) = 62.30 g/mol
• 3.89 grams of MgF2 precipitates

Question 8b

• The mass of MgF2 that re-dissolves is 3.79 × 10-3 grams MgF2

Explore chemical equilibrium, focusing on Le Chatelier's principle, the reaction quotient Q, and the equilibrium constant K. Learn how pressure, temperature, and catalysts affect equilibrium position and constant values determining reaction direction.