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AP®ChemistryCollege BoardExam QuestionsUnit 7: EquilibriumEquilibrium Representation & Calculations

Equilibrium Representation & Calculations (College Board AP® Chemistry): Exam Questions

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Equilibrium Representation & Calculations

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1a2 marks

Ba2+ (aq) + EDTA4− (aq)  ⇌ Ba(EDTA)2− (aq)                         K = 7.7 x 107

The polyatomic ion C10H12N2O84− is commonly abbreviated as EDTA4−. The ion can form complexes with metal ions in aqueous solutions. A complex of EDTA4− with Ba2+ ion forms according to the equation above. A 50.0 mL volume of a solution that has an EDTA4− (aq) concentration of 0.30 M is mixed with 50.0 mL of 0.20 M Ba(N03)2 to produce 100.0 mL of solution.

Considering the value of K for the reaction, determine the concentration of Ba(EDTA)2− (aq)  in the 100.0 mL of solution. Justify your answer.

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1b2 marks

The solution is diluted with distilled water to a total volume of 1.00 L. After equilibrium has been reestablished, is the number of moles of Ba2+ (aq) present in the solution greater than, less than, or equal to the number of moles of Ba2+ (aq) present in the original solution before it was diluted? Justify your answer.

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2a1 mark

The following reaction takes place in a closed container at a constant temperature:

A (g) + B (g) ⇌ C (g)

The equilibrium constant is Kc = 2.50 at this temperature. An equilibrium mixture in a 1.00 L container is found to contain 0.40 mol of A, 0.10 mol of B, and 0.20 mol of C.

Write the expression for the equilibrium constant, Kc​, for the reaction.

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2b1 mark

Calculate the value of the reaction quotient, Qc​.

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2c2 marks

Predict whether the system is at equilibrium. If the system is not at equilibrium, describe what will happen to the concentration of C (g) as the system moves toward equilibrium. Justify your answer in terms of Qc​ and Kc​.

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3a1 mark

A mixture of gases in a sealed container is represented by the particulate diagram below. Each particle represents 1 molecule. Assume the reaction below is at equilibrium:

2D (g) ⇌ E (g)

(Assume the diagram shows 4 particles of D and 3 particles of E.)

Based on the particulate diagram, determine the ratio of molecules of D to E at equilibrium.

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3b1 mark

Write the expression for the equilibrium constant, Kc​, for the reaction.

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3c2 marks

The volume of the container is suddenly halved. Predict how the number of molecules of D at equilibrium will change. Justify your prediction in terms of the shift in equilibrium position.

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1a1 mark

A student mixes 0.500 mol of hydrogen gas and 0.500 mol of iodine gas in a 1.00 L flask at 700 K. The system reaches equilibrium according to the following reaction:

H2 (g) + I2 (g) ⇌ 2HI (g)

At equilibrium, the concentration of HI is measured to be 0.800 M.

Write the equilibrium constant Kc for the reaction.

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1b2 marks

Determine the equilibrium concentrations of H2 and I2.

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1c1 mark

Calculate the value of Kc​ for this equilibrium.

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2a1 mark

A chemist introduces 1.00 atm of dinitrogen tetroxide gas (N2O4) into a sealed 2.00 L container at a constant temperature of 298 K. The system reaches equilibrium according to the reaction:

N2O4 (g) ⇌ 2NO2 (g)

At equilibrium, the total pressure in the container is measured to be 1.36 atm.

Write the equilibrium constant, Kp, expression for this reaction.

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2b2 marks

i) Determine the equilibrium partial pressures of N2O44​ and NO2​.

ii) Calculate Kp for this reaction.

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2c1 mark

The equilibrium constant Kp​ for this reaction at 298 K is 4.66. Explain whether the system is at equilibrium.

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3a1 mark

A 0.680 mol sample of sulfur trioxide (SO3​) is introduced into a 3.04 L reaction vessel and allowed to reach equilibrium at temperature T. The decomposition reaction is:

2SO3 (g) ⇌ 2SO2 (g) + O2 (g)     ΔH = + 196 kJ mol-1

At equilibrium, 32% of the SO3​ has decomposed.

Write the equilibrium expression Kc​ for this reaction.

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3b1 mark

Calculate the equilibrium concentrations of all species.

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3c1 mark

Determine the value of Kc​, giving your answer to two significant figures.

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4a1 mark

A chemist investigates the decomposition of dinitrogen tetroxide in a closed reaction vessel:

N2O4 (g) ⇌ 2NO2 (g)

A sample of pure N2O4 is introduced into a 1.50 L reaction vessel at a specific temperature. At equilibrium, the system contains 0.280 moles of N2O4 and 0.440 moles of NO2​.

Write the expression for the equilibrium constant, Kc​, for this reaction.

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4b2 marks

Calculate the value of Kc​ at this temperature.

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4c1 mark

The volume of the reaction vessel is increased at constant temperature.

How does the reaction quotient Qc compare to the equilibrium constant Kc immediately after this change. Justify your answer.

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