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

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 below.

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

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.

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.

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

A (g) + B (g) ⇌ C (g)

The equilibrium constant is K_c = 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, K_c​, for the reaction.

Calculate the value of the reaction quotient, Q_c​.

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 Q_c​ and K_c​.

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)

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

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

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.

At 700 K, carbon monoxide gas reacts reversibly with steam in a closed container according to the following balanced equation:

CO (g) + H₂O (g) ⇌ CO₂ (g) + H₂ (g)

An experiment is conducted where 0.400 mol of each species is placed in a 2.00 L container. The system reaches equilibrium at constant temperature.

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

Determine the initial concentrations (in mol L^-1) of all four species in the container.

At equilibrium, the concentration of H₂O (g) is found to be 0.160 mol L⁻¹. Calculate the equilibrium concentrations of the other three species.

Using your answers from parts (a) and (c), calculate the value of the equilibrium constant, K_c​, for the reaction.

A second reaction mixture is prepared at the same temperature with the following concentrations:

Determine whether the system will shift to the left, right, or remain unchanged. Justify your answer using a calculation.

The reaction vessel is opened briefly, allowing some H₂ gas to escape before being resealed. Explain how this change affects the concentrations of all four species after the system re-establishes equilibrium.

Chlorine gas reacts reversibly with iodine monochloride gas in a sealed container at constant pressure:

Cl₂ (g) + ICl (g) ⇌ ICl₃ (g)

This reaction is studied at two different temperatures. The equilibrium concentrations of each species are recorded for each trial in identical 2.00 L containers:

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

Calculate the value of K_c​ at 400 K.

Calculate the value of K_c​ at 500 K.

Determine whether the forward reaction is endothermic or exothermic. Justify your answer using the values of K_c​.

Each of the following particle diagrams shows a 2.00 L container at equilibrium.

Which diagram best represents the system at 400 K? Justify your answer based on the value of K_c​.

A new mixture is prepared at 400 K with the following concentrations:

[Cl₂] = 0.10 M, [ICl] = 0.10 M, [ICl₃] = 1.20 M

Determine whether the system will shift toward the reactants, the products, or remain unchanged. Justify your answer with a calculation.

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 K_c for the reaction.

Determine the equilibrium concentrations of H₂ and I₂.

Calculate the value of K_c​ for this equilibrium.

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, K_p, expression for this reaction.

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

ii) Calculate K_p for this reaction.

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

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 K_c​ for this reaction.

Calculate the equilibrium concentrations of all species.

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

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, K_c​, for this reaction.

Calculate the value of K_c​ at this temperature.

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

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