Equilibrium Constants (College Board AP® Chemistry): Exam Questions

CaCO₃ (s)  ⇌ CaO (s) + CO₂ (g)

When heated, calcium carbonate decomposes according to the equation above. In a study of the decomposition of calcium carbonate, a student added a 50.0 g sample of powdered CaCO₃ (s) to a 1.00 L rigid container. The student sealed the container, pumped out all the gases, then heated the container in an oven at 1100 K . As the container was heated, the total pressure of the CO₂ (g) in the container was measured over time. The data are plotted in the graph below.

The student repeated the experiment, but this time the student used a 100.0 g sample of powdered CaCO₃ (s). In this experiment, the final pressure in the container was 1.04 atm, which was the same final pressure as in the first experiment.

Calculate the number of moles of CO₂ (g) present in the container after 20 minutes of heating.

The student claimed that the final pressure in the container in each experiment became constant because all of the CaCO₃ (s) had decomposed. Based on the data in the experiments, do you agree with this claim? Explain.

After 20 minutes some CO₂ (g) was injected into the container, initially raising the pressure to 1.5 atm. Would the final pressure inside the container be less than, greater than, or equal to 1.04 atm? Explain your reasoning.

Are there sufficient data obtained in the experiments to determine the value of the equilibrium constant, K_p , for the decomposition of CaCO₃ (s) at 1100 K ? Justify your answer.

Methanol vapor decomposes to form carbon monoxide gas and hydrogen gas at high temperatures in the presence of a platinum catalyst, as represented by the balanced chemical equation given.

CH₃OH (g) ⇌ CO (g) + 2H₂ (g)          ∆H ° = 90.0 kJ / mol_rxn

Are the hydrogen atoms oxidized or are they reduced in the forward reaction? Justify your answer in terms of oxidation numbers.

In the following box, draw the complete Lewis electron-dot diagram for the carbon monoxide molecule in which every atom obeys the octet rule. Show all bonding and nonbonding valence electrons.

The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.

i) Use the data in the table to calculate the value of the standard entropy change, ∆S°, in J/(K·mol_rxn), for the reaction.

ii) Calculate the value of ∆G°, in kJ/mol_rxn, for the reaction at 375 K. Assume that ∆H ° and ∆S° are independent of temperature.

The following particle-level diagram shows a representative sample of the equilibrium mixture represented by the equation given.

Use information from the particle diagram to calculate the partial pressure of CO at equilibrium when the total pressure of the equilibrium mixture is 12.0 atm.

Write the expression for the equilibrium constant, K_p, for the reaction.

CH₃OH (g) ⇌ CO (g) + 2 H₂ (g)

The reaction system represented by the equation is allowed to achieve equilibrium at a different temperature. The following table gives the partial pressure of each species in the equilibrium mixture.

Use the information in the table to calculate the value of the equilibrium constant, K_p, at the new temperature.

The volume of the container is rapidly doubled with no change in temperature. As equilibrium is re-established, does the number of moles of CH₃OH (g) increase, decrease, or remain the same? Justify your answer by comparing the value of the reaction quotient, Q, with the value of the equilibrium constant, K_p.

Answer the following questions relating to the chemistry of the halogens.

The molecular formulas of diatomic bromine, chlorine, fluorine, and iodine are written below. Circle the formula of the molecule that has the longest bond length. Justify your choice in terms of atomic structure.

Br₂                            Cl₂                            F₂                              I₂

A chemistry teacher wants to prepare Br₂. The teacher has access to the following three reagents: NaBr (aq) , Cl₂ (g) , and I₂ (s).

Using the data in the table above, write the balanced equation for the thermodynamically favorable reaction that will produce Br₂ when the teacher combines two of the reagents. Justify that the reaction is thermodynamically favorable by calculating the value of E° for the reaction.

Br₂ and Cl₂ can react to form the compound BrCl .

The boiling point of Br₂ is 332 K, whereas the boiling point of BrCl is 278 K. Explain this difference in boiling point in terms of all the intermolecular forces present between molecules of each substance.

The compound BrCl can decompose into Br₂ and Cl₂ , as represented by the balanced chemical equation below.

2 BrCl (g)  ⇌ Br₂ (g) + Cl₂ (g)               ΔH° = 1.6 kJ/mol_rxn

A 0.100 mole sample of pure BrCl (g) is placed in a previously evacuated, rigid 2.00 L container at 298 K. Eventually the system reaches equilibrium according to the equation above.

Calculate the pressure in the container before equilibrium is established.

Write the expression for the equilibrium constant, K_eq ,for the decomposition of BrCl.

After the system has reached equilibrium, 42 percent of the original BrCl sample has decomposed.

Determine the value of K_eq  for the decomposition reaction of BrCl at 298 K.

Calculate the bond energy of the Br-Cl bond, in kJ/mol, using ΔH° for the reaction (1.6 kJ/mol_rxn) and the information in the following table.

The following reaction occurs in a sealed 5.00 L container at temperature T:

H₂ (g) + I₂ (g) ⇌ 2HI (g)

The equilibrium constant at this temperature is K_c = 50.0. The system is initially prepared with the following concentrations:

[H₂] = 0.250 M, [I₂] = 0.250 M, [HI] = 0.100 M

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

Calculate the reaction quotient Q_c​.

Determine whether the system is at equilibrium by comparing Q_c​ and K_c​​. If the system is not at equilibrium, state whether the forward or reverse reaction will be favored and justify your answer.

A reaction involving sulfur dioxide (SO₂​) reaches equilibrium at a temperature where K_c = 85.0:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

A reaction vessel initially contains 0.500 M SO₂​, 0.300 M O₂​, and 0.200 M SO₃​. At equilibrium, the concentration of SO₃​ is 0.350 M.

Write the expression for the equilibrium constant K_c​.

Determine the equilibrium concentrations of SO₂​ and O₂.

The volume of the container is doubled while maintaining constant temperature. Explain how this change will affect the equilibrium position.

A chemist studies the following equilibrium reaction in a sealed reaction vessel at 900 K:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

The equilibrium constant K_p​ for this reaction is 3.45 × 10⁴ at 900 K.

Write the expression for the equilibrium constant K_p​ for this reaction.

Calculate the reaction quotient Q_p​. Compare your result to K_p​ and explain whether the system is at equilibrium.

Predict whether the reaction will shift toward the reactants or the products to reach equilibrium. Justify your answer.

A chemical reaction involving gaseous species is represented as follows:

2A (g) + B (g) ⇌ C (g)

At a certain temperature, the equilibrium concentrations are:

• [A] = 0.30 mol/L

• [B] = 0.40 mol/L

• [C] = 0.20 mol/L

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

Calculate the value of K_c​.

If the reaction is reversed, write the expression for the new equilibrium constant.

Predict whether the value of the equilibrium constant, K_c​, will change if the initial concentration of A is increased. Justify your answer.