AP Chemistry Unit 7 Review

• 
  Some of the Cu+ and Cl− ions combine to form CuCl(s) because the K_sp will be lower than 1.6×10⁻⁷.

• Some of the Cu+ and Cl− ions combine to form CuCl(s) because the molar solubility will be lower than 4×10⁻⁴ M

• More Cu+ and Cl− ions will be in solution because the molar solubility will be higher than 4×10⁻⁴ M

• More Cu+ and Cl− ions will be in solution because the K_sp will be higher than 1.6×10⁻⁷.

Answer explanation

The increase in the concentration of Cl− ions in solution with the addition of NaCl will disrupt the equilibrium, the rate of the reverse reaction will increase, and the system will reach a new equilibrium position. As a result, Cu+ and Cu− ions will combine to form CuCl(s) and the molar solubility will be lower than 4×10⁻⁴M.

• The particle diagram shows that the dissociation of CuCl(s) into ions contributes to an increase in the entropy for the dissolution.

• The particle diagram shows that the dissociation of CuCl(s) into ions contributes to a decrease in the entropy for the dissolution.

• The particle diagram shows that there is no reorganization of the water molecules around the ions and the change in entropy for the dissolution is zero.

• The particle diagram shows that there are no interactions between the water molecules and the change in entropy for the dissolution is zero.

Answer explanation

As a result of the dissociation of CuCl(s), two particles form, Cu+ and Cl−. This is shown in the particle diagram, and this increase in the number of particles contributes to an increase in entropy.

• The value of the enthalpy change for the dissolution of CuCl(s) cannot be predicted from the particle diagram because it fails to illustrate the amount of energy required to overcome the forces between solute particles and between solvent particles.

• The value of the enthalpy change for the dissolution of CuCl(s) cannot be predicted from the particle diagram because it fails to illustrate the amount of energy released when the water molecules form hydrogen bonds with Cl− ions.

• The value of the enthalpy change for the dissolution of CuCl(s) is positive (endothermic) because energy is released to overcome the forces between solute particles, as shown in the particle diagram

• The value of the enthalpy change for the dissolution of CuCl(s) is negative (exothermic) because energy is required when the bonds between the ions are broken, as shown in the particle diagram.

Answer explanation

The particle diagram does not provide enough information to predict if the dissolution of CuCl is endothermic or exothermic. The enthalpy change for the dissolution can be evaluated only if the relative energy of these processes are known: (1) the energy required to overcome the interactions between solute particles, (2) the energy required to overcome the interactions between solvent particles, and (3) the energy released when new solute-solvent interactions are formed.

The graph above represents the data collected under certain conditions for the decomposition of N2O4(g) according to the chemical equation above. Based on the graph, at approximately which time is equilibrium established?

• At time A, because N₂O₄(g) is expanding to fill the container.

• At time B, because the reaction is reversible and [NO₂]=[N₂O₄].

• 
  At time C, because the reaction is about to reach completion and [NO₂]>[N₂O₄].

• At time D, because there are no observable changes in [NO₂] and [N₂O₄].

Answer explanation

At equilibrium, N2O4 is transformed into NO2 at the same rate at which NO2 is transformed into N2O4, and no changes in [NO2] and [N2O4] are observed

A sample of acetone is placed into a container. The container is sealed and attached to a pressure sensor, as shown in the diagram above. The container is allowed to sit on the lab table for a few minutes as the pressure in the container is monitored at regular intervals. At the end of 2.00 minutes, some acetone liquid remains in the container. Which of the following best explains the pressure data presented in the table above?

• The acetone heats up over time, causing more of it to vaporize at an increasing rate.

• The acetone has completely vaporized after 1.50 minutes, so the pressure becomes constant.

• The acetone vaporizes from the liquid at a constant rate, the rate of condensation increases until it becomes equal to the rate of evaporation, and then the pressure stays constant.

• The acetone vaporizes from the liquid at a rate that is fast in the beginning but then slows down until the vaporization process stops completely.

Answer explanation

A constant equilibrium pressure is achieved once the rate of condensation of the acetone vapor matches the rate of evaporation of the acetone from the liquid.

A sample of pure NO₂(g) in a sealed tube at 20°C is placed in a temperature bath at 30°C. Observations of changes in the color, pressure, and mass of the mixture are recorded as a function of time. Which of the following is an observation that would best support the claim that the reaction represented above has reached equilibrium at 30°C?

• The total mass of the system remains constant because the amounts of reactant and product do not change with time at equilibrium.

• The color of the system changes from brown to completely colorless because only the product will be present at equilibrium.

• The total pressure of the system decreases then reaches a constant value because the amounts of reactant and product no longer change at equilibrium.

• The temperature of the system remains constant because the temperature must be constant at equilibrium.

Answer explanation

As the reaction proceeds to the right, two NO2 molecules are converted into one N2O4 molecule, resulting in a decrease in the number of molecules in a fixed volume and a decrease in pressure. At equilibrium, the pressure will be constant since the number of reactant and product molecules will be constant.

• It is a good representation because it shows that the dipoles of the H2O molecules are oriented around the Ag+ and Cl− ions in solution.

• It is a good representation because it shows that the concentrations of Ag+ ions and Cl− ions are equal at equilibrium.

• It is not a good representation because it does not show that the concentration of AgCl(s) is constant at equilibrium.

• 
  It is not a good representation because it does not illustrate the dynamic equilibrium in which the rates of the forward and reverse reactions are equal.

Answer explanation

Since at equilibrium the rates of the forward and reverse reactions are the same, one single representation cannot capture the dynamic nature of this process. Multiple representations of the reaction before and after equilibrium would be needed to show that the concentrations of the ions have remained constant over time.

The particle diagram above shows the system represented by the equation 2X(g)+Y2(g)⇄2XY(g). Which of the following explains whether the particle diagram indicates that the system is at equilibrium?

• The particle diagram does not indicate that the system is at equilibrium because it shows the system only at one point in time.

• The particle diagram does not indicate that the system is at equilibrium because the ratio [XY]_eq/[X]_eq is not equal to 1.

• The particle diagram indicates that the system is at equilibrium because the value of K is small.

• The particle diagram indicates that the system is at equilibrium because [X]_eq=2×[Y2]_eq.

Answer explanation

Equilibrium is a dynamic process in which the rates of the forward reaction and the reverse reaction are equal and the concentrations of reactants and products remain constant over time. A single representation cannot capture this dynamic process.

• The rate of the reverse reaction is zero because the concentration of XY keeps increasing.

• The reverse reaction has a higher rate than the forward reaction between 200s and 300s because [XY]>>[X].

• The forward reaction has a faster rate than the reverse reaction between 0s and 300s because more products were being formed.

• The rates of the forward and reverse reactions were the same after 100s because the rate of formation of XY was constant.

Answer explanation

Based on the particle diagrams, the amount of XY kept increasing between 0s and 300s, indicating that the forward reaction occurred at a faster rate than the reverse reaction.

In an experiment involving the reaction shown above, a sample of pure HI was placed inside a rigid container at a certain temperature. The table above provides the initial and equilibrium concentrations for some of the substances in the reaction. Based on the data, which of the following is the value of the equilibrium constant (K_eq) for the reaction, and why?

• 
  K_eq=2.5×10⁻¹, because [I₂]_eq=2×[HI]_eq

• K_eq=6.3×10⁻², because [I₂]_eq=1/2[HI]_eq

• K_eq=1.6×10⁻², because [I₂]_eq=[HI]_eq

• K_eq=3.1×10⁻², because [I₂]_eq=2 ^_x [HI]_eq

Answer explanation

RICE Chart