AP Chemistry Practice Test Questions: Kinetics & Thermochemistry

Which of the following equations represents a reaction for which the standard entropy change is positive (DS° > 0) ?

(A) 3 O2(g) → 2 O3(g) (B) 2 H2(g) + O2(g) → 2 H2O(l) *(C) CaCO3(s) → CaO(s) + CO2(g)* (D) I2(g) + 2 K(s) → 2 KI(s)

2 H2O2(aq) —> 2 H2O(l) + O2(g) DH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. Time (s) — [H2O2] 0.00 — 2.7 200 — 2.1 400 — 1.7 600 — 1.4 Which of the following identifies the element(s) being oxidized and reduced in the reaction?

(A) Hydrogen is oxidized and oxygen is reduced. (B) Oxygen is oxidized and hydrogen is reduced. *(C) Oxygen is both oxidized and reduced.* (D) No elements are oxidized or reduced; the reaction is not a redox reaction.

When water is added to a mixture of Na2O2(s) and S(s), a redox reaction occurs, as represented by the equation below. 2NaO(s) + S(s) + 2H2O(l) —> 4NaOH(aq) + SO2(aq) DH = 298 kJ/mol DS = -7.3 J/(K•mol) Which of the following statements about the thermodynamic favorability of the reaction at 298 K is correct?

(A) It is thermodynamically unfavorable. (B) It is thermodynamically favorable and is driven by DS only. *(C) It is thermodynamically favorable and is driven by DH only.* (D) It is thermodynamically favorable and is driven by both DH and DS

CH3OH(g) —> CO(g) + 2 H2(g) DH° = +91 kJ/molrxn The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K. What can be inferred about DS° for the reaction at 600 K?

(A) It must be positive, since the reaction is thermodynamically unfavorable at 600 K. (B) It must be negative, since there are more moles of products than reactants. *(C) It must be positive, since DG° is negative and DH° is positive.* (D) It must be negative, since DG° is positive and DH° is positive.

At 1.0 atm a sample of ice is heated to liquid water and then to water vapor. The heating curve is shown in the graph above. Which of the following lists the signs for the changes in enthalpy and entropy for the process corresponding to segment X, going from left to right?

(A) Positive; Negative *(B) Positive; Positive* (C) Negative; Negative (D) Negative; Positive

When water is added to a mixture of Na2O2(s) and S(s), a redox reaction occurs, as represented by the equation below. 2NaO(s) + S(s) + 2H2O(l) —> 4NaOH(aq) + SO2(aq) DH = 298 kJ/mol DS = -7.3 J/(K•mol) Atoms of which element are reduced in the reaction?

(A) S; each atom loses four electrons (B) Na in Na2O2; each atom loses one electron *(C) O in Na2O2; each atom gains one electron* (D) O in H2O; each atom gains one electron

N2(g) + 3 H2(g) <—> 2 NH3(g) DH = −92 kJ/mol ; DG = -33 kJ/mol Consider the reaction represented above at 298 K. When equal volumes of N2(g) and H2(g), each at 1 atm, are mixed in a closed container at 298 K, no formation of NH3(g) is observed. Which of the following best explains the observation?

(A) The N2(g) and the H2(g) must be mixed in a 1:3 ratio for a reaction to occur. *(B) A high activation energy makes the forward reaction extremely slow at 298 K.* (C) The reaction has an extremely small equilibrium constant, thus almost no product will form. (D) The reverse reaction has a lower activation energy than the forward reaction, so the forward reaction does not occur.

CH3OH(g) —> CO(g) + 2 H2(g) DH° = +91 kJ/molrxn The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K. What happens to the temperature of the contents of the vessel as the reaction occurs?

(A) The temperature must increase, because according to Le Châtelier's principle, an increase in temperature causes more products to form. (B) The temperature must decrease, because the reaction takes place at a temperature above room temperature. *(C) The temperature must decrease, because the reaction is endothermic.* (D) The temperature does not change, because the vessel is insulated.

A student conducted an experiment to determine DHrxn for the reaction between HCl(aq) and NaOH(aq). The student ran two trials using the volumes of HCl(aq) and NaOH(aq) indicated in the table above, and determined the amount of heat released. Which of the following best explains the relationship between X and Y?

(A) Y = 2X, because the volume of HCl(aq) used in trial 2 is twice the volume used in trial 1. *(B) Y = X, because the number of moles of acid and base reacting with each other is the same in both trials.* (C) Y = 2X , because the heat is distributed over more 3 particles in trial 2 than in trial 1. (D) The relationship between X and Y cannot be predicted.

CH3OH(g) —> CO(g) + 2 H2(g) DH° = +91 kJ/molrxn The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K. Which of the following statements about the bonds in the reactants and products is most accurate?

* (A) The sum of the bond enthalpies of the bonds in the reactant is greater than the sum of the bond enthalpies of the bonds in the products.* (B) The sum of the bond enthalpies of the bonds in the reactant is less than the sum of the bond enthalpies of the bonds in the products. (C) The length of the bond between carbon and oxygen in CH3OH is shorter than the length of the bond between carbon and oxygenin CO. (D) All of the bonds in the reactant and products are polar.

A student mixes a 10.0 mL sample of 1.0 M NaOH(aq) with a 10.0 mL sample of 1.0 M HCl(aq) in a polystyrene container. The temperature of the solutions before mixing was 20.0°C. If the final temperature of the mixture is 26.0°C, what is the experimental value of DHrxn? (Assume that the solution mixture has a specific heat of 4.2 J/(g•K) and a density of 1.0 g/mL.)

*(A) -50. kJ/molrxn* (B) -25 kJ/molrxn (C) -5.0 × 104 kJ/molrxn (D) -5.0 × 102 kJ/molrxn

2 H2O2(aq) —> 2 H2O(l) + O2(g) DH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. Time (s) — [H2O2] 0.00 — 2.7 200 — 2.1 400 — 1.7 600 — 1.4 The O2(g) produced from the decomposition of the 1.0 L sample of H2O2(aq) is collected in a previously evacuated 10.0 L flask at 300. K. What is the approximate pressure in the flask after 400. s? (For estimation purposes, assume that 1.0 mole of gas in 1.0 L exerts a pressure of 24 atm at 300. K.)

*(A) 1.2 atm* (B) 2.4 atm (C) 12 atm (D) 24 atm

Which of the following arranges the molecules N2 , O2 , and F2 in order of their bond enthalpies, from least to greatest?

*(A) F2<O2<N2* (B) O2<N2<F2 (C) N2<O2<F2 (D) N2 < F2 < O2

The heating curve for a sample of pure ethanol is provided above. The temperature was recorded as a 50.0 g sample of solid ethanol was heated at a constant rate. Which of the following explains why the slope of segment T is greater than the slope of segment R ?

*(A) The specific heat capacity of the gaseous ethanol is less than the specific heat capacity of liquid ethanol.* (B) The specific heat capacity of the gaseous ethanol is greater than the specific heat capacity of liquid ethanol. (C) The heat of vaporization of ethanol is less than the heat of fusion of ethanol. (D) The heat of vaporization of ethanol is greater than the heat of fusion of ethanol.

4 NH3(g) + 3 O2(g) —> 2 N2(g) + 6 H2O(g) If the standard molar heats of formation of ammonia, NH3(g), and gaseous water, H2O(g), are -46 kJ/mol and -242 kJ/mol, respectively, what is the value of DH represented above?

(A) -190 kJ/molrxn (B) -290 kJ/molrxn (C) -580 kJ/molrxn *(D) -1,270 kJ/molrxn* (E) -1,640 kJ/molrxn

1/2 H2(g) + 1/2 I2(s) —> HI(g) ΔH = 26 kJ/molrxn 1/2 H2(g) + 1/2 I2(g) —> HI(g) ΔH = -5.0 kJ/molrxn Based on the information above, what is the enthalpy change for the sublimation of iodine, represented below? I2(s) → I2(g)

(A) 15 kJ/molrxn (B) 21 kJ/molrxn (C) 31 kJ/molrxn (D) 42 kJ/molrxn *(E) 62 kJ/molrxn*

A hot iron ball is dropped into a 200. g sample of water initially at 50.° C. If 8.4 kJ of heat is transferred from the ball to the water, what is the final temperature of the water? (The specific heat of water is 4.2 J/(g·°C).)

(A) 40.°C (B) 51°C *(C) 60.°C* (D) 70.°C

Which of the following is true for the decomposition of H2O2(aq) ?

(A) DG°>0 and Keq>1 (B) DG°>0 and Keq <1 *(C) DG°<0 and Keq>1* (D) DG°<0 and Keq <1

CH3OH(g) —> CO(g) + 2 H2(g) DH° = +91 kJ/molrxn The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K. A sample of CH3OH(g) is placed in the previously evacuated vessel with a pressure of P1 at 600 K. What is the final pressure in the vessel after the reaction is complete and the contents of the vessel are returned to 600 K?

(A) P1/9 (B) P1/3 (C) P1 *(D) 3P1*

2 H2O2(aq) —> 2 H2O(l) + O2(g) DH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. Time (s) — [H2O2] 0.00 — 2.7 200 — 2.1 400 — 1.7 600 — 1.4 The reaction is thermodynamically favorable. The signs of ΔG° and ΔS° for the reaction are which of the following?

(A) Positive; Positive *(B) Negative; Positive* (C) Positive; Negative (D) Negative; Negative

When water is added to a mixture of Na2O2(s) and S(s), a redox reaction occurs, as represented by the equation below. 2NaO(s) + S(s) + 2H2O(l) —> 4NaOH(aq) + SO2(aq) DH = 298 kJ/mol DS = -7.3 J/(K•mol) Two trials are run, using excess water. In the first trial, 7.8 g of Na2O2(s) (molar mass 78 g/mol) is mixed with 3.2 g of S(s). In the second trial, 7.8 g of Na2O2(s) is mixed with 6.4 g of S(s). The Na2O2(s) and S(s) react as completely as possible. Both trials yield the same amount of SO2(aq). Which of the following identifies the limiting reactant and the heat released, q, for the two trials at 298 K?

(A) S; 30. kJ (B) S; 61 kJ *(C) Na2O2; 30 kJ* (D) Na2O2; 61 kJ

A 1.0 g sample of a cashew was burned in a calorimeter containing 1000. g of water, and the temperature of the water changed from 20.0°C to 25.0°C. In another experiment, a 3.0 g sample of a marshmallow was burned in a calorimeter containing 2000. g of water, and the temperature of the water changed from 25.0°C to 30.0°C. Based on the data, which of the following can be concluded about the energy content for 1.0 g of each of the two substances? (The specific heat of water is 4.2 J/(g⋅°C).)

(A) The combustion of 1.0 g of cashew releases less energy than the combustion of 1.0 g of marshmallow. (B) The combustion of 1.0 g of cashew releases the same amount of energy as the combustion of 1.0 g of marshmallow. *(C) The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow.* (D) No comparison can be made because the two systems started with different masses of food, different masses of water, and different initial temperatures.

2 H2O2(aq) —> 2 H2O(l) + O2(g) DH° = −196 kJ/molrxn The decomposition of H2O2(aq) is represented by the equation above. A student monitored the decomposition of a 1.0 L sample of H2O2(aq) at a constant temperature of 300. K and recorded the concentration of H2O2 as a function of time. The results are given in the table below. Time (s) — [H2O2] 0.00 — 2.7 200 — 2.1 400 — 1.7 600 — 1.4 Which of the following statements is a correct interpretation of the data regarding how the order of the reaction can be determined?

(A) The reaction must be first order because there is only one reactant species. *(B) The reaction is first order if the plot of ln [H2O2] versus time is a straight line.* (C) The reaction is first order if the plot of 1/[H2O2] versus time is a straight line. (D) The reaction is second order because 2 is the coefficient of H2O2 in the chemical equation.

Sr 2+(aq) + F 1-(aq) —> SrF 1+(aq) At 25°C, the equilibrium constant for the reaction represented above has a value of 1.3. At 50°C, the value of the equilibrium constant is less than 1.3. Based on this information, which of the following must be correct?

(A) The reaction rate decreases as the temperature is increased. (B) The reaction is thermodynamically favorable only at temperatures above 25°C. (C) At 25°C, Δ G° for the reaction is positive. (D) At 25°C, Δ S ° for the reaction is positive. *(E) At 25°C, Δ H ° for the reaction is negative.*

Under which of the following conditions can an endothermic reaction be thermodynamically favorable?

(A) ΔG is positive (B) ΔS is negative *(C) TΔS > ΔH* (D) TΔS = 0 (E) There are no conditions under which an endothermic reaction can be thermodynamically favorable.

A 10. g cube of copper at a temperature T1 is placed in an insulated cup containing 10. g of water at a temperature T2 . If T1 > T2 , which of the following is true of the system when it has attained thermal equilibrium? (The specific heat of copper is 0.385 J/(g·°C) and the specific heat of water is 4.18 J/(g·°C).)

*(A) The temperature of the copper changed more than the temperature of the water.* (B) The temperature of the water changed more than the temperature of the copper. (C) The temperature of the water and the copper changed by the same amount. (D) The relative temperature changes of the copper and the water cannot be determined without knowing T1 and T2 .