AP Chemistry Unit 6: Thermochemistry Practice Exam

Assessment for Unit 6: Thermochemistry

Estimated Time: 60 minutes

Exam Details

Questions: 20 total (20 multiple choice)
Time: 60 minutes (recommended)
Topics Covered: Unit 6: Thermochemistry
Format: Multiple Choice questions matching real exam difficulty
Scoring: Instant results with detailed explanations

A. Multiple-Choice Questions (20 Questions)

Select the one best answer for each question.

1. 1. [Skill: 3.A | Topic: 6.1] A student investigates an acid-base reaction in a coffee-cup calorimeter. Equal volumes of 1.0 M HCl(aq) and 1.0 M NaOH(aq) are mixed. The data are shown. - Initial temperature of both solutions: 22.0 °C - Highest temperature observed after mixing: 28.5 °C Which statement best describes the energy change for the reaction and the direction of energy transfer?

(A)The reaction is exothermic; thermal energy is transferred from the system (reaction) to the surroundings (solution/calorimeter), increasing the measured temperature.(B)The reaction is endothermic; thermal energy is transferred from the surroundings (solution/calorimeter) to the system (reaction), increasing the measured temperature.(C)The reaction is exothermic; thermal energy is transferred from the surroundings (solution/calorimeter) to the system (reaction), increasing the measured temperature.(D)The reaction is endothermic; thermal energy is transferred from the system (reaction) to the surroundings (solution/calorimeter), increasing the measured temperature.

Refer to the figure below.

2. 2. [Skill: 3.A | Topic: 6.1] A pure liquid is placed in a freezer, and its temperature is recorded as a function of time. The resulting cooling curve includes a flat (constant-temperature) region. Which claim about the flat region is most consistent with energy changes in the system and observations on the cooling curve?

(A)The flat region corresponds to freezing; the process is exothermic because the system releases energy to the surroundings while temperature remains constant.(B)The flat region corresponds to freezing; the process is endothermic because the system absorbs energy from the surroundings while temperature remains constant.(C)The flat region corresponds to cooling of the solid; the process is exothermic because the system releases energy and temperature decreases steadily.(D)The flat region corresponds to melting; the process is exothermic because the system releases energy while temperature remains constant.

3. 3. [Skill: 4.A | Topic: 6.1] Two salts are each dissolved separately in identical cups containing the same mass of water, initially at 23.0 °C. Equal masses of each salt are used. - When salt X dissolves, the water temperature decreases to 18.5 °C. - When salt Y dissolves, the water temperature increases to 29.0 °C. Which explanation best accounts for the difference in temperature change?

(A)Dissolving X is endothermic because more energy is required to separate solute particles and disrupt solvent-solvent interactions than is released when solute-solvent interactions form; dissolving Y is exothermic because forming solute-solvent interactions releases more energy than is required to separate particles.(B)Dissolving X is exothermic because breaking solute-solute interactions releases energy; dissolving Y is endothermic because forming solute-solvent interactions requires energy input.(C)Both dissolutions are endothermic because dissolving always requires energy to break interactions; the temperature increase for Y is due to measurement error.(D)Both dissolutions are exothermic because forming interactions always releases energy; the temperature decrease for X is due to evaporation of water.

Refer to the figure below.

4. [Skill: 1.B | Topic: 6.2] An energy diagram for a reaction is shown. [Image Cue]: Reaction energy diagram, Potential energy (kJ) vs Reaction progress, reactants start at 150 kJ, products end at 90 kJ; show a vertical arrow labeled ΔH from reactant level down to product level. Which statement best describes the process represented by the diagram?

(A)The reaction is endothermic and ΔH is positive because the products have lower potential energy than the reactants.(B)The reaction is exothermic and ΔH is negative because the products have lower potential energy than the reactants.(C)The reaction is endothermic and ΔH is negative because the products have higher potential energy than the reactants.(D)The reaction is exothermic and ΔH is positive because the products have higher potential energy than the reactants.

Refer to the figure below.

5. [Skill: 2.A | Topic: 6.2] Two processes are shown as energy diagrams. [Image Cue]: Two-panel energy diagrams (Diagram 1 and Diagram 2), Potential energy (kJ) vs Reaction progress. Diagram 1: reactants at 40 kJ, products at 120 kJ (products higher). Diagram 2: reactants at 140 kJ, products at 60 kJ (products lower). No activation energy details needed. Which claim correctly matches each diagram with the type of process?

(A)Diagram 1 is exothermic; Diagram 2 is endothermic.(B)Diagram 1 is endothermic; Diagram 2 is exothermic.(C)Both Diagram 1 and Diagram 2 are endothermic.(D)Both Diagram 1 and Diagram 2 are exothermic.

6. [Skill: 1.B | Topic: 6.2] A student sketches an energy diagram (Potential energy vs Reaction progress) for a physical process. The diagram shows the reactants at 25 kJ and the products at 85 kJ. Based on the diagram, which set of conclusions is correct?

(A)ΔH = +60 kJ and the process is endothermic.(B)ΔH = -60 kJ and the process is endothermic.(C)ΔH = +110 kJ and the process is exothermic.(D)ΔH = -110 kJ and the process is exothermic.

Refer to the figure below.

7. [Skill: 4.A | Topic: 6.3] A student places a 50.0 g metal block initially at 80.0°C into a beaker containing 100.0 g of water initially at 20.0°C. The beaker is well insulated from the surroundings. [Image Cue]: Line graph, "Temperature vs. Time for Metal Block and Water", x-axis: time (min) from 0 to 6, y-axis: temperature (°C) from 15 to 85. Two curves: (1) metal starts at 80°C at t = 0 and decreases smoothly toward 30°C; (2) water starts at 20°C at t = 0 and increases smoothly toward 30°C. Both curves level off and meet at 30°C around t ≈ 5 min. At time t = 1 min, which statement best describes the energy transfer occurring between the metal block and the water?

(A)Net thermal energy is transferred from the metal to the water because particles in the warmer metal have a greater average kinetic energy, and collisions transfer energy until thermal equilibrium is reached.(B)Net thermal energy is transferred from the water to the metal because water has a higher specific heat capacity than most metals.(C)No net thermal energy is transferred at t = 1 min because energy is exchanged in both directions during collisions.(D)Net thermal energy is transferred from the metal to the water only until the metal reaches room temperature; after that, thermal energy flows from the water to the metal.

8. [Skill: 2.B | Topic: 6.3] Two rigid containers are separated by a thin metal wall that allows thermal energy to transfer but prevents gases from mixing. Container 1 holds He(g) at 300 K. Container 2 holds Ar(g) at 300 K. The containers are placed in contact through the metal wall and left undisturbed. Which statement best describes the situation at the molecular level?

(A)Helium atoms have a greater average kinetic energy than argon atoms because helium atoms are lighter, so net thermal energy will transfer from container 1 to container 2.(B)Argon atoms have a greater average kinetic energy than helium atoms because argon has more electrons, so net thermal energy will transfer from container 2 to container 1.(C)Helium atoms and argon atoms have the same average kinetic energy because they are at the same temperature, so there is no net thermal energy transfer between the containers.(D)Thermal energy transfer occurs only if the gases are allowed to mix, so there will be no collisions that can transfer energy through the wall.

9. **1.** [Skill: 2.B | Topic: 6.4] A student places a 50.0 g sample of an unknown metal, initially at 100.0°C, into 100.0 g of water initially at 22.0°C in a well-insulated coffee-cup calorimeter. The final temperature of the water-metal mixture is 27.0°C. Assume no heat is absorbed by the calorimeter and that the specific heat capacity of water is 4.184 J/(g·°C). What is the specific heat capacity of the metal?

(A)0.13 J/(g·°C)(B)0.39 J/(g·°C)(C)0.57 J/(g·°C)(D)0.90 J/(g·°C)

10. **2.** [Skill: 4.A | Topic: 6.4] A student investigates the enthalpy change of dissolution for ammonium nitrate, $NH_4NO_3(s)$, using a coffee-cup calorimeter. The student dissolves 5.00 g of $NH_4NO_3(s)$ in 50.0 g of water. The initial temperature of the water is 24.0°C, and after the solid dissolves completely, the final temperature of the solution is 20.5°C. Assume the specific heat capacity of the solution is 4.184 J/(g·°C), and use the total mass of solution as 55.0 g. Which statement best describes the process and the value of $q_{rxn}$ for dissolving the 5.00 g sample?

(A)The dissolution is exothermic; $q_{rxn} = -8.0 \times 10^2$ J.(B)The dissolution is exothermic; $q_{rxn} = +8.0 \times 10^2$ J.(C)The dissolution is endothermic; $q_{rxn} = -8.0 \times 10^2$ J.(D)The dissolution is endothermic; $q_{rxn} = +8.0 \times 10^2$ J.

11. [Skill: 2.A | Topic: 6.5] A student investigates the energy change when steam condenses. Water has a molar enthalpy of vaporization of 40.7 kJ/mol at 100°C. A sample of 9.00 g of H_2O(g) at 100°C is completely condensed to H_2O(l) at 100°C. What is the heat q for the system during this phase change?

(A)-20.4 kJ(B)+20.4 kJ(C)-40.7 kJ(D)+40.7 kJ

Refer to the figure below.

12. [Skill: 5.A | Topic: 6.5] A student heats a pure solid sample of substance Y at a constant rate and records the heating curve shown. [Image Cue]: Heating curve graph, "Temperature vs. Heat Added for Substance Y"; x-axis labeled "Heat added (kJ)" from 0 to 8; y-axis labeled "Temperature (°C)" from 0 to 80. The curve rises from (0, 10) to (2.0, 30), then is horizontal (plateau) at 30°C from 2.0 kJ to 5.6 kJ, then rises again from (5.6, 30) to (8.0, 70). The sample mass is 12.0 g, and the molar mass of Y is 48.0 g/mol. Based on the heating curve, what is the molar enthalpy of fusion, ΔH_fus, of substance Y?

(A)3.6 kJ/mol(B)14.4 kJ/mol(C)57.6 kJ/mol(D)0.30 kJ/mol

13. [Skill: 5.A | Topic: 6.6] A student studies the combustion of ethanol using the thermochemical equation below, which is written for constant pressure conditions. $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$ $\Delta H = -1367\ \text{kJ}$ per 1 mol $C_2H_5OH(l)$ reacted The student completely combusts 5.00 g of ethanol. Which of the following is the best estimate of the heat $q$ released by the system?

(A)-148 kJ(B)-74.0 kJ(C)+148 kJ(D)-410 kJ

14. [Skill: 5.A | Topic: 6.6] A student heats calcium carbonate and models the process with the thermochemical equation below, written for constant pressure conditions. $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$ $\Delta H = +178\ \text{kJ}$ per 1 mol $CaCO_3(s)$ decomposed The student starts with 15.0 g of $CaCO_3(s)$, and 60.0% of the sample decomposes. Which of the following best describes the heat $q$ for the system and the direction of heat flow during the decomposition?

(A)$q = +16.0\ \text{kJ}$; heat flows from the surroundings into the system(B)$q = -16.0\ \text{kJ}$; heat flows from the surroundings into the system(C)$q = +26.7\ \text{kJ}$; heat flows from the system into the surroundings(D)$q = +44.4\ \text{kJ}$; heat flows from the surroundings into the system

15. [Skill: 4.A | Topic: 6.7] A student estimates the enthalpy change for the gas-phase reaction below using average bond enthalpies. $H_2(g) + Cl_2(g) \rightarrow 2HCl(g)$ Average bond enthalpies (kJ/mol): - $H-H$: 436 - $Cl-Cl$: 243 - $H-Cl$: 431 Based on these data, which of the following is the best estimate of $\Delta H_{rxn}$ for the reaction, and what does the sign indicate?

(A)$\Delta H_{rxn} \approx +183\ \text{kJ/mol}$; the reaction is endothermic(B)$\Delta H_{rxn} \approx -183\ \text{kJ/mol}$; the reaction is exothermic(C)$\Delta H_{rxn} \approx +862\ \text{kJ/mol}$; the reaction is endothermic(D)$\Delta H_{rxn} \approx -862\ \text{kJ/mol}$; the reaction is exothermic

16. [Skill: 4.A | Topic: 6.7] A student estimates the enthalpy change for the combustion of methane (all gases) using average bond enthalpies. $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$ Average bond enthalpies (kJ/mol): - $C-H$: 413 - $O=O$: 498 - $C=O$ (in $CO_2$): 799 - $O-H$: 463 Which of the following is the best estimate of $\Delta H_{rxn}$ for the reaction as written?

(A)$\Delta H_{rxn} \approx -802\ \text{kJ/mol}$(B)$\Delta H_{rxn} \approx +802\ \text{kJ/mol}$(C)$\Delta H_{rxn} \approx -401\ \text{kJ/mol}$(D)$\Delta H_{rxn} \approx -1604\ \text{kJ/mol}$

17. [Skill: 2.B | Topic: 6.8] A student uses a table of standard enthalpies of formation, ΔHf°, to determine the standard enthalpy change for the combustion of methanol. Thermochemical data (kJ/mol): - ΔHf°[$CO_2(g)$] = -393.5 - ΔHf°[$H_2O(l)$] = -285.8 - ΔHf°[$CH_3OH(l)$] = -238.7 - ΔHf°[$O_2(g)$] = 0 Reaction: $2CH_3OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 4H_2O(l)$ What is $\Delta H^\circ_{\text{rxn}}$ for the reaction as written?

(A)-1453 kJ(B)-726 kJ(C)+1453 kJ(D)+726 kJ

18. [Skill: 2.B | Topic: 6.8] A researcher measures the standard enthalpy change for the thermal decomposition of calcium carbonate: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$ The measured value is $\Delta H^\circ_{\text{rxn}} = +178.3\ \text{kJ/mol}$. Thermochemical data (kJ/mol): - ΔHf°[$CaO(s)$] = -635.1 - ΔHf°[$CO_2(g)$] = -393.5 Based on these data, what is the standard enthalpy of formation, ΔHf°, of $CaCO_3(s)$?

(A)-1206.9 kJ/mol(B)+1206.9 kJ/mol(C)-850.3 kJ/mol(D)-1028.6 kJ/mol

19. **1.** [Skill: 4.A | Topic: 6.9] A student is determining the enthalpy change for the formation of methane using Hess’s law. The student is given the following thermochemical equations: 1. $C(s,\ \text{graphite}) + O_2(g) \rightarrow CO_2(g)$ $\Delta H^\circ = -393.5\ \text{kJ}$ 2. $H_2(g) + \tfrac{1}{2}O_2(g) \rightarrow H_2O(l)$ $\Delta H^\circ = -285.8\ \text{kJ}$ 3. $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$ $\Delta H^\circ = -890.3\ \text{kJ}$ What is $\Delta H^\circ$ for the reaction $C(s,\ \text{graphite}) + 2H_2(g) \rightarrow CH_4(g)$ ?

(A)$-74.8\ \text{kJ}$(B)$+74.8\ \text{kJ}$(C)$-965.1\ \text{kJ}$(D)$-890.3\ \text{kJ}$

20. **2.** [Skill: 2.B | Topic: 6.9] A process converts substance $A$ to substance $D$ and can occur by either pathway shown below. Pathway 1: - Step 1: $A \rightarrow B$ $\Delta H^\circ=-50\ \text{kJ}$ - Step 2: $B \rightarrow D$ $\Delta H^\circ=-20\ \text{kJ}$ Pathway 2: - Step 1: $A \rightarrow C$ $\Delta H^\circ=-10\ \text{kJ}$ - Step 2: $C \rightarrow D$ $\Delta H^\circ=-60\ \text{kJ}$ Which choice correctly states the overall $\Delta H^\circ$ for $A \rightarrow D$ and provides the best justification?

(A)$\Delta H^\circ=-70\ \text{kJ}$, because enthalpy is a state function and the overall change equals the sum of the steps.(B)$\Delta H^\circ=-70\ \text{kJ}$, because the pathway with more steps is always more exothermic.(C)$\Delta H^\circ=-80\ \text{kJ}$, because the overall enthalpy depends on which intermediate is formed.(D)The overall $\Delta H^\circ$ cannot be determined without knowing the reaction mechanism.

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