--- title: "AP Chemistry Unit 6 Practice Test" description: "A timed AP Chemistry Unit 6 (Thermochemistry) practice test in the AP format, drawn from 3,009 multiple-choice questions and 99 FRQs written for the unit. The standard version takes about 60 minutes." canonical: "https://fiveable.me/ap-chem/unit-test/6" type: "unit-test" subject: "AP Chemistry" unit: "Unit 6" --- # AP Chemistry Unit 6 Practice Test ## Overview A timed AP Chemistry Unit 6 (Thermochemistry) practice test in the AP format, drawn from 3,009 multiple-choice questions and 99 FRQs written for the unit. The standard version takes about 60 minutes. ## Test Format - 20 Multiple Choice Questions (30 minutes) - 1 FRQ – FRQ 1 – Long Answer (15 minutes) - 1 FRQ – FRQ 4 – Short Answer (15 minutes) ## Unit FRQs by Type - [FRQ 1 – Long Answer](/ap-chem/frqs/frq-1?unit=6): 10 questions - [FRQ 2 – Long Answer](/ap-chem/frqs/frq-2?unit=6): 12 questions - [FRQ 3 – Long Answer](/ap-chem/frqs/frq-3?unit=6): 9 questions - [FRQ 4 – Short Answer](/ap-chem/frqs/frq-4?unit=6): 21 questions - [FRQ 5 – Short Answer](/ap-chem/frqs/frq-5?unit=6): 9 questions - [FRQ 6 – Short Answer](/ap-chem/frqs/frq-6?unit=6): 18 questions - [FRQ 7 – Short Answer](/ap-chem/frqs/frq-7?unit=6): 20 questions ## Practice Questions ### Question 1 A student dissolves solid calcium chloride (CaCl2) in water to measure the heat released and evaluate the energy of ion-dipole formation. The student uses a glass beaker instead of an insulated calorimeter, allowing significant heat transfer to the surroundings. How does this error affect the calculated magnitude of the enthalpy of solution and the evaluation of the energy released? - (A) The magnitude of the enthalpy will be lower than the actual value, leading to an underestimation of the energy released. - (B) The magnitude of the enthalpy will be lower than the actual value, leading to an overestimation of the energy released. - (C) The magnitude of the enthalpy will be higher than the actual value, leading to an underestimation of the energy released. - (D) The magnitude of the enthalpy will be lower than the actual value, but the evaluation of energy released remains accurate because the error affects all trials equally. **Answer: (A)** Tracing the heat flow reveals that energy lost to the air is not measured by the thermometer, resulting in a smaller temperature change. This leads to a smaller calculated heat value (q), causing the student to underestimate the magnitude of the exothermic energy change. ### Question 2 A proposed mechanism for the reduction of nitrogen monoxide by hydrogen gas proceeds in two steps. Step 1 is 2NO(g) + H2(g) → N2(g) + H2O2(g). Step 2 is H2O2(g) + H2(g) → 2H2O(g). Which equation represents the overall chemical phenomenon? - (A) 2NO(g) + 2H2(g) → N2(g) + 2H2O(g) - (B) 2NO(g) + H2(g) → N2(g) + H2O2(g) - (C) 2NO(g) + H2(g) → N2O(g) + H2O(g) - (D) N2(g) + 2H2O(g) → 2NO(g) + 2H2(g) **Answer: (A)** Summing the two elementary steps and canceling the intermediate H2O2 yields the overall balanced equation. ### Question 3 The industrial production of nitric acid begins with two oxidation steps. First, ammonia reacts with oxygen: 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g). Second, the nitrogen monoxide produced reacts with more oxygen: 2NO(g) + O2(g) → 2NO2(g). Which equation represents the overall reaction for the complete conversion of ammonia to nitrogen dioxide? - (A) 4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(g) - (B) 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) - (C) 4NH3(g) + 6O2(g) → 4NO2(g) + 6H2O(g) - (D) 4NH3(g) + 8O2(g) → 4NO2(g) + 6H2O(g) **Answer: (A)** Multiplying the second step by 2 allows the intermediate NO to cancel, resulting in the correct stoichiometric sum. ### Question 4 The formation of nitrogen dioxide from atmospheric gases is represented by the overall equation N2(g) + 2O2(g) → 2NO2(g). If the first step of the mechanism is N2(g) + O2(g) → 2NO(g), which equation represents the second step? - (A) 2NO(g) + O2(g) → 2NO2(g) - (B) 2NO(g) + 2O(g) → 2NO2(g) - (C) NO(g) + O(g) → NO2(g) - (D) N2(g) + 2O2(g) → 2NO2(g) **Answer: (A)** Subtracting the first step from the overall equation reveals the second step needed to complete the stoichiometry. ### Question 5 A multistep reaction is overall exothermic with $$\Delta H = -50$$ kJ/mol. A student draws an energy profile with two peaks where the potential energy of the products is higher than the potential energy of the reactants. How well does this model describe the reaction thermodynamics? - (A) It incorrectly represents the thermodynamics because the products should be at lower energy than the reactants. - (B) It incorrectly represents the thermodynamics because an exothermic reaction must have only one transition state. - (C) It correctly represents the thermodynamics because the activation energy raises the final potential energy. - (D) It correctly represents the thermodynamics because the intermediate state must be lower than the final state. **Answer: (A)** Evaluating the thermodynamic consistency requires comparing reactant and product energies. An exothermic reaction ($$\Delta H < 0$$) releases energy, meaning the final product state must be lower in potential energy than the initial reactant state. ### Question 6 A reaction is endothermic with an enthalpy change of $$+50$$ kJ/mol. A student's energy profile shows an activation energy of $$+30$$ kJ/mol. Which statement correctly evaluates the physical possibility of this model? - (A) The model is impossible because the activation energy must be at least equal to the enthalpy change. - (B) The model is impossible because endothermic reactions must have negative activation energy values. - (C) The model is possible because the transition state energy is independent of the product energy. - (D) The model is possible because the activation energy is determined solely by the reactant stability. **Answer: (A)** Analyzing the energy profile requires understanding the conservation of energy. In an endothermic reaction, the products are at a higher energy than the reactants, so the transition state must be at least as high as the products ($$E_a \ge \Delta H$$). ### Question 7 A potential energy diagram for a multistep reaction displays reactants at $$150$$ kJ/mol and final products at $$80$$ kJ/mol. The highest peak in the profile reaches $$250$$ kJ/mol. What is the value of the overall enthalpy change ($$\Delta H^\circ$$)? - (A) $$-70$$ kJ/mol, calculated as the energy of products minus reactants - (B) $$+70$$ kJ/mol, calculated as the energy of reactants minus products - (C) $$+100$$ kJ/mol, calculated as the peak energy minus reactants - (D) $$+170$$ kJ/mol, calculated as the peak energy minus products **Answer: (A)** Identifying the overall enthalpy change requires subtracting the energy of the reactants from the energy of the products ($$80 - 150 = -70$$ kJ/mol). The peak energy represents the transition state and is not used for calculating $$\Delta H^\circ$$. ### Question 8 A reaction profile shows reactants at $$50$$ kJ, a transition state at $$150$$ kJ, and products at $$100$$ kJ. Which option correctly describes the thermodynamics and kinetics of this reaction? - (A) Endothermic with an activation energy of $$100$$ kJ - (B) Endothermic with an activation energy of $$50$$ kJ - (C) Exothermic with an activation energy of $$100$$ kJ - (D) Exothermic with an activation energy of $$50$$ kJ **Answer: (A)** Analyzing the profile reveals two key properties: the reaction is endothermic because products ($$100$$ kJ) are higher than reactants ($$50$$ kJ), and the activation energy is the height of the barrier ($$150 - 50 = 100$$ kJ). ### Question 9 A reaction profile shows reactants at 0 kJ/mol, a transition state at 100 kJ/mol, and products at -20 kJ/mol. Which statement best explains the macroscopic exothermicity using particulate-level bond energies? - (A) Energy released forming product bonds exceeds energy required to break reactant bonds. - (B) Energy required to break reactant bonds exceeds energy released forming product bonds. - (C) Product bonds are weaker and less stable than the reactant bonds that were broken. - (D) Product bonds have higher potential energy than the reactant bonds that were broken. **Answer: (A)** Connects the net release of energy (exothermic) to the fundamental difference between the energy input for bond breaking and energy output from bond formation. ### Question 10 Consider a reaction where reactants $$R$$ form products $$P$$ through a single transition state $$TS$$. If a different conformer of $$R$$ with higher potential energy is used, while the energy of $$TS$$ remains constant, how do the activation energy ($$E_a$$) and reaction enthalpy ($$\Delta H$$) change? - (A) $$E_a$$ decreases and $$\Delta H$$ becomes more exothermic - (B) $$E_a$$ increases and $$\Delta H$$ becomes more endothermic - (C) $$E_a$$ decreases and $$\Delta H$$ becomes less exothermic - (D) $$E_a$$ increases and $$\Delta H$$ becomes less endothermic **Answer: (A)** Analyzing the profile variables shows that $$E_a = E_{TS} - E_R$$ and $$\Delta H = E_P - E_R$$. Increasing $$E_R$$ reduces the distance to the peak (lower $$E_a$$) and increases the distance down to the products (more negative $$\Delta H$$). ## Review the Unit - [6.1 Endothermic and Exothermic Processes](/ap-chem/unit-6/endothermic-exothermic-processes/study-guide/IBke22j0ulvsM9ps3QPJ) - [6.2 Energy Diagrams ](/ap-chem/unit-6/energy-diagrams/study-guide/oASenD5gSuLH8VI0Yrxa) - [6.3 Heat Transfer and Thermal Equilibrium](/ap-chem/unit-6/heat-transfer-thermal-equilibrium/study-guide/WJd1kCvPeSS08sVOaDPt) - [6.4 Heat Capacity and Calorimetry](/ap-chem/unit-6/heat-capacity-calorimetry/study-guide/jShImkrhZMnPWxlEjdwN) - [6.5 Energy of Phase Changes](/ap-chem/unit-6/energy-phase-changes/study-guide/kAXAzHrD24XL6LdpMFHh) - [6.6 Introduction to Enthalpy of Reaction](/ap-chem/unit-6/intro-enthalpy-reaction/study-guide/MZToO4yTqx0BDiUxmvhN) - [6.7 Bond Enthalpies](/ap-chem/unit-6/bond-enthalpies/study-guide/Q35odLQQyndAgpsA3iwn) - [6.8 Enthalpy of Formation](/ap-chem/unit-6/enthalpy-formation/study-guide/glO3L5mcfcUwCd0ODBej) - [6.9 Hess’s Law](/ap-chem/unit-6/hess-law/study-guide/p9ryCGfaOvpZj0Qye5eT) ## Other Unit Tests - [Unit 1: Atomic Structure & Properties](/ap-chem/unit-test/1) - [Unit 2: Compound Structure and Properties](/ap-chem/unit-test/2) - [Unit 3: Properties of Substances and Mixtures](/ap-chem/unit-test/3) - [Unit 4: Chemical Reactions](/ap-chem/unit-test/4) - [Unit 5: Kinetics](/ap-chem/unit-test/5) - [Unit 7: Equilibrium](/ap-chem/unit-test/7) - [Unit 8: Acids & Bases](/ap-chem/unit-test/8) - [Unit 9: Thermodynamics and Electrochemistry](/ap-chem/unit-test/9) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)