--- title: "AP Chemistry Unit 9 Practice Test" description: "A timed AP Chemistry Unit 9 (Thermodynamics and Electrochemistry) practice test in the AP format, drawn from 4,101 multiple-choice questions and 114 FRQs written for the unit. The standard version takes about 60 minutes." canonical: "https://fiveable.me/ap-chem/unit-test/9" type: "unit-test" subject: "AP Chemistry" unit: "Unit 9" --- # AP Chemistry Unit 9 Practice Test ## Overview A timed AP Chemistry Unit 9 (Thermodynamics and Electrochemistry) practice test in the AP format, drawn from 4,101 multiple-choice questions and 114 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=9): 12 questions - [FRQ 2 – Long Answer](/ap-chem/frqs/frq-2?unit=9): 16 questions - [FRQ 3 – Long Answer](/ap-chem/frqs/frq-3?unit=9): 10 questions - [FRQ 4 – Short Answer](/ap-chem/frqs/frq-4?unit=9): 15 questions - [FRQ 5 – Short Answer](/ap-chem/frqs/frq-5?unit=9): 17 questions - [FRQ 6 – Short Answer](/ap-chem/frqs/frq-6?unit=9): 29 questions - [FRQ 7 – Short Answer](/ap-chem/frqs/frq-7?unit=9): 15 questions ## Practice Questions ### Question 1 A researcher plots the standard Gibbs free energy change (ΔG°) on the y-axis versus temperature (T) on the x-axis for a chemical reaction. The resulting line has a negative slope and a positive y-intercept. Based on the linear relationship ΔG° = ΔH° − TΔS°, which statement correctly identifies the signs of the enthalpy and entropy changes indicated by the graph? - (A) The enthalpy change is positive and the entropy change is positive. - (B) The enthalpy change is positive and the entropy change is negative. - (C) The enthalpy change is negative and the entropy change is positive. - (D) The enthalpy change is negative and the entropy change is negative. **Answer: (A)** Connecting the linear equation y = mx + b to the thermodynamic function shows that the y-intercept corresponds to ΔH° and the slope corresponds to −ΔS°. A positive intercept indicates ΔH° > 0, and a negative slope indicates −ΔS° < 0, meaning ΔS° must be positive. ### Question 2 A heating curve plots temperature versus heat added for a pure substance. The horizontal segment representing vaporization is significantly longer than the horizontal segment representing melting. Which conclusion about the standard entropy of vaporization (ΔS°vap) compared to the standard entropy of fusion (ΔS°fus) is best supported by this graphical difference? - (A) ΔS°vap > ΔS°fus because the gas phase has greater matter dispersal than the liquid. - (B) ΔS°vap < ΔS°fus because the higher temperature reduces the calculated entropy change. - (C) ΔS°vap = ΔS°fus because the mass of the substance remains constant during heating. - (D) ΔS°vap ≈ 0 because the temperature remains constant throughout the phase transition. **Answer: (A)** Interpreting the graph shows that vaporization requires more energy (longer segment) than fusion. Since ΔS = q/T, the large heat input combined with the massive increase in disorder (dispersal of matter) when forming a gas results in ΔS°vap being significantly larger than ΔS°fus. ### Question 3 A graph of ln K versus 1/T for the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) yields a straight line with a negative y-intercept. Based on the relationship ln K = (−ΔH°/R)(1/T) + (ΔS°/R), what does the y-intercept indicate about the entropy change of the system? - (A) The entropy change is negative, consistent with the decrease in moles of gas. - (B) The entropy change is negative, consistent with the increase in moles of gas. - (C) The entropy change is positive, consistent with the decrease in moles of gas. - (D) The entropy change is positive, consistent with the increase in moles of gas. **Answer: (A)** Identifying the y-intercept as ΔS°/R allows for the determination of the sign of ΔS°. A negative intercept means ΔS° is negative, which aligns with the reaction stoichiometry showing a reduction from 3 moles of gas reactants to 2 moles of gas products. ### Question 4 Consider the phase changes of water: fusion ($$H_2O(s) \rightarrow H_2O(l)$$) and vaporization ($$H_2O(l) \rightarrow H_2O(g)$$). Based on the dispersal of matter, which statement correctly compares the magnitude of the standard entropy change ($$\Delta S^\circ$$) for these processes? - (A) $$\Delta S^\circ_{vap}$$ is larger than $$\Delta S^\circ_{fus}$$ because the volume increase from liquid to gas is much greater. - (B) $$\Delta S^\circ_{fus}$$ is larger than $$\Delta S^\circ_{vap}$$ because the structural disruption from solid to liquid is greater. - (C) $$\Delta S^\circ_{vap}$$ is smaller than $$\Delta S^\circ_{fus}$$ because the intermolecular forces in the liquid state are stronger. - (D) $$\Delta S^\circ_{fus}$$ is equal to $$\Delta S^\circ_{vap}$$ because both processes involve breaking intermolecular forces. **Answer: (A)** Comparing the relative dispersal of matter in phase changes reveals that the transition from liquid to gas involves a massive increase in volume and particle freedom compared to solid to liquid. ### Question 5 The decomposition of solid calcium carbonate is represented by the equation $$CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$$. Based on the states of matter involved, which statement best predicts the sign of the standard entropy change ($$\Delta S^\circ$$) for this reaction? - (A) Positive, because the production of a gas from a solid significantly increases matter dispersal. - (B) Negative, because the formation of a solid oxide creates a highly ordered crystal lattice structure. - (C) Positive, because the breaking of ionic bonds in the reactant requires a large input of energy. - (D) Negative, because the reactant is a complex solid structure that decomposes into simpler substances. **Answer: (A)** Analyzing the states of matter shows that generating a gaseous product from a solid reactant drastically increases the number of accessible microstates and disorder. ### Question 6 When solid ammonium nitrate ($$NH_4NO_3$$) dissolves in water, the particles dissociate into ions. Which explanation best describes the primary driving force for the increase in entropy during this process? - (A) Entropy increases because the crystal lattice breaks down, dispersing ions throughout the solvent. - (B) Entropy decreases because water molecules form ordered hydration shells around the dissolved ions. - (C) Entropy increases because the endothermic dissolution process absorbs heat from the surroundings. - (D) Entropy decreases because the ions lose their vibrational energy upon entering the liquid phase. **Answer: (A)** Connecting the dissolution process to particle dispersal reveals that breaking the ordered solid lattice to allow ions to move freely in the solvent volume drives the entropy increase. ### Question 7 A rigid container is divided into two equal bulbs by a valve. One bulb contains 1.0 mol of $$He(g)$$ at 298 K, and the other is evacuated. When the valve is opened, the gas expands to fill both bulbs. Which statement best explains the change in entropy? - (A) Entropy increases because the gas occupies a larger volume, increasing the number of accessible microstates. - (B) Entropy remains constant because the temperature of the gas is unchanged during the expansion into a vacuum. - (C) Entropy decreases because the pressure of the gas drops significantly as it fills the total volume. - (D) Entropy increases because the gas molecules gain kinetic energy while moving into the empty bulb. **Answer: (A)** Applying statistical probability principles indicates that expanding into a larger volume increases the number of possible positions (microstates) for the particles. ### Question 8 Aqueous solutions of lead(II) nitrate and potassium iodide are mixed, forming a precipitate of lead(II) iodide according to the equation $$Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)$$. Which statement correctly predicts the sign of $$\Delta S^\circ$$ for this reaction? - (A) Negative, because mobile ions in solution combine to form a rigid, highly ordered solid structure. - (B) Positive, because mixing two aqueous solutions increases the total volume available to the ions. - (C) Negative, because the molar mass of the solid product is greater than that of the individual ions. - (D) Positive, because the reaction is exothermic and releases energy that increases solvent disorder. **Answer: (A)** Predicting the entropy change involves recognizing that transitioning from free-moving aqueous ions to a fixed solid lattice significantly reduces particle freedom. ### Question 9 The synthesis of methanol is represented by the reaction $$CO(g) + 2H_2(g) \rightarrow CH_3OH(g)$$. Which of the following best explains the sign of $$\Delta S^\circ$$ for this reaction? - (A) Negative, because three moles of gaseous reactants combine to form one mole of gaseous product. - (B) Positive, because the product molecule is more complex and has more internal vibrational modes. - (C) Negative, because the formation of stronger covalent bonds releases energy to the surroundings. - (D) Positive, because the product has a higher molar mass than either of the individual reactants. **Answer: (A)** Counting gas moles reveals a decrease from 3 to 1, which corresponds to a significant reduction in the number of particles and thus a negative entropy change. ### Question 10 Which of the following gas-phase reactions results in the largest increase in the standard entropy ($$\Delta S^\circ$$) of the system? - (A) The decomposition of $$2H_2O(g)$$ into $$2H_2(g)$$ and $$O_2(g)$$ - (B) The synthesis of $$2NH_3(g)$$ from $$N_2(g)$$ and $$3H_2(g)$$ - (C) The oxidation of $$2NO(g)$$ by $$O_2(g)$$ to form $$2NO_2(g)$$ - (D) The combination of $$H_2(g)$$ and $$Cl_2(g)$$ to form $$2HCl(g)$$ **Answer: (A)** Analyzing the stoichiometry shows this reaction produces a net increase of +1 mole of gas (2 mol $\to$ 3 mol), resulting in the most positive entropy change. ## Review the Unit - [9.1 Introduction to Entropy](/ap-chem/unit-9/intro-entropy/study-guide/rrwnj8YrrJ2xOtgibBsL) - [9.2 Absolute Entropy and Entropy Change](/ap-chem/unit-9/absolute-entropy-entropy-change/study-guide/RhcmO6DuTuEoGzcCk2zG) - [9.3 Gibbs Free Energy and Thermodynamic Favorability](/ap-chem/unit-9/gibbs-free-energy-thermodynamic-favorability/study-guide/hCJVI2XJaSGmj1c3zvrO) - [9.4 Thermodynamic and Kinetic Control](/ap-chem/unit-9/thermodynamic-kinetic-control/study-guide/hRZ0V3goVueXCw1JeUdA) - [9.5 Free Energy and Equilibrium](/ap-chem/unit-9/free-energy-equilibrium/study-guide/ConxkA0PUZM7vwf8nPNr) - [9.6 Free Energy of Dissolution](/ap-chem/unit-9/free-energy-dissolution/study-guide/ci7mbcleKOYTH8HZ) - [9.7 Coupled Reactions](/ap-chem/unit-9/coupled-reactions/study-guide/iNXDbmYm10NlojcZ3cu4) - [9.8 Galvanic (Voltaic) and Electrolytic Cells](/ap-chem/unit-9/galvanic-voltaic-electrolyticlls/study-guide/egSkWaC0jSmJvCszUAkK) - [9.9 Cell Potential and Free Energy](/ap-chem/unit-9/cell-potential-free-energy/study-guide/GLRagoPDoMJ35XxbRbdb) - [9.10 Cell Potential Under Nonstandard Conditions](/ap-chem/unit-9/cell-potential-under-non-standard-conditions/study-guide/iVpmBAtYorzFr6ljbcdT) - [9.11 Electrolysis and Faraday's Law](/ap-chem/unit-9/electrolysis-faradays-law/study-guide/v39gKqGDszHzsS5opd6x) ## 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 6: Thermochemistry](/ap-chem/unit-test/6) - [Unit 7: Equilibrium](/ap-chem/unit-test/7) - [Unit 8: Acids & Bases](/ap-chem/unit-test/8) ## 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`)