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AP Chemistry Unit 6 practice questions

Try 10 multiple-choice questions from Unit 6 here. Pick an answer to see why it's right or wrong, then take the timed test above for a full AP-style set.

  1. 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?

    The magnitude of the enthalpy will be lower than the actual value, leading to an underestimation of the energy released.

    The magnitude of the enthalpy will be lower than the actual value, leading to an overestimation of the energy released.

    The magnitude of the enthalpy will be higher than the actual value, leading to an underestimation of the energy released.

    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.

    Show answer

    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.

  2. 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?

    2NO(g) + 2H2(g) → N2(g) + 2H2O(g)

    2NO(g) + H2(g) → N2(g) + H2O2(g)

    2NO(g) + H2(g) → N2O(g) + H2O(g)

    N2(g) + 2H2O(g) → 2NO(g) + 2H2(g)

    Show answer

    Answer: (A)

    Summing the two elementary steps and canceling the intermediate H2O2 yields the overall balanced equation.

  3. 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?

    4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(g)

    4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)

    4NH3(g) + 6O2(g) → 4NO2(g) + 6H2O(g)

    4NH3(g) + 8O2(g) → 4NO2(g) + 6H2O(g)

    Show answer

    Answer: (A)

    Multiplying the second step by 2 allows the intermediate NO to cancel, resulting in the correct stoichiometric sum.

  4. 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?

    2NO(g) + O2(g) → 2NO2(g)

    2NO(g) + 2O(g) → 2NO2(g)

    NO(g) + O(g) → NO2(g)

    N2(g) + 2O2(g) → 2NO2(g)

    Show answer

    Answer: (A)

    Subtracting the first step from the overall equation reveals the second step needed to complete the stoichiometry.

  5. Question 5

    A multistep reaction is overall exothermic with ΔH=−50\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?

    It incorrectly represents the thermodynamics because the products should be at lower energy than the reactants.

    It incorrectly represents the thermodynamics because an exothermic reaction must have only one transition state.

    It correctly represents the thermodynamics because the activation energy raises the final potential energy.

    It correctly represents the thermodynamics because the intermediate state must be lower than the final state.

    Show answer

    Answer: (A)

    Evaluating the thermodynamic consistency requires comparing reactant and product energies. An exothermic reaction (ΔH<0\Delta H < 0) releases energy, meaning the final product state must be lower in potential energy than the initial reactant state.

  6. Question 6

    A reaction is endothermic with an enthalpy change of +50+50 kJ/mol. A student's energy profile shows an activation energy of +30+30 kJ/mol. Which statement correctly evaluates the physical possibility of this model?

    The model is impossible because the activation energy must be at least equal to the enthalpy change.

    The model is impossible because endothermic reactions must have negative activation energy values.

    The model is possible because the transition state energy is independent of the product energy.

    The model is possible because the activation energy is determined solely by the reactant stability.

    Show answer

    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 (Ea≥ΔHE_a \ge \Delta H).

  7. Question 7

    A potential energy diagram for a multistep reaction displays reactants at 150150 kJ/mol and final products at 8080 kJ/mol. The highest peak in the profile reaches 250250 kJ/mol. What is the value of the overall enthalpy change (ΔH∘\Delta H^\circ)?

    −70-70 kJ/mol, calculated as the energy of products minus reactants

    +70+70 kJ/mol, calculated as the energy of reactants minus products

    +100+100 kJ/mol, calculated as the peak energy minus reactants

    +170+170 kJ/mol, calculated as the peak energy minus products

    Show answer

    Answer: (A)

    Identifying the overall enthalpy change requires subtracting the energy of the reactants from the energy of the products (80−150=−7080 - 150 = -70 kJ/mol). The peak energy represents the transition state and is not used for calculating ΔH∘\Delta H^\circ.

  8. Question 8

    A reaction profile shows reactants at 5050 kJ, a transition state at 150150 kJ, and products at 100100 kJ. Which option correctly describes the thermodynamics and kinetics of this reaction?

    Endothermic with an activation energy of 100100 kJ

    Endothermic with an activation energy of 5050 kJ

    Exothermic with an activation energy of 100100 kJ

    Exothermic with an activation energy of 5050 kJ

    Show answer

    Answer: (A)

    Analyzing the profile reveals two key properties: the reaction is endothermic because products (100100 kJ) are higher than reactants (5050 kJ), and the activation energy is the height of the barrier (150−50=100150 - 50 = 100 kJ).

  9. 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?

    Energy released forming product bonds exceeds energy required to break reactant bonds.

    Energy required to break reactant bonds exceeds energy released forming product bonds.

    Product bonds are weaker and less stable than the reactant bonds that were broken.

    Product bonds have higher potential energy than the reactant bonds that were broken.

    Show answer

    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.

  10. Question 10

    Consider a reaction where reactants RR form products PP through a single transition state TSTS. If a different conformer of RR with higher potential energy is used, while the energy of TSTS remains constant, how do the activation energy (EaE_a) and reaction enthalpy (ΔH\Delta H) change?

    EaE_a decreases and ΔH\Delta H becomes more exothermic

    EaE_a increases and ΔH\Delta H becomes more endothermic

    EaE_a decreases and ΔH\Delta H becomes less exothermic

    EaE_a increases and ΔH\Delta H becomes less endothermic

    Show answer

    Answer: (A)

    Analyzing the profile variables shows that Ea=ETS−ERE_a = E_{TS} - E_R and ΔH=EP−ER\Delta H = E_P - E_R. Increasing ERE_R reduces the distance to the peak (lower EaE_a) and increases the distance down to the products (more negative ΔH\Delta H).

What's on the AP Chemistry Unit 6 practice test

The standard version takes about 60 minutes and follows the timing and directions of the AP exam, sized to one unit:

  • 20 Multiple Choice Questions (30 minutes)
  • 1 FRQ – FRQ 1 – Long Answer (15 minutes)
  • 1 FRQ – FRQ 4 – Short Answer (15 minutes)

Questions are drawn from 3009 multiple-choice questions and 99 FRQs written for Unit 6, Thermochemistry.

Practice AP Chemistry Unit 6 FRQs

Answer the unit's free-response questions one at a time and get a score on every part.

Review Unit 6 before you start

Other AP Chemistry unit tests