AP Chemistry — Unit 6 Progress Check: MCQ

3. An advertisement for a commercial hand warmer claims that the hand warmer works because a chemical reaction in the hand warmer draws out the body's own natural heat, causing a warming effect. Which of the following states the accuracy of the claim in the advertisement and best provides a correct scientific justification of the claim?

A. The advertisement's claim is inaccurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was endothermic, which would cause the hands to feel colder.

17. The combustion of methane proceeds according to the chemical equation CH₄ + 2O₂ → CO₂ + 2H₂O. The table above provides the average bond enthalpy for selected bonds. The calculated enthalpy change for the reaction, ∆Hrxn, is -802 kJ/mol. Which of the following provides the mathematical procedure to estimate the bond enthalpy per mole of O₂?

A. ½[-802 + 2(799) + 4(464) - 4(414)] kJ/mol

19. CaCO₃ decomposes according to the balanced equation CaCO₃(s) → CaO(s) + CO₂(g). Based on the standard enthalpies of formation provided in the table above, what is the approximate enthalpy change of the reaction?

B. +180 kJ

18. The formation of hydrogen chloride gas is represented by the chemical equation H₂(g) + Cl₂(g) → 2HCl(g). Based on the bond enthalpy data in the table above, what is the approximate enthalpy change for the reaction?

B. -185 kJ/molrxn

20. Based on the information above, what is ∆H° for the reaction SO₂(g) + ½O₂(g) → SO₃(g)?

B. -99.1 kJ/molrxn

7. The graph above shows the changes in temperature recorded for the 2.00 L of H₂O surrounding a constant-volume container in which a 1.00 g sample of benzoic acid was combusted. Assume that heat was not absorbed by the container or lost to the surroundings, that the density of H₂O is 1.00g/mL, and that the specific heat capacity of H₂O is about 4.2J/g⋅°C. Based on this information, estimate how much heat was released from the combustion of the benzoic acid sample.

B. 25 kJ

15. 2CO(g) + C(g) → C₃O₂(g) ∆H° = 127.3 kJ/molrxn The equation shown above represents an endothermic reaction between CO(g) and C(g). What is the amount of heat absorbed when 1.00 mol of CO(g) reacts with an excess C(g).

B. 63.7 kJ

12. The heating curve for 1.0 mole of Na, initially at 25.0°C, is shown above at the left. Which of the following best explains the change in heat when 0.50 mole of Na undergoes the trans

B. Approximately 49 kJ of heat are absorbed to overcome the attractive forces acting between Na atoms.

4. A student carefully drops a 9.0g solid Zn pellet initially at 50.0°C into an insulated cup containing 30.0g of water at 27.8°C. The student predicts that the temperature of the water will increase after the pellet is added. Which of the following statements is the best justification for the student's prediction?

B. Collisions between the water molecules and the surface of the Zn pellet will result in the transfer of energy, increasing the average kinetic energy of the water molecules.

5. Two aqueous solutions of equal volume and concentration were kept in flasks and held at different temperatures. The two solutions were combined in a larger flask. Based on this information, which of the following predictions is correct?

B. The average kinetic energy of the particles in the cooler solution will increase as they collide with the particles from the warmer solution.

24. The table above provides the values for the standard enthalpy change for two chemical reactions. A student need to calculate the enthalpy change for the reaction 2HNO₃(l) → N₂O₅(g) + H₂O(l) under standard conditions using Hess's law. In addition to the values given in the table, what information will the student need to perform the calculation?

B. ∆H°f for N₂O₅(g)

9. A student dissolves 8.00 g of NH₄NO₃(s) in 100. g of H₂O(l) in a coffee-cup calorimeter. Based on the initial and final temperatures of the mixture in degrees Celsius shown in the diagram above the laboratory setup, what is the calculated ∆T of the water reported to the appropriate number of significant figures?

C. -5.5°C

11. The table above provides the values for some physical properties of H₂S and H₂O. Approximately, how many moles of H₂S must be condensed to release as much heat as would be released when 1 mole of H₂O is condensed?

C. 2 moles of H₂S

23. Reaction 1: 2O₃(g) → 3O₂(g) Reaction 2: ? Overall Reaction: 2O₃(g) + 2NO(g) → 2NO(g) + 2O₂(g) The enthalpy of the overall reaction represented above can be determined by adding the enthalpies of reactions 1 and 2. Which of the following could be reaction 2?

C. 2NO(g) + O₂(g) → 2NO₂(g)

16. 2O₃(g) → 3O₂(g) ∆H° = -300 kJ/molrxn The conversion of ozone to diatomic oxygen is represented by the equation above. Based on the data in the table above, what is the approximate average bond enthalpy for the oxygen-to-oxygen bonds in ozone?

C. 300 kJ/mol

13. Na₂S₂O₃(aq) + 4NaOCl(aq) + 2NaOH(aq) → 2Na₂SO₄(aq) + 4NaCl(aq) + H₂O(l) The enthalpy change for the reaction represented by the chemical equation shown above is ∆H° = -1236 kJ/molrxn. When 2.00 mol of NaCl(aq) reacts completely with an excess of Na₂S₂O₃(aq) and NaOH(aq), which of the following enthalpy changes occurs

C. 618 kJ of heat is released.

22. C₂H₆(g) + ⁷⁄₂O₂(g) → 2CO₂(g) + 3H₂O(l) ∆H° = -1560 kJ C(graphite) + O₂(g) → CO₂(g) ∆H° = -394 kJ Given the equations and the values of ∆H° for the combustion of C₂H₆(g) and the combustion of C(graphite) represented above, which of the following additional information is needed to determine the values ∆H° for the overall reaction 2C(graphite) + 3H₂(g) → C₂H₆(g)?

C. H₂(g) + ½O₂(g) → H₂O(l) ∆H° = -286 kJ

2. When pellets of NaOH(s) are added to a flask of water, it is observed that the temperature of the water increases as the pellets dissolve. Which of the following claims about the observed dissolution of NaOH(s) in water is most accurate?

C. It is an exothermic process because heat energy is absorbed by the water as the NaOH(s) dissolves in it.

1. The chemical reaction between HCl(aq) and NH3(aq) is represented above. A student combines equimolar amounts of HCl(aq) and NH₃(aq), both solutions initially at 24°C, in a coffee-cup calorimeter. The student observes that the mixture reaches a temperature of 28°C. Based on the experimental results, which of the following can be concluded about the reaction?

C. It is an exothermic process, because energy is released by the reaction and is gained by the reaction mixture.

10. Particulate models of the evaporation of liquid water and the condensation of water vapor are shown above. Based on these models, which of the following accurately compares the energy changes associated with each of the phase changes?

C. The amount of energy absorbed by one mole of water molecules as they escape the liquid is equal to the amount of energy released by one mole of water molecules as they come together to form a liquid.

21. Natural gas consists primarily of CH₄, which is combusted according to the following chemical equation? CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) Based on the standard enthalpies of formation in the table above, which of the following expressions give the approximate enthalpy change for the reaction (∆H°rxn)?

C. ∆H°rxn = [(-390 kJ/mol) + 2(-240 kJ/mol)] - (-75 kJ/mol)

8. For an experiment, a 20.0 g piece of Al was placed inside an insulated cup filled with 40.0 g of H₂O. The data are given in the table above. Assuming no heat is absorbed by the container or the surroundings, which of the following equations provides the correct mathematical relationship for the transfer of heat q between Al and H₂O?

D. -qAl = qH₂O

14. 2Al(s) + Fe₂O₃(s) → 2Fe(s) + Al₂O₃(s) ∆H° = -850 kJ/molrxn The chemical equation shown above represents the thermite reaction. What is the approximate amount of heat released when 108 g of Al(s) reacts with excess Fe₂O₃(s)?

D. 1700 kJ

6. The table provides data for two CH₃OH(l) samples. Based on this information, which of the following statements describes what happens when these samples are initially mixed, and why?

D. The CH₃OH molecules from sample 1 transfer thermal energy to the CH₃OH molecules from sample 2 through collisions because the average kinetic energy of the molecules in sample 1 is greater.