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Chapter Six

Chapter Six. Thermochemistry. Question. Label the following process as exothermic or endothermic: The freezing of water. a) Exothermic b) Endothermic. Answer. a) Exothermic For water to freeze, heat must be released from the water to the surroundings. Question.

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Chapter Six

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  1. Chapter Six Thermochemistry

  2. Question • Label the following process as exothermic or endothermic: • The freezing of water. a) Exothermic b) Endothermic

  3. Answer • a) Exothermic • For water to freeze, heat must be released from the water to the surroundings.

  4. Question • Label the following process as exothermic or endothermic: • Your hand gets cold when you touch ice. a) Exothermic b) Endothermic

  5. Answer • a) Exothermic • For your hand to get cold, heat must be transferred from your hand to the ice.

  6. Question • Label the following process as exothermic or endothermic: • The ice increases in temperature when you touch it. a) Exothermic b) Endothermic

  7. Answer • b) Endothermic • Heat is transferred from the hand (surroundings) to the ice (system).

  8. Question • Label the following process as exothermic or endothermic: • Water vapor condenses on a cold pipe. a) Exothermic b) Endothermic

  9. Answer • a) Exothermic • For water vapor to condense, heat must be transferred from the water vapor to the cold pipe.

  10. Question • Which of the following statements is true? a) Change in enthalpy is a state function. b) In exothermic reactions, the reactants are lower in potential energy than the products. c) A chemist takes the point of view of the surroundings when determining the sign for heat, q. d) The heat of reaction and change in enthalpy can always be used interchangeably. e) More than one of the above statements is true.

  11. Answer • a) Change in enthalpy is a state function. • Change in enthalpy does not depend on the pathway, making it a state function.

  12. Question • Given the equation: • S (s) + O2 (g)  SO2 (g), H = –296 kJ, • which of the following statement(s) is (are) true? • I. The reaction is exothermic. • II. When 0.500 mol sulfur is reacted, • 148 kJ of energy is released. • III. When 32.0 g of sulfur is burned, 2.96  105 J of energy is released. • a) All are true. • b) None is true. • c) I and II are true. • d) I and III are true. • e) Only II is true.

  13. Answer • a) All are true. • A reaction with a negative change in enthalpy is exothermic. • Based on the stoichiometry, if 0.500 mol of sulfur was consumed, H = (½)(–296 kJ). If 1.00 mol (32.0 g) of sulfur is burned, 296 kJ of energy is released.

  14. Question • Two different metals of equal mass with different specific heat capacities absorb the same amount of heat. Which undergoes the smallest change in temperature? a) The metal with the higher specific heat capacity b) The metal with the lower specific heat capacity c) Because they have equal mass, both metals undergo the same change in temperature.

  15. Answer • a) The metal with the higher specific heat • capacity • Since each metal absorbs the same amount of heat: qmetal1 = qmetal2. Since the mass of each metal is the same, • (specific heat capacitymetal1)(tmetal1) = • = (specific heat capacitymetal2)(tmetal2), and since the heat capacity and the change in temperature are inversely related, for a smaller change in temperature, the heat capacity would have to be higher.

  16. Question • If 5.0 kJ of energy is added to a 15.5-g sample of water at 10.°C, the water is a) boiling. b) completely vaporized. c) frozen solid. d) still a liquid.

  17. Answer • d) still a liquid. • In order to decide on an answer, use the equation q = s×m× Δt (where Δt = tf−ti) so • Since tf is 87°C, the water is still a liquid.

  18. Question • A 50.0-g sample of water at 80°C is added to a 50.0-g sample of water at 20°C. The final temperature of the water should be a) between 20°C and 50°C. b) 50°C. c) between 50°C and 80°C.

  19. Answer • b) 50°C. • In both samples, mass and specific heat capacity are the same. So, the final temperature should be the average of the two temperatures.

  20. Question • A 50.0-g sample of water at 80°C is added to a 100.0-g sample of water at 20°C. The final temperature of the water should be a) between 20°C and 50°C. b) 50°C. c) between 50°C and 80°C.

  21. Answer • a) between 20°C and 50°C. • The mass of the cooler water is greater, so the final temperature should be lower than the average of the two temperatures.

  22. Question • You have a 50.0-g sample of water at 20°C and add a 50.0-g sample of iron at 80°C. The final temperature of the water should be a) between 20°C and 50°C. b) 50°C. c) between 50°C and 80°C.

  23. Answer • a) between 20°C and 50°C. • Iron has a lower specificheat capacity than water (and the masses are the same). Therefore, the change in temperature of the water will be less than that of the iron. So, the final temperature will be less than the average temperature of the iron and the water.

  24. Question • Hypothetical elements A2 and B2 react according to the following reaction, forming the compound AB: • A2 (aq) + B2 (aq)  AB (aq) H° = −265 kJ/mol • If solutions A2 (aq) and B2 (aq), starting at the same temperature, are mixed in a coffee-cup calorimeter, the reaction that occurs is a) exothermic, and the temperature of the resulting solution rises. b) endothermic, and the temperature of the resulting solution rises. c) exothermic, and the temperature of the resulting solution falls. d) endothermic, and the temperature of the resulting solution falls.

  25. Answer • a) exothermic, and the temperature of the resulting solution rises. • Since the H° is negative, the reaction is exothermic. Exothermic reactions release heat to their surroundings (the water), and so the temperature of the solution rises.

  26. Question • Given the following reaction: • 2CO (g) + O2 (g)  2CO2 (g) H° = −566.0 kJ • Calculate the enthalpy change for the reaction: • CO (g) + ½ O2 (g)  CO2 (g). a) −566.8 kJ b) −283.0 kJ c) 283.0 kJ d) 566.0 kJ e) −1132 kJ

  27. Answer b) −283.0 kJ • Since each of the stoichiometric coefficients in the reaction is cut in half, the enthalphy change is also cut in half.

  28. Question • Given the following reaction: • 2CO (g) + O2 (g)  2CO2 (g) H° = −566.0 kJ • Calculate the enthalpy change for the reaction: • 2CO2 (g)  2CO (g) + O2 (g). a) −566.8 kJ b) −283.0 kJ c) 283.0 kJ d) 566.0 kJ e) −1132 kJ

  29. Answer • d) 566.0 kJ • Since the reaction is reversed, the sign of the enthalphy change is (−1) times the original reaction.

  30. Question • Use the following equations: • C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l)= −2219.9 kJ • CO (g) + ½ O2 (g)  CO2 (g) = −283.0 kJ • to calculate the enthalpy change for the reaction: • C3H8 (g) + O2 (g)  3CO (g) + 4H2O (l)= ? a) −1936.9 kJ b) −1370.9 kJ c) 1370.9 kJ d) −3068.9 kJ e) 3068.9 kJ

  31. Answer • b) −1370.9 kJ • C3H8 (g) + 5O2 (g)  3CO2 (g) + 4H2O (l)H = −2219.9 kJ • 3CO2 (g)  3CO (g) + 3/2O2 (g) H = −3(−283.0 kJ) • C3H8 (g) + O2 (g)  3CO (g) + 4H2O (l)H = −1370.9 kJ • Since C3H8 is a reactant in the first reaction and the desired reaction, the reaction can be used as is. Since 3 moles of CO are required as a product in the desired reaction and the second reaction has only 1 mole of CO as a reactant, the reaction must be reversed and multiplied by 3. Thus, the H = − 3. Adding the reactions cancels CO2 and excess O2.

  32. Question • The standard enthalpy of formation of sulfuric acid, H2SO4 (l), refers to the H° of which of the following reactions? • a) H2SO4 (l)  H2 (g) + S (s) + 2O2 (g) • b) H2 (g) + S (g) + 2O2 (g)  H2SO4 (l) • c) H2SO4(l)  2H (g) + S (s) + 4O (g) • d) H2 (g) + S (s) + 2O2 (g)  H2SO4 (l) • e) 2H (g) + S (g) + 4O (g)  H2SO4 (l)

  33. Answer • d) H2 (g) + S (s) + 2O2 (g)  H2SO4 (l) • The standard enthalpy of formation of a substance is the H° of the reaction where 1 mol of the compound is formed from its elements in their reference form and in their standard states. For sulfuric acid, H2SO4 (l), the elements are H2 (g), S (s), and O2 (g).

  34. Question • Calculate for the combustion reaction of ethanol, C2H5OH, shown below given the following data: • C2H5OH(l) = −276.98 kJ/mol •  H2O (l) = −285.5 kJ/mol •  H2O (g) = −241.8 kJ/mol • CO2 (g) = −393.5 kJ/mol • C2H5OH (l) + 3O2 (g)  2CO2 (g) + 3H2O (l). a) 1356.5 kJ b) 993.6 kJ c) −1356.5 kJ d) −993.6 kJ e) −1197.8 kJ

  35. Answer • c) −1356.5 kJ • Use Hess’s Law to calculate the enthalpy change: • Be sure to choose the proper phase for each species and the proper stoichiometric coefficient. Combustion reactions are exothermic.

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