# Chapter 5 Thermochemistry

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1 Chapter 5 Thermochemistry 1. The ΔE of a system that releases 14.4 J of heat and does 4.8 J of work on the surroundings is J. (a) J (b) J (c). 4.8 J (d) J Explanation: The ΔE can be calculated by using the equation ΔE = q + w. Since the system is releasing heat and is doing work on the surroundings both of these numbers will have negative signs. Here the system is losing its internal energy. Thus ΔE = (-4.8) = J 2. The value of ΔE for a system that performs 23 kj of work on its surroundings and loses 79 kj of heat is kj. (a). +23 kj (b) kj (c) kj (d). -79 kj Explanation: The ΔE can be calculated by using the equation ΔE = q + w. Since the system is losing heat and is doing work on the surroundings both of these numbers will have negative signs. Here the system is losing its internal energy. Thus ΔE = (-79) = -102 kj 3. Calculate the value of ΔE in joules for a system that loses 50 J of heat and has 150 J of work performed on it by the surroundings. (a). +50 J (b) J (c) J (d) J Explanation: The ΔE can be calculated by using the equation ΔE = q + w. Since the system is releasing heat the numerical value of the heat will be negative. At the same time the surroundings are doing work on the system and the work will have a positive sign. Here the system is gaining internal energy. Thus ΔE = +150 J + (-50 J) = +100 J Copyright 2006 Dr. Harshavardhan D. Bapat 1

2 4. The change in the internal energy of a system that releases 2,500 J of heat and that does 7,655 J of work on the surroundings is J. (a). +10,155 J (b). -5,155 J (c) J (d). -10,155 J Explanation: The ΔE can be calculated by using the equation ΔE = q + w. Since the system is releasing heat and is doing work on the surroundings both of these numbers will have negative signs. Here the system is losing its internal energy. Thus ΔE = J + (-7655 J) = J 5. The value of ΔH ο for the reaction below is -72 kj. How many kj of heat will be released when 1.0 mol of HBr is formed in this reaction? H 2 (g) + Br 2 (g) 2HBr (g) (a) kj (b). +72 kj (c). +36 kj (d). -36 kj Explanation: The ΔH ο for the reaction indicates the amount of heat released by the reaction where 2 moles of HBr (from the balanced equation) are formed. If only one mole of HBr is formed then the reaction should release only 36 kj (72/2) of heat. 6. The value of ΔH o for the reaction below is -126 kj. How many kj of heat will be released when 2.00 mol of NaOH is formed in the reaction? 2Na 2 O 2 (s) + 2H 2 O (l) 4NaOH (s) + O 2 (g) (a) kj (b). +63 kj (c) kj (d) kj Explanation: The ΔH ο for the reaction indicates the amount of heat released by the reaction where 4 moles of NaOH (from the balanced equation) are formed. If only two moles of NaOH are formed then the reaction should release only 63 kj (126/2) of heat. Copyright 2006 Dr. Harshavardhan D. Bapat 2

3 7. The value of ΔH o for the reaction below is -790 kj. Calculate the enthalpy change accompanying the complete reaction of 0.75 g of S 2S (s) + 3O 2 (g) 2SO 3 (g) (a). +23 kj (b) kj (c). -18 kj (d). +12 kj Explanation: The balanced equation shows a reaction in which 2 moles of S are reacting. This reaction has a value of ΔH o = -790 kj. Need to convert the grams of sulfur to moles of sulfur and then calculate the value of ΔH o for that number of moles of sulfur. Remember to factor in the 2 moles that are reacting in the equation. Failure to do this will give you an incorrect answer of -18 kj. 1moleS! 790 kj 0.75 g S" " =! 9.2 kj g S 2 moles S 8. The value of ΔH o for the reaction below is -482 kj. Calculate the heat (kj) released to the surroundings when 12.0 g of CO (g) reacts completely. 2CO (g) + O 2 (g) 2CO 2 (g) (a) x 10 3 kj (b) kj (c) kj (d) kj Explanation: The balanced equation involves 2 moles of CO reacting which releases 482 kj. To calculate the amount of heat released by the reaction of 12.0 g of CO, need to convert the grams of CO to moles of CO and then calculate the amount of heat released. Do not forget to factor in the 2 moles not doing this will give you the incorrect answer of -207 kj. 1mole CO! 482 kj 12.0 g CO " " =! 103 kj 28.01g 2 moles CO 9. The value of ΔH o for the reaction below is -336 kj. Calculate the heat (kj) released to the surroundings when 23.0 g of HC1 is formed. CH 4 (g) + 3C1 2 (g) CHC1 3 (l) + 3HCl (g) (a). 177 (b) x 10 3 (c) (d). 211 Copyright 2006 Dr. Harshavardhan D. Bapat 3

4 Explanation: The balanced equation involves the formation of 3 moles of HCl, which releases 336 kj. The grams of HCl need to be converted to moles and then the amount of heat released by this reaction can be calculated. Do not forget to factor in the 3 moles not doing this will give you the incorrect answer of -211 kj. 1mole HCl! 336 kj 23.0 g HCl " " =! 70.7 kj g 3 mole HCl 10. The value of ΔH o for the reaction below is +128.l kj: CH 3 OH (l) CO (g) + 2H 2 (g) How many kj of heat are consumed when 5.10 g of H 2 (g) is formed as shown in the equation? (a). 653 (b) (c). 324 (d). 162 Explanation: The balanced equation involves the formation of 2 moles of H 2, which consumes kj. Covert the grams of H 2 to moles of H 2 and then calculate the amount of heat consumed in this reaction. Do not forget to factor in the 2 moles of H 2, not doing this will give you the incorrect answer of 324 kj 1mole H kj 5.10 g H 2!! = kj g 2 moles H 11. The value of ΔH o for the reaction below is kj: 2 CH 3 OH (l) CO (g) + 2H 2 (g) How many kj of heat are consumed when 5.10 g of CO (g) is formed as shown in the equation? (a) kj (b). 162 kj (c) kj (d) kj Explanation: The balanced equation involves the formation of 1 mole of CO, which consumes kj. Covert the grams of CO to moles of CO and then calculate the amount of heat consumed in this reaction. 1mole CO kj 5.10 g CO!! = 23.3 kj 28.01g 1mole CO Copyright 2006 Dr. Harshavardhan D. Bapat 4

5 12. The value of ΔH o for the reaction below is kj: 2Ba (s) + O 2 (g) 2 BaO (s) How many kj of heat are released when 5.75 g of BaO (s) is produced? (a) kj (b) kj (c) kj (d). 193 kj Explanation: The balanced equation involves the formation of 2 moles of BaO, which releases 1107 kj of heat. Convert the grams of BaO produced and then calculate the amount of heat released. 1mole BaO! 1107 kj 5.75 g BaO" " =! 20.8 kj g 2 moles BaO 13. The molar heat capacity of a compound with the formula C 2 H 6 SO is 88.0 J/mol-K. The specific heat capacity of this substance is J/g-K. (a) (b) (c) (d) x 10 3 Explanation: This question is based on the definitions of the 2 quantities involved. The molar heat capacity of any substance is the heat capacity of one mole of the substance while the specific heat capacity involves only 1 gram of the substance. J Molar heat capacity 88.0 Specific heat capacity = = mol K = 1.13 Molar mass g mol J g K 14. A sample of aluminum metal absorbs 9.86 J of heat, upon which the temperature of the sample increased from 23.2 o C to 30.5 o C. Since the specific heat capacity of aluminum is 0.90 J/g-K, the mass of the sample is g. (a). 72 (b). 65 (c) (d). 8.1 Explanation: This question is based on the formula used to calculate the amount of heat q involved. The formula for this calculation is: q = m x c x ΔT, where m is the mass of the substance, c is its specific heat capacity and ΔT is the change in temperature. In this question the ΔT is a positive number as the T final ( Copyright 2006 Dr. Harshavardhan D. Bapat 5

6 = K) is higher than the T initial.( = K). It is important that the temperatures are converted to Kelvin and not left in the Celsius scale. q J m = = = 1.50 g c! ÄT 0.90 J/g! 7.30 K 15. The specific heat capacity of lead is 0.13 J/g-K. How much heat (in J) is required to raise the temperature of g of lead from o C to o C? (a). 2.0 J (b) J (c). 5.8 x 10-4 J (d) J Explanation: This question is based on the formula used to calculate the amount of heat q needed here. The sample would have to absorb heat to raise its temperature. q = m x c x ΔT q = (150 g) x (0.13 J/g K) x (15.00K) = J 16. Objects can possess energy as. (a) a only (b) a and b (c) a and c (d) b and c (a). endothermic energy (b). potential energy (c). kinetic energy Explanation: Only potential and kinetic are types of energy while endothermic relates to a process in which a system absorbs energy. 17. The internal energy of a system is always increased by. (a). adding heat to the system (b). having the system do work on the surroundings (c). withdrawing heat from the system (d). adding heat to the system and having the system do work on the surroundings. Explanation: Addition of heat is the only process listed here that will result in increasing the internal energy of a system. Copyright 2006 Dr. Harshavardhan D. Bapat 6

7 18. Which one of the following conditions would always result in an increase in the internal energy of a system? (a). The system loses heat and does work on the surroundings. (b). The system loses heat and has work done on it by the surroundings (c). The system gains heat and has work done on it by the surroundings. (d). None of the above is correct. 19. When a system ΔE is always negative. (a). absorbs heat and does work (b). gives off heat and does work (c). absorbs heat and has work done on it (d). none of the above is always negative 20. Which one of the following is an exothermic process? (a). ice melting (b). water evaporating (c). boiling soup (d). condensation of water vapor Explanation: All the processes except condensation of water vapor will require that hat is absorbed. In an exothermic process heat is always released. 21. Of the following, which one is a state function? (a). H (b). q (c). w (d). All of the above 22. Which of the following is a statement of the first law of thermodynamics? (a). E k = ½mv 2 (b). A negative ΔH corresponds to an exothermic process (c). ΔE = q - w (d). Energy lost by the system must be gained by the surroundings. Explanation: The first law of thermodynamics is essentially the law of conservation of energy, more specifically regards a system and surroundings. Copyright 2006 Dr. Harshavardhan D. Bapat 7

8 23. A ΔH corresponds to an process. (a). negative, endothermic (b). negative, exothermic (c). positive, exothermic (d). zero, exothermic Explanation: An exothermic system will lose its enthalpy corresponding to a negative ΔH. 24. The internal energy of a system can be increased by. (a). transferring heat from the surroundings to the system (b). transferring heat from the system to the surroundings (c). doing work on the system (a). a only (b). b only (c). c only (d). a and c Explanation: An increase in the internal energy will be possible only of the system gains energy from the surroundings and has work done on it by the surroundings. 25. ΔH for an endothermic process is while ΔH for an exothermic process is. (a). zero, positive (b). zero, negative (c). positive, negative (d). negative, positive Explanation: In an endothermic process the enthalpy of the system will increase as the system will gain energy while in an endothermic process the system will lose energy resulting in a loss in enthalpy. 26. For a given process at constant pressure, ΔH is negative. This means that the process is. (a). endothermic (b). a static process (c). exothermic Explanation: An exothermic process will lose energy and hence have a negative value of ΔH. Copyright 2006 Dr. Harshavardhan D. Bapat 8

9 27. Which one of the following statements is true? (a). Enthalpy is an intensive property. (b). The enthalpy change for a reaction is independent of the state of the reactants and products. (c). Enthalpy is a state function. (d). H is the value of q measured under conditions of constant volume. Explanation: Enthalpy is dependent on the amount of and state of the reactants and products and is measured at constant pressure, making (a), (b) and (d) false. 28. A chemical reaction that absorbs heat from the surroundings is said to be and has a ΔH at constant pressure. (a). endothermic, positive (b). endothermic, negative (c). exothermic, negative (d). exothermic, positive Copyright 2006 Dr. Harshavardhan D. Bapat 9

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