CHEMISTRY 3310 PROBLEM SHEET #4

Transcription

1 CHEMISTRY 3310 PROBLEM SHEET #4 1. The specific heats of a number of materials are listed below. Calculate the molar heat capacity for each. (a) gold, (b) rust (Fe 2 O 3 ) (c) sodium chloride 2. Calculate the final temperature that results from each of the following processes. (a) A 8.82 g sample of water at 20.3 C absorbs 112 calories of heat. (b) A 6.25 kg sample of solid sulfur at 45.3 C gives off 165 kcal of heat. 3. What is the quantity of work performed in the expansion of the gas that is confined in a pistoncylinder configuration if the mass of the piston is 500. g and a height of 16.8 cm is lifted? 4. Derive the expressions for the pressure volume work involved in the following processes: (a) any process which occurs with no change in volume, (b) expansion of an ideal gas into a vacuum, (c) any process occurring with a constant external pressure, (d) expansion of an ideal case during which the temperature is held constant and the external pressure is constantly adjusted to match the pressure while the gas itself exerts. 5. The combustion of benzoic acid is often used to establish the heat capacity of a bomb calorimeter. If the combustion of a g sample of benzoic acid (HC 7 H 5 O 2 ) causes a temperature increase to 4.96 C, what is the heat capacity of the bomb calorimeter assembly? 6. Calculate q, w, ΔH, and ΔE for the heating of 10 moles of carbon dioxide gas from 0 C to 100 C at constant pressure. Assume that the C p of carbon dioxide for this temperature range is constant. 7. In a particular change in state, a system absorbed 200 cal of heat. When the system was returned to its initial state by a different process, 100 cal of heat were emitted and the system did the equivalent of 400 cal of work on the surroundings. How much work was involved in the first process? 8. A glass containing 200 g of tap water is at 15 C. How many ice cubes weighing 10 g each at 5 C are required to bring the water temperature to 0 C? Assume specific heat of ice to be the same as that of water. 9. When food is heated in a metal pot on a stove, heat must be added to warm the pot as well as the food. Calculate the average specific heat of a pot which weighs 0.50 kg if 2.54 kilowatt are needed to heat the pot from 25 C to 100 C in 1 minute when it contains 375 ml of water. Assume 100% heat transfer from the unit to the pot. What material is the pot made of?

2 10. Suppose a weighed specimen of excised fatty tissue is sealed in a vessel filled with oxygen. The vessel is then immersed in 1000 ml of water in a calorimeter. The combustion reaction is initiated by an electric current. The completed reaction was observed to raise the water temperature from 25.0 to 38.8 C. A previous calibration has shown that the heat capacity of those calorimeter parts undergoing temperature rise may be expressed as a "water equivalent" of 210 ml. What is the heat given off or absorbed in this process? 11. A man who weighs 68.0 kg produces about 3000 kcal of heat every day as a result of his normal metabolism processes. If we assume that his heat capacity to be the same as that of water, how hot would he get if his body retained all the heat produced in a day? 12. A 1.50 g sample of NH 4 NO 3 is added to 35 g of H 2 O in a Dewar flask and stirred until it dissolves. The temperature of the solution drops from 22.7 to 19.4 C. (a) Is the process endothermic or exothermic? (b) What is the enthalpy change of NH 4 NO 3 in kj/mole. Assume that the specific heat of the solution is 4.18 J/g C. 13. Consider a system consisting of 1 mole of a monatomic gas contained in a piston. What is the temperature change of the gas if q = 50 joules and w = 100 joules? 14. How much heat is evolved from 50.0 g of hydrogen gas is burned to water in a rocket engine? H 2 (g) + ½O 2 (g) H 2 O (l) ΔH = 68.3 kcal 15. Calculate the heat evolved when 50.0 g of H 2 gas is mixed with equal amount of N 2 gas. The 3 H 2 (g) + N 2 (g) 2 NH 3 (g) ΔH = kcal reaction is carried out at constant pressure condition. 16. Predict ΔH for the following reaction from N (g) + O (g) NO (g) ½ N 2 (g) + ½ O 2 (g) NO (g) N (g) ½ N 2 (g) O (g) ½ O 2 (g) ΔH = 21.6 kcal ΔH = kcal ΔH = 59.6 kcal 17. Give the thermal equations C (graphite) C (diamond) C (diamond) + O 2 (g) CO 2 (g) ΔH = kcal ΔH = kcal Write the thermochemical equation for the combustion of C (graphite).

3 18. Predict ΔH for the following reaction from C 6 H 4 (OH) 2 (aq) + H 2 O 2 (aq) C 6 H 4 O 2 (aq) + 2 H 2 O (l) C 6 H 4 (OH) 2 (aq) C 6 H 4 O 2 (aq) + H 2 (g) ΔH = 42.2 kcal ½ O 2 (g)+ H 2 O (l) H 2 O 2 (aq) ΔH = 22.6 kcal H 2 (g) + ½ O 2 (g) H 2 O (l) ΔH = 68.3 kcal 19. Calculate the heat of reaction for 2 SO 2 (g) + O 2 (g) 2SO 3 (g) 20. From the following data, calculate the molar heat of formation of solid PCl 5 (s). P 4 (s) + 6 Cl 2 (g) 4 PCl 3 (l) PCl 3 (l) + Cl 2 (g) PCl 5 (s) ΔH = kcal ΔH = 32.8 kcal 21. Calculate the molar heat of formation of methyl alcohol from the following equations: CH 3 OH (l) + 3 / 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) C (s) + O 2 (g) CO 2 (aq) H 2 (g) + 1 / 2 O 2 (g) H 2 O (l) ΔH = kcal ΔH = 94.0 kcal ΔH = 68.3 kcal 22. Use the values of bond energies to calculate ΔH for each of the following reactions: (a) H 2 (g) + Cl 2 (g) 2 HCl (g) (b) CH 4 (g) + 4 F 2 (g) CF 4 (g) + 4 HF (g) (c) C 2 H 4 (g) + Cl 2 (g) C 2 H 4 Cl 2 (g) (d) cyclopentane (g) 1 pentene (g) 23. From the following equations, determine the molar heat of formation of HNO 2 (aq): NH 4 NO 2 (aq) N 2 (g) + 2 H 2 O (l) ΔH = 76.5 kcal NH 3 (aq) + HNO 2 (aq) NH 4 NO 2 (aq) ΔH = 9.00 kcal 2 NH 3 (aq) N 2 (g) + 3 H 2 (g) ΔH = 40.6 kcal 2 H 2 (g) + O 2 (g) 2 H 2 O (l) ΔH = kcal 24. Calculate the heat of vaporization of 1 gram of water at body temperature (37 C). The heat capacity of liquid water may be taken as constant 18.0 cal mole 1 deg 1 ; the heat capacity of water vapor varies with temperature: C p (H 2 O, g) = x 10 3 T x 10 7 T 2 (in cal mole 1 deg 1 ) H 2 O (l) H 2 O (g) ΔH 373 = 9.70 kcal

4 25. Predict the enthalpy of reaction for the oxidation of CH 3 OH (l) CH 3 OH (l) + 3 / 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) using the given table of mean bond energies. The enthalpy of vaporization at 25 C is kj/mole for CH 3 OH and kj/mole for H 2 O. 26. For the constant pressure process, C 2 H 6 (g) + 7 / 2 O 2 (g) 2 CO 2 (g) + 3 H 2 O (l) ΔH at 298 K for the above reaction is kcal/mole. (a) Find w, q, and ΔE for the process. (b) Using Table I to help you, find ΔH 298 for the process C 2 H 6 (g) + 7 / 2 O 2 (g) 2 CO 2 (g) + 3 H 2 O (g) 27. (a) Calculate the enthalpy of formation, per mole, of ZnS (s) by using the following information, and Table I. 2 ZnS (s) + 3 O 2 (g) 2 ZnO (s) + 2 SO 2 (g) ΔH = kj (b) What reaction and conditions are implied when the enthalpy of formation of ZnS (s) in (a) is calculated.

5 Some useful data: Specific heats (cal g 1 deg 1 ): water gold Fe 2 O 3 NaCl sulfur aluminum steel copper Heat of fusion (joules/g): ice 320 Density (g/cc): water steel Heat of combustion (kj/g): benzoic acid C p (cal/mole deg): CO Acceleration due to gravity 9.81 m s 2 Normal body temperature 37 C 1 calorie = joules 1 Joule = 1 watt sec = 1 kg m 2 s 2 Table 1 ΔH f for selected substances ΔH f (kj/mole) ΔH f (kj/mole) Al 2 O 3 (s) 1676 CuO (s) BaCO 3 (s) 1216 HBr (g) Br (g) HCl (g) Br 2 (g) HF (g) C (diamond) 1.90 HI (g) CCl 4 (g) H 2 O (g) CO (g) H 2 O (l) CO 2 (g) H 2 S (g) CH 4 (g) Hg (g) C 2 H 4 (g) KCl (s) C 2 H 6 (g) MnO 2 (s) C 3 H 8 (g) NH 3 (g) C 6 H 6 (g) N 2 O (g) C 6 H 6 (l) NO 2 (g) CH 3 OH (g) N 2 O 4 (g) 9.16 CH 3 OH (l) O 3 (g) C 2 H 5 OH (g) SO 2 (g) C 2 H 5 OH (l) SO 3 (g) Ca(OH) 2 (s) ZnO (s) 348.3

6 Table 2 Mean bond energies