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Home , Calorimetry, Thermochemistry

Thermochemistry Problems

The First Law 1. The complete combustion of acetic acid, HC2H3O2(l), to form water, H2O(l), and CO2(g), at constant releases 871.7 kJ of per mol of acetic acid. Write a balanced thermochemical equation for this reaction. Draw an diagram for the reaction.

2. Consider the following reaction, which occurs at standard state:

2Cl(g)® Cl2(g) ∆H° = –-243.4 kJ a. Which has the higher, more positve, enthalpy under these conditions, 2Cl

(g) or Cl2(g)?

b. Consider the following reaction: 2 Mg(s) + O2(g) ® 2 MgO(s) ∆H = -1204 kJ. c. Is the reaction exothermic or endothermic?

d. Calculate the amount of heat transferred when 2.4 g of Mg(s) reacts at constant pressure. How many grams of MgO are produced during an enthalpy change of 96.0 kJ?

e. How many kilo of heat are absorbed when 7.50 g of MgO(s) are

decomposed into Mg(s) and O2(g) at constant pressure?

Enthalpy

Calorimetry Problems 3. Determine the specific heat of a sample of Cu from the fact that 64.0J are needed to raise the of 15.0 g of Cu metal from 22.0°C to 33.0°C. [0.388 J/g°C] 4. The specific heat of a sample of copper is 0.385 J/g-K. How many joules of heat are necessary to raise the temperature of a 1.42 kg block of copper from 25.0 °C to 88.5°C? 5. A 50.0-g sample of water at 100.00°C was placed in an insulated cup. Then 25.3-g of at 25.00°C was added to the water. The temperature of the water dropped to 96.68°C. What is the specific heat of the zinc?[0.388 J/g°C] 6. When a 6.50 g sample of solid NaOH dissolves in 100.0g of water in a coffee-cup , the temperature rises from 21.6°C to 37.8°C. Calculate the ∆H (in kJ/mol NaOH) for the solution process. (Assume the specific heat of the solution formed is 4.18 J/g-K) + – NaOH(s) ® Na (aq)+ OH (aq) 7. A house is designed to have passive solar energy features, Brickwork is to be incorporated into the interior of the house to act as a heat absorber. Each brick

Thermochemistry Problems Page 1 of 7 weighs approximately 1.8 kg. The specific heat of the brick is 0.85 j/g-K. How many bricks must be incorporated into the interior of the hose to provide the same total as 1.0 103 gal of water?

Bomb calorimetery 8. When 15.3 g of sodium nitrate was dissolved in water in a calorimeter, the temperature fell from 25.00 °C to 21.56 °C. If the heat capacity of the solution and the calorimeter is 1071 J/°C, what is the enthalpy change for the process when 1 mol of sodium nitrate dissolves in water?

9. Camphor (C10H16O) has an energy of combustion of -5903.6 kJ/mol. When a sample of camphor with mass 0.1204 g is burned in a bomb calorimeter, the temperature increases by 2.28°C. Calculate the heat capacity of the calorimeter.

Hesses Law 10. Consider the following hypothetical reactions A ® B ∆H = 30 kJ B ® C ∆H = 60 kJ Use Hess’s law to calculate the enthalpy change for the reaction A ® C. Construct an enthalpy diagram for substances A, B, and C and show how Hess’s law applies.

11. Given the following of reaction: P4(s)+3O2(g)®P4O6(s) ∆H = –1640.1 kJ P4(s)+ 5O2(g) ®P4H10(s) ∆H = –2940.1 kJ Calculate the enthalpy change for the reaction: P4O6(s)+ 2 O2(g) ®P4H10(s)

12. From the following enthalpies of reaction H2(g) + F2(g) ® 2HF(g) ∆H = –537 kJ C(s) + 2 F2(g) ® CF4(g) ∆H = –680 kJ 2 C(s) + 2 H2(g)® C2H4(g) ∆H = –52.3 kJ Calculate the ∆H for the reaction of C2H4(g) with F2(g) to make CF4(g) and HF(g) of formation 13. Many cigarette lighters contain liquid butane, C4H10(l), which has a ∆H°ƒ = –147.6 kJ/mol. Using enthalpies of formation, calculate the quantity of heat produced when 1.0 g of butane is completely combusted in air. (Write the equation for the process first1) 14. Gasoline is composed primarily of hydrocarbons with eight carbon . These hydrocarbons are called octanes. One of the cleanest burning octanes is a

Thermochemistry Problems Page 2 of 7 compound called 2,3,4-trimethylpentane. The complete combustion of 1 mol of this compound to CO2(g) and water leads to ∆H°rxn = –5461 kJ. Write a balanced equation for the combustion of 1 mole of C8H18(l). Write a balanced equation for the formation of C8H18 from its elements. By using the information in this problem, calculate the heat of formation for 2,3,4-trimethylpentane.

Phase changes ∆Hfus= 6.01 kJ/mol or 333 J/g, ∆Hvap= 40.67 kJ/mol or 2257 J/g; Specific heat of water = 4.184 J/g°C.

15. A bag of ice was placed on a patient’s head. The ice bag contained 220.0g of ice at 0.00°C. When the ice bag was removed, all of the ice inside had melted and the liquid had a temperature of 21.00°C. How many joules of heat were added? [92.59 kJ]

16. How many kJ of heat are needed to completely vaporize 50.0g of water at 100°C? [113. kJ]

17. How many joules are required to convert 10.0g of solid ethyl alcohol at -180.3°C to the vapor state at the boiling point of 78.3°C? (C2H60) a. C [solid EtOH] = 0.971J/g°C b. C [liquid EtOH] = 2.30J/g°C c. The point of alcohol is -117.3°C d. ∆Hfus= 218J/g e. ∆Hvap= 854 J/g.} [13.1 kJ]

18. When ice at 0.0°C melts to liquid water at 0.0°C, it absorbs 0.334kJ of heat per gram. Suppose the heat needed to melt 35.0-g of ice is absorbed from the water contained in a glass. If this water has a mass of 0.210kg and a temperature of 21.0°C, what is the final temperature of the water? [Note that you will have 35.0- g of water at 0°C from the ice.][6.63°C] 19. boils at -33.°C; at this temperature the density of liquid ammonia is 3 0.81 g/cm . Calculate enthalpy change when 1.000 L of liquid NH3 is burned in air to give N2(g) and H2O(g).The ∆H°ƒ NH3(g) = –46.2 kJ/mol; and the heat of vaporization of ammonia∆H°vap = 23.2kJ/mol. 20. When steam condenses to liquid water, 2.26 kJ of heat is released per gram, the heat from 124 g of steam is used to heat a room (20.0 ftx12.0 ftx8.00ft) containing 6.44x 104 g of air. The specific heat of air at normal pressure is 1.015 j/g•°C. What is the change in air temperature assuming all the heat from the steam is absorbed only by the air? 21. An ice cube tray contains enough water at 22.0°C to make 18 ice cubes that each have a mass of 30.0 g. The tray is placed in a freezer that uses CF2Cl2 as a refrigerant. The heat of vaporization CF2Cl2 is 158 kJ/g. What mass of CF2Cl2 Thermochemistry Problems Page 3 of 7 must be vaporized in the refrigeration cycle to convert all of the water at 22.0°C to ice at -5.0°C. The heat capacities for water and ice are 4.18 J/g-°C and 2.08 J/g- °C respectively, and the enthalpy of fusion for water is 6.02 kJ/mol

ANSWERS

1. HC2H3O2(l) + 2O2(g) ® CO2(g + 2H2O(l), HC2H3O2(l)+ 4O2(g) – 871.7 kJ ¯ 2CO2(g + 2H2O(l)

2. The enthalpy of reaction is ∆H° prod -∆H° react. The reactant is at a higher potential than the product. When the reaction occurs, heat is released, stabilizing the system and the system moves to a lower energy potential. 3. The reaction is exothermic, the enthalpy is negative [1] HC2H3O2(l) + 2O2(g) ® 2 CO2(g) + 2 H2O(l) HC2H3O2(l) + 4 O2(g) – 871.7 kJ ¯ 2CO2(g) + 2 H2O(l)

[2] The enthalpy of reaction is ∆H° prod -∆H° react. The reactant is at a higher potential than the product. When the reaction occurs, heat is released, stabilizing the system and the system moves to a lower energy potential.

[3] The reaction is exothermic. The ∆H is negative. 1 mol Mg –1204 kJ 2.4 g MgX = 0.10 mol Mg X = –60.2 kJ 24.0 g Mg 2 mol Mg 2 mol MgO 40.0 g MgO 96 kJ X X = 6.4 g MgO –1204 kJ 1 mol MgO 1 mol MgO +1204 kJ 7.50 g MgO X X = 113 Kj 40.0 g MgO 2 mol MgO 0.385 J 1 K [4] q = X 1.42 X 103 g Cu X (88.5°C – 25.0°C) X = 3.47 X104J g–K 1°C [5] First, find the energy that the solution either absorbed or released. Then relate that amount of energy to the moles of NaOH 4.18 J q sol = (100.0 g water + 6.50 g NaOH) X (37.8°C - 21.6°C) X = 7.211 X 103J g°C –7.211 kJ 39.90 g NaOH ∆H = X = 44.3 kJ/mol 6.50 g 1 mol NaOH [6] A ® B A B ® C 60 kJ B ­ 30 kJ ­ 90 kJ Total 90 kJ C ­ 60 kJ ­ [7] Hess’s law is a law of summations. Partial paths add to the desired path

Thermochemistry Problems Page 4 of 7 Given the following enthalpies of reaction: P4O6(s) ® P4(s) + 3 O2(g) ∆H = +1640.1 kJ P4(s) + 5 O2(g) ® P4O10(s) ∆H = -2940.1 kJ P4O6(s) + 2 O2(g) ® P4O10(s) + 1640.1 kJ + –2940.1 kJ = – 1300.0 kJ

[8] H2(g) + F2(g) ® 2HF(g) ∆H = -537 kJ 2 C(s) + 4 F2(g) ® 2 CF4(g) ∆H = -680 kJ 2 C2H4(g) ® 2 C(s) + 2 H2(g) ∆H = – 52.3 kJ C2H4(g) + 5 F2(g) ® 2 CF4(g) + 2 HF(g) ∆H = –1949.3 kJ

Thermochemistry Problems Page 5 of 7 [9] 13 C H + O ® 4 CO + 5 H O 4 10(l) 2 2(g) 2(g) 2 (l) -147.6 0 -393.5 -285.8 ° 1 mol 1) ∆H = 4 –393.5 + 5 X –285.8 –[–147.6] = –3151 kJ X X 1.0 g = -54.2 kJ reaction 58.08 g 25 [10] C H + O ® 8 CO + 9 H O ∆ H° = -5461kJ 8 18 (l) 2 2(g) 2(g) 2 (l) 8 C(s) + 9 H2(g) ® C8H18(l) ∆H =?? –5461 kJ – [8 X –393.5 kJ + 9 X–285.8 kJ] = 259.2 kJ [11] For this problem, we do not know the temperature change that either the brick or the water is undergoing. We do know that q water = –q bricks. First, find the heat capacity, not the specific heat of 1000 gal of water. Then, use that number to find the number of bricks. 4 qt 1 L 1000 mL 1.00 g 4.18 J 7 C = 1.0 X 103 gal X X X X X = 1.582 X 10 J/°C 1 gal 1.0567 qt L 1 mL g°C This is the same energy that many bricks absorbed. 1 brick 1g °C 1.582 ¥ 107J no. of bricks = X X = 1.0 104 bricks 1.8¥103 g 0.85 J °C [12] Write the equations for the energies given, find the moles of liquid ammonia

2 NH3(l) + 3 O2(g) ® N2(g) + 3 H2O(g) ∆H = ?? NH3(l) ® NH3(g) ∆H° = 23.2 kJ 1N2(g) + 3 H2(g) ® NH3(g) ∆H° = -46.2 kJ/mol

NH3(g) ® NH3(l); 1 N2(g) + 3 H2(g) ® NH3(g); 1 N2(g) + 3H2(g) ® NH3(l) ∆H° = -69.4 kJ/mol ∆H rxn = 3 X -241.8 –[2 -69.4] = 586.6 kJ/mol X 47.6 mol = -2.79 X 104 kJ

[13] 124 g steam X –2.26 kJ/g = -280.2 kJ released by steam 1.015 J 6.44 X 104 g air X X ∆T; ∆T = 4.28 °C MM camphor = 152.2g/mol; g°C

[14]

1 mol [15] 0.1204 g X X –5903.6 kJ, mol) = -4.670 kJ 152.2 g +4.670 kJ absorbed by calorimeter = 2.05 kJ/°C 2.28 °C temp change [16] Total mass of water in the ice tray: 18 X 30.0 = 540. g 4 H2O(l) 22 °C ® H2O(l) 0°C 4.185J/g°C X 540. g X (-22°C) = -4.97 10 J 3 H2O(l) 0°C ® H2O(s)0°C 540. g X 6.02 kJ/g = -3.25 X 10 kJ 3 H2O(s) 0°C ® H2O(s) -5°C 540.g X (–5°C)X 2.08 J/g °C =-5.62 X 10 J –3.30 X 106 J 1 g 3.30 X 106 J absorbed X = 20.9 g refrigerant 158 kJ

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