Prentice Hall Chemistry (c) 2005

1. Prentice Hall Chemistry(c) 2005 By Daniel R. Barnes Init: 3/14/2011 Section Assessment Answers Chapter 17 WARNING: some images and content in this presentation may have been taken without permission from the world wide web. It is intended for use only by Mr. Barnes and his students. It is not meant to be copied or distributed. Thermochemistry

2. 4. How much heat is required to raise the temperature of 250.0 g of mercury 52 oC? Q = ? J m = 250.0 g DT = 52 oC Q = mDTCp Cp = 0.14 J/goC (from Table 17.1 on page 508) Q = (250.0 g)(52 oC)(0.14 J/goC) 17.1 Practice Problems Q = 1820 J = 1.82 kJ

3. 5. In what direction does heat flow between two objects? Heat flows from the hotter object to the colder object. 17.1 Section Assessment

4. 6. How do endothermic processes differ from exothermic processes? Endothermic processes absorb heat from the surroundings; exothermic processes release heat into the surroundings. 17.1 Section Assessment

5. 7. What units are used to measure heat flow? Calories and joules 17.1 Section Assessment

6. 8. On what factors does the heat capacity of an object depend? Mass and chemical composition. . . . in other words, how heavy it is and what it’s made of. 17.1 Section Assessment

7. 9. Using calories, calculate how much heat 32.0 g of water absorbs when it is heated from 25.0oC to 80.0oC. How many joules is this? m = 32 g Ti = 25 oC Tf = 80 oC Q = ? cal = ? J DT = Tf – Ti = 80 oC – 25 oC = 55 oC = DT Q = mDTCp = (32 g)(55 oC)(1 cal/goC) 17.1 Section Assessment Q = 1760 cal = 1.76 x 103 cal = 1.76 kcal Q = mDTCp = (32 g)(55 oC)(4.18 J/goC) Q = 7356.8 J = 7.36 x 103 J = 7.36 kJ

8. 10. A chunk of silver has a heat capacity of 42.8 J/oC and a mass of 181 g. Calculate the specific heat of silver. 42.8 J/oC = 0.236 J/goC = 2.36 x 10-1 J/(goC) 181 g 17.1 Section Assessment If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

9. 11. How many kilojoules of heat are absorbed when 1.00 L of water is heated from 18oC to 85oC? 2.8 x 102 J = 280 J 17.1 Section Assessment If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

10. 16. Calorimetry is based on what basic concepts? The heat released by the system is equal to the heat absorbed by the surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings. 17.2 Section Assessment

11. 17. How are enthalpy changes treated in chemical equations? The enthalpy change in a chemcial reaction can be written as either a reactant (to the left of the arrow) or a product (to the right of the arrow). 17.2 Section Assessment

12. 18. When 2 mol of solid magnesium (Mg) combines with 1 mole of oxygen gas (O2), 2 mol of solid magnesium oxide (MgO) is formed and 1204 kJ of heat is released. Write the thermochemical equation for this combustion reaction. 2Mg(s) + O2(g)  2MgO(s) + 1204 kJ Or 2Mg(s) + O2(g)  2MgO(s) DH = -1204 kJ 17.2 Section Assessment Note the negative sign on DH. DH is negative because the reaction is exothermic. (If the reaction were endothermic, “+1204 kJ” would have been on the left side of the arrow and DH would have been positive.)

13. 19. Gasohol contains ethanol (C2H5OH(l)), which, when burned, reacts with oxygen to produce CO2(g) and H2O(g). How much heat is released when 12.5 g of ethanol burns? C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g) DH = -1368 kJ 3.72 x 102 kJ = 372 kJ 17.2 Section Assessment If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

14. 20. Explain the term heat of combustion. Heat of combustion is the heat of reaction for the complete burning of one mole of a substance. 17.2 Section Assessment

15. 27. How does the molar heat of fusion of a substance compare to its molar heat of solidification? Molar heat of fusion and molar heat of solidification have an identical numerical value but are opposite in sign. 17.3 Section Assessment

16. 28. How does the molar heat of vaporization compare to its molar heat of condensation? Molar heat of vaporization and molar heat of condensation have an identical numerical value but are opposite in sign. Is there an echo in here? 17.3 Section Assessment

17. 29. What enthalpy changes occur when a solute dissolves in a solvent? Heat is either released or absorbed in the formation of a solution. 17.3 Section Assessment

18. 30. Identify each enthalpy change by name and classify each changes exothermic or endothermic. a. 1 mol C3H8(l)  1 mol C3H8(g) Molar heat of vaporization; endothermic. b. 1 mol Hg(l)  1 mol Hg(s) Molar heat of solidification; exothermic. c. 1 mol NH3(g)  1 mol NH3(l) 17.3 Section Assessment Molar heat of condenstion; exothermic. d. 1 mol NaCl(s) + 3.88 kJ/mol  1 mol NaCl(aq) Molar heat of solution; endothermic. e. 1 mol NaCl(s)  1 mol NaCl(l) Molar heat of fusion; endothermic.

19. 31. Why is a burn from steam potentially far more serious than a burn from very hot water? “When a mole of steam condenses on your skin, it releases a substantial amount of heat – its molar heat of condensation.” A mole of steam releases its molar heat of condensation as it turns from gas to liquid. This is in addition to the heat it gives you as it cools down to your skin temperature. Hot water gives off heat as it cools down to your skin temperature, but it gives off no “bonus energy” because it doesn’t change state. Plus, steam is probably at a temperature above the boiling point of water, whereas liquid water probably isn’t. 17.3 Section Assessment On the other hand, water is much denser than steam, since it’s a liquid, so burns from hot water can still be devastating. Be very careful around both steam AND hot water. They’re both very dangerous. It’s even worse if they’re under pressure.

20. 34. What are two ways that heat of reaction can be determined when it can not be directly measured? Use Hess’ law of heat summation or use standard heats of formation. 17.4 Section Assessment

21. 35. Calculate the enthalpy change (DH) in kJ for the following reaction: 2Al(s) + Fe2O3(s)  2Fe(s) + Al2O3(s) Use the enthalpy changes for the combustion of aluminum and iron: 2Al(s) + 3/2O2(g)  Al2O3(s) DH = -1669.8 kJ 2Fe(s) + 3/2O2(g)  Fe2O3(s) DH = -824.2 kJ 17.4 Section Assessment -8.456 x 102 kJ If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

22. 36. What is the formula for calculating the standard heat of reaction? DH0 = DHf0(products) – DHf0(reactants) 17.4 Section Assessment

23. What is the standard heat of reaction (DH0) for the decomposition of hydrogen peroxide? • 2H2O2(l)  2H2O(l) + O2(g) 17.4 Section Assessment -1.960 x 102kJ If you would like Mr. Barnes to explain how this answer is calculated, ask him at school and he can show you on paper OR e-mail him at barnesd@centinela.k12.ca.us and he can make a click-by-click explanation on this very presentation and post the updated version of this power point.

24. The rest of this presentation is UNDER CONSTRUCTION Let Mr. Barnes know if you really need a certain part of it to get done. barnesd@centinela.k12.ca.us