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Chapter 17 Thermochemistry StudyGuide17.1 – 17.2 Thermochemical Equations Honors Chemistry1. Make the following conversions: a. 444 cal to joules = 1.86 x 103 J b. 1.8 kJ to joules = 1.8 x 103 J c. 0.45 kJ to calories = 1.1 x 102 cal2. Classify each of these processes as endothermic or exothermic: a. condensing steam – exo c. evaporating alcohol – endo b. burning alcohol – exo d. baking a potato – endo3. The specific heat capacity for silver is 0.24 J/goC. Calculate the energy required to raise the temperature of 150.0 g Ag from 273 K to 298 K. Calculate the molar heat capacity of silver. Energy: q = (150.0)(.24)(298-273) = 9.0 x 102 J Molar heat capacity: (0.24 J/goC)(107.87 g/mol Ag) = 26 J/mol4. It takes 585 J of energy to raise the temperature of 125.6 g Hg from 20.0oC to 53.5oC. Calculate the specific heat capacity and the molar heat capacity of Hg. Specific heat capacity: C = q/mΔt = 585/(125.6)(53.5-20) = 0.139 J/goC Molar heat capacity: (0.139 J/goC)(200.59 g/mol Hg) = 27.9 J/mol5. A 46.2-g sample of copper is heated to 95.4oC and then placed in a calorimeter containing 75.0 g water at 19.6oC. The equilibrium temperature in the calorimeter is 21.8oC. Calculate the specific heat capacity of copper; assuming that all the heat lost by the copper is gained by the water. C = 0.203 J/goC6. A 15.0-g sample of nickel metal is heated to 100.0oC and dropped into 55.0 g of water, initially at 23.0oC. Assuming that all the heat lost by the nickel is absorbed by the water; calculate the final temperature of the nickel and the water. The specific heat of nickel is 0.444 J/goC. –15.0(.444)( Tf – 100.0) = 55.0(4.184)(Tf – 23.0) – 6.66Tf + 666 = 230.12Tf – 5292.76 5958.76 = 236.78 Tf 25.2oC = Tf7. Chloe was running bath water and realized it was too hot. If she has 20.0 L of water in the tub at 95oC and then adds 15.8 L of water at 75oC, what will the final temperature of the water be? –20(Tf – 95) = 15.8(Tf – 75) –20Tf + 1900 = 15.8 Tf – 1185 3085 = 35.8 Tf
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86.2 oC = Tf8. For the combustion of methane, calculate the enthalpy change for the burning of 10.00 g of methane. CH4(g)+ 2O2(g) CO2(g) + 2H2O(l) + 890 kJ 1molCH 4 890 kJ H 10 .00 gCH 4 16 .05 gCH 4 1molCH 4 ΔH = - 555 kJ17.3 Heat in Changes of State9. Read heating curves (See Changes of State w/s) Label the phases, boiling point and melting point here: boiling point = 20oC , melting point = 10oC a = solid b = melting c = liquid d = boiling e = gas Indicate whether a heating curve would be flat or rising in 10-14. 10. liquid is boiling - 13. PE is increasing – flat flat 11. solid is warming – 14. KE is increasing – rising risingHeat of Fusion and Heat of 12. solid is melting –Vaporization Problems flatValues for Water: (SHOW YOUR WORK FOR FULL CREDIT!!) Hfus = 6.02 kJ/mol Hvap = 40.7 kJ/mol oCH2O(s) = 2.03 J/g C CH2O(l) = 4.184 J/g C CH2O(g) = 2.02 J/goC o15. How much heat needs to be added to 10090 grams of ice at -10.0oC so that it all becomes a vapor at 120.0oC? A q = mCΔT = (10090g)(2.03 J/goC)(0-(-10.0)) = 204827 J B ΔH = 10090 g H 2 O 1 mol H 2 O 6.02 kJ 1000 J = 3370799.1 J 1 18.02 g H 2 O 1 mol H 2 O 1kJ C q = mCΔT = (10090g)(4.184 J/goC)(100-0) = 4221656 J D ΔH = 10090 g H 2 O 1 mol H 2 O 40 .7 kJ 1000 J = 22789289.7 J 1 18.02 g H 2 O 1 mol H 2 O 1kJ E q = mCΔT = (10090g)(2.02 J/goC)(120-100) = 407636 J
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A + B + C + D + E = 30994207.8 J = 3.10 x 107 J16. If you are freezing special green ice cubes for St. Patrick’s Day, how much energy do you need to remove from 2 ice trays of 12 cubes if each cube holds 24.2 grams of water at 0oC? 194 kJ17. Anand has a cold and wants to make steam in a vaporizer to help clear his congestion. How much energy will be required to change one gallon of water at 100oC to steam at 100oC? (The density of water is 1.00g/ml) 8547 kJ (no sig figs specified)18. Chelsea’s prom dress is all wrinkled, but she doesn’t have the time or patience to iron it. How much energy will be required to fire up the steamer to remove wrinkles if she add 521 grams of water at 25.0oC and changes it all to steam at 100.oC? C q = mCΔT = (521g)(4.184 J/goC)(100-25) = 163489.8 J D ΔH = 521 g H 2 O 1 mol H 2 O 40 .7 kJ 1000 J = 1176731.41 J 1 18.02 g H 2 O 1 mol H 2 O 1kJ C + D = 1340221.21 J = 1.34 x 106 J19. The steamer Andrew is using to strip the NASCAR wallpaper off his bedroom walls has used 32,967 kJ of energy. How many grams of water at 100 oC has he changed to steam at 100oC? 14596 g20. Using a coffee cup calorimeter, Abby finds a balance that works and adds 1.60 g of NH4NO3 to 75.0 g of water at an initial temperature of 25.00oC. After the salt completely dissolves, the final temperature of the solution is 23.34oC. Assuming no heat is lost to the calorimeter; calculate the enthalpy change for the dissolution of NH4NO3 in kJ/mol. ΔH = 26.6 kJ/mol17.4 Hess’s Law and Standard Enthalpies of Formation21. Given the following data: H2(g) + ½ O2(g) → H2O(l) ΔH = -285.8 kJ N2O5(g) + H2O(l) → 2HNO3(l) ΔH = -76.6 kJ ½ N2(g) + 3/2 O2(g) + ½ H2(g) → HNO3(l) ΔH = -174.1 kJ Calculate the ΔH for the reaction
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2N2(g) + 5O2(g) → 2N2O5(g) ΔH = 28.4 kJ22. Use the values of ΔHof from Appendix A handout to calculate the ΔHo for this reaction: SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(aq) o ΔH = - 20 kJ23. Use the values of ΔHof from Appendix A handout to calculate the ΔHo for this reaction: (balance first!) 4 NH3(g) + 7 O2(g) → 4 NO2(g) + 6 H2O(l) o ΔH = - 1396 kJ24. Write out the equation for the combustion of C3H6(g). Use the values of ΔHof from the Appendix A handout to calculate the ΔHo for the reaction. 2 C3H6(g) + 9 O2(g) → 6 CO2(g) + 6 H2O(g) ΔH = – 3854.8 kJ25. Use the values of ΔHof from Appendix A and write each thermochemical equation used to calculate the ΔHo for problem 24. 3 C(s) + 3 H2(g) → C3H6(g) ΔH = 20.9kJ C(s) + O2(g) → CO2(g) ΔH = -393.5 kJ H2(g)+ ½ O2(g) → H2O(g) ΔH = -242 kJ
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