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1 November 2011

1 November 2011. Objective : You will be able to: describe enthalpy and differentiate between endothermic and exothermic reactions and calculate the enthalpy of a reaction. Homework : p. 263 #21, 22, 23, 24, 25: tomorrow. Thermochemistry.

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1 November 2011

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  1. 1 November 2011 • Objective: You will be able to: • describe enthalpy and differentiate between endothermic and exothermic reactions and calculate the enthalpy of a reaction. • Homework: p. 263 #21, 22, 23, 24, 25: tomorrow

  2. Thermochemistry

  3. Thermodynamics: study of interconversion of heat and other kinds of energy • First Law of Thermodynamics: energy can be converted from one form to another, but can not be created or destroyed • We can not accurately measure total energy of a system • Instead, we measure changes in energy ΔE

  4. 2H2(g) + O2(g) 2H2O (l) + energy H2O (g) H2O (l) + energy energy + 2HgO (s) 2Hg (l) + O2(g) energy + H2O (s) H2O (l) Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. Endothermic processis any process in which heat has to be supplied to the system from the surroundings.

  5. Enthalpy of Chemical Reactions Specifically, we’re interested in heat flow that occurs under constant pressure

  6. Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. Enthalpy change (∆H) is the heat absorbed during a physical or chemical process. DH = H (products) – H (reactants) Hproducts < Hreactants Hproducts > Hreactants DH < 0 DH > 0 6.4

  7. Enthalpies of Reaction • Expressed in kJ or kJ/mol • endothermic: ∆H is always positive • endothermic changes absorb heat • exothermic: ∆H is always negative • exothermic changes liberate heat • Calculating ∆H is the same for a wide variety of processes (see table)

  8. H2O (s) H2O (l) DHfus= 6.01 kJ Thermochemical Equations Is DH negative or positive? System absorbs heat Endothermic DH > 0 6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.

  9. DHcomb= -890.4 kJ CH4(g) + 2O2(g) CO2(g) + 2H2O (l) Thermochemical Equations Is DH negative or positive? System gives off heat Exothermic DH < 0 890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.

  10. H2O (s) H2O (l) H2O (l) H2O (s) 2H2O (s) 2H2O (l) DH = -6.01 kJ DH = 6.01 kJ/mol ΔH = 6.01 kJ DH = 2 mol x 6.01 kJ/mol= 12.0 kJ Thermochemical Equations • The stoichiometric coefficients always refer to the number of moles of a substance • If you reverse a reaction, the sign of DH changes • If you multiply both sides of the equation by a factor n, then DH must change by the same factor n. The physical states of all reactants and products must be specified in thermochemical equations.

  11. Note on “implied signs” • In a sentence, the sign of ∆H is often implied. • “6.9 kJ of heat was liberated from the system” = −6.9 kJ • “6.9 kJ of heat was absorbed by the system” = +6.9 kJ

  12. Phase transitions • solid to liquid; liquid to gas • absorb heat • endothermic • positive ∆H • H2O(l) → H2O(g) ∆Hvap= +44 kJ • gas to liquid; liquid to solid • liberate heat • exothermic • negative ∆H • H2O(g) → H2O(l) ∆Hvap= −44 kJ

  13. Example 1 • Given the thermochemical equation 2SO2(g) + O2(g)  2SO3(g) DH = -198.2kJ/mol calculate the heat evolved when 87.9 g of SO2 is converted to SO3.

  14. Example 2 • Calculate the heat evolved with 266 g of white phosphorus (P4) burns in air: P4(s) + 5O2(g)  P4O10(s) DH= -3013kJ/mol

  15. Example 3 • How many kJ of heat are absorbed by the surroundings when 25.0 g of methane (CH4) burns in air?

  16. Problem Determine the amount of heat (in kJ) given off when 1.26x104 g of NO2 are produced according to the equation 2NO(g) + O2(g) → 2NO2(g) ∆H = −114.6 kJ/mol

  17. What scientific law requires that the magnitude of the heat change for forward and reverse processes be the same with opposite signs? Explain.

  18. Why are phase changes from solid to liquid and from liquid to gas always endothermic? • Is the process of sublimation endothermic or exothermic? Explain.

  19. 2 November 2011 • Objective: You will be able to: • describe calorimetry and calculate heat change and specific heat. • Homework Quiz: Week of Oct. 31 Determine the amount of heat (in kJ) given off when 1.26x104 g of NO2 are produced according to the equation 2NO(g) + O2(g) → 2NO2(g) ∆H = −114.6 kJ/mol

  20. Agenda • Homework Quiz • Go over homework • Calorimetry calculations examples • Practice Problems • Pre-lab questions and lab set up Homework: Finish pre-lab questions, lab notebook set up p. #

  21. Calorimetry • Calorimetry: measurement of heat flow • Calorimeter: an apparatus that measures heat flow • Heat capacity, C, heat required to raise the temperature of an object by 1 K (units are J/K) • Molar heat capacity, Cmolar: amount of heat absorbed by one mole of a substance when it experiences a one degree temperature change (units are J/mol K)

  22. Specific heat capacity: the heat capacity of one gram of a substance (units: J/g K) • water: 1 cal/g K = 4.184 J/ gK

  23. Calorimetry • q=CΔt • q = heat absorbed in Joules • C = specific heat capacity of a substance: the amount of heat required to raise the temperature of a given quantity of a substance by 1 degree. (=mass*specific heat) • q=msΔt Ex. 1: A 466 g sample of water is heated from 8.50oC to 74.60oC. Calculate the amount of heat absorbed (in kJ) by the water.

  24. Ex. 2 • 1.435 grams of naphthalene (C10H8) was burned in a constant-volume bomb calorimeter. The temperature of the water rose from 20.28oC to 25.95oC. If the heat capacity of the bomb plus water was 10.17 kJ/oC, calculate the heat of combustion of naphthalene on a molar basis (find the molar heat of combustion).

  25. Ex. 3 • A lead pellet having a mass of 26.47 g at 89.98oC was placed in a constant pressure calorimeter of negligible heat capacity containing 100.0 mL of water. The water temperature rose from 22.50oC to 23.17oC. What is the specific heat of the lead pellet?

  26. Ex. 4 • What is the molar heat of combustion of liquid ethanol if the combustion of 9.03 grams of ethanol causes a calorimeter to increase in temperature by 3.19 K? • heat capacity of the calorimeter is 75.8 kJ/K

  27. Pre-Lab • Molar heat of crystallization • a.k.a. Latent heat of fusion • Set up your lab notebook and answer the pre-lab questions (refer to your textbook!) • Make a procedure summary and set up a data table.

  28. Homework

  29. 3 November 2011 • Objective: You will be able to: • calculate the molar heat of crystallization for a chemical handwarmer • Homework Quiz: A 10.0 gram piece of copper (specific heat = 0.385 J/g oC) has been heated to 100oC. It is then added to a sample of water at 22.0oC in a calorimeter. If the final temperature of the water is 28.0oC, calculate the mass of the water in the calorimeter. specific heat of water = 4.184 J/ g oC

  30. Lab • With your partner, carefully follow the directions. Hand warmer pouch and metal disk: 3.42 grams • Collect all data, results and answers to questions in your lab notebook.

  31. 7 November 2011 • Objective: You will be able to • calculate the standard enthalpy of formation and reaction for compounds. • Homework Quiz: (Week of Nov. 7) An iron bar of mass 869 g cools from 94oC to 5oC. Calculate the heat released (in kJ) by the metal. specific heat of iron=0.444 J/g·oC

  32. Agenda • Homework Quiz • Return Tests • Standard Enthalpy of Formation/Reaction notes/problems • Homework: p. 264 #46, 49, 51, 53, 56, 57, 59, 62, 64: Tuesday • Test on Thermochem Thurs. • Lab notebook: due tomorrow (checklist!) • Test Corrections: Mon.

  33. 8 November 2011 • Objective: You will be able to • review thermochemistry • Homework Quiz: (Week of Nov. 7) Calculate the heat of combustion for the following reaction from the standard enthalpies of formation in Appendix 3. 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)

  34. Agenda • Homework quiz • Collect lab notebooks • Homework answers and more enthalpy problems • Problem set work time • Homework: Problem Set: Thurs. Test on Thermochem Thurs. Test Corrections: Mon.

  35. C(graphite) + O2(g) CO2(g)DH0 = -393.5 kJ S(rhombic) + O2(g) SO2(g)DH0 = -296.1 kJ CS2(l) + 3O2(g) CO2(g) + 2SO2(g)DH0= -1072 kJ 9 November 2022 • Objective: You will be able to: • practice thermochemistry for a test. • Homework Quiz: Calculate the standard enthalpy of formation of CS2(l) given that:

  36. Agenda • Homework Quiz • Problem Set work time Homework: Problem set due tomorrow Thermochemistry test tomorrow

  37. Test Corrections • Required. 2nd quarter quiz grade • Show work, including for multiple choice questions. • On a separate sheet of paper or using a different color pen. • Due Mon.

  38. Standard Enthalpy of Formation and Reaction

  39. Formation Reaction • a reaction that produces one mole of a substance from its constituent elements in their most stable thermodynamic state ½H2(g) + ½I2(s) → HI(g) ∆H = +25.94 KJ

  40. DH0 (O2) = 0 f DH0 (O3) = 142 kJ/mol DH0 (C, graphite) = 0 DH0 (C, diamond) = 1.90 kJ/mol f f f Standard heat of formation (DH0) is the heat absorbed when one mole of a compound is formed from its elements at a pressure of 1 atm at 25oC. f The standard heat of formation of any element in its most stable form is zero.

  41. Example • Write the thermochemical equation associated with the standard heat of formation of AlCl3(s) using the values in the table. (Hint: Be sure to make only one mole of product!)

  42. Based on the definition of a formation reaction, explain why the standard heat of formation of an element in its most stable thermodynamic state is zero. Write a chemical equation to illustrate your answer.

  43. Hess’s Law Hess’s Law:if a reaction is carried out in a series of steps, DH of the overall reaction is equal to the sum of the DH’s for each individual step. (Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end.)

  44. Example • Calculate DH for the following reaction 2S(s) + 3O2(g) → 2SO3(g) from the enthalpies of these related reactions S(s) + O2(g) → SO2(g) DH = −296.9 kJ 2SO2(g) + O2(g) → 2SO3(g) DH = −196.6 kJ

  45. The standard heat of reaction (DH0 ) is the heat change of a reaction carried out at 1 atm. rxn aA + bB cC + dD - [ + ] [ + ] = DH0 DH0 rxn rxn aDH0 (A) dDH0 (D) bDH0 (B) cDH0 (C) DH0 (products) f f f f f Standard Heat of Reaction - DH0 (reactants) S S = f

  46. Example 1 Calculate the standard enthalpy change for the combustion of one mole of liquid ethanol. (Hint: Be really careful with your math, and the + and − signs!)

  47. Example 2 Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49.04 kJ/mol. 2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O (l)

  48. 2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O (l) - S S = DH0 DH0 DH0 - [ ] [ + ] = rxn rxn rxn [ 12 × -393.5 + 6 × -285.8 ] – [ 2 × 49.04 ] = -6535 kJ = 12DH0 (CO2) 2DH0 (C6H6) f f = - 3267 kJ/mol C6H6 6DH0 (H2O) -6535 kJ f 2 mol DH0 (reactants) DH0 (products) f f Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49.04 kJ/mol.

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