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Thermochemistry

Thermochemistry. Chapters 6 and11. TWO Trends in Nature. ____________  Disorder   ______ energy ____ energy . 2H 2 ( g ) + O 2 ( g ) 2H 2 O ( l ) + energy. H 2 O ( g ) H 2 O ( l ) + energy.

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Thermochemistry

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  1. Thermochemistry Chapters 6 and11

  2. TWO Trends in Nature • ____________ Disorder   • ______ energy ____ energy 

  3. 2H2(g) + O2(g) 2H2O (l) + energy H2O (g) H2O (l) + energy energy + 2HgO (s) 2Hg (l) + O2(g) energy + H2O (s) H2O (l) ___________ process is any process that gives ___ heat – transfers thermal energy from the system to the surroundings. ________ process is any process in which heat has to be _____to the system from the surroundings. 6.2

  4. Enthalpy (H) is used to quantify the _______flow into or out of a system in a process that occurs at ______ pressure. DH = H (products) – H (reactants) DH = heat given off or absorbed during a reaction at constant pressure Hproducts ? Hreactants Hproducts ? Hreactants DH < 0 DH > 0 6.4

  5. H2O (s) H2O (l) DH = 6.01 kJ Thermochemical Equations Is DH negative or positive? System absorbs heat _____thermic DH ? 0 6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm. 6.4

  6. DH = -890.4 kJ CH4(g) + 2O2(g) CO2(g) + 2H2O (l) Thermochemical Equations Is DH negative or positive? System gives off heat _____thermic DH ? 0 890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm. 6.4

  7. 2H2O (s) 2H2O (l) H2O (s) H2O (l) H2O (l) H2O (s) 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 _____________always refer to the number of _______ of a substance • If you ________ 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. 6.4

  8. How much heat is evolved when 266 g of white phosphorus (P4) burn in air? x H2O (l) H2O (g) H2O (s) H2O (l) 3013 kJ 1 mol P4 x DH = 44.0 kJ DH = 6.01 kJ 1 mol P4 123.9 g P4 Thermochemical Equations • The physical states of all reactants and products must be specified in thermochemical equations. P4(s) + 5O2(g) P4O10(s)DHreaction = -3013 kJ = 6470 kJ 266 g P4 6.4

  9. DH0 (O2) = 0 DH0 (O3) = 142 kJ/mol DH0 (C, graphite) = 0 DH0 (C, diamond) = 1.90 kJ/mol f f f f Standard enthalpy of formation (DH0) is the heat change that results when one mole of a compound is formed from its ________ at a pressure of 1 atm. f The standard enthalpy of formation of any element in its most stable form is ______. 6.6

  10. 6.6

  11. The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm. rxn aA + bB cC + dD - [ + ] [ + ] = DH0 DH0 rxn rxn DH0 (_______) dDH0 (D) aDH0 (A) bDH0 (B) cDH0 (C) f f f f f - DH0 (_______) S S = f Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a _______of steps. (Enthalpy is a ______ function. It doesn’t matter how you get there, only where you start and end.) 6.6

  12. 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 = - ________ 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. 6.6

  13. C(graphite) + O2(g) CO2(g)DH0 = -393.5 kJ rxn S(rhombic) + O2(g) SO2(g)DH0 = -296.1 kJ rxn CS2(l) + 3O2(g) CO2(g) + 2SO2(g)DH0 = -1072 kJ rxn 2S(rhombic) + 2O2(g) 2SO2(g)DH0 = -296.1x2 kJ C(graphite) + 2S(rhombic) CS2 (l) C(graphite) + 2S(rhombic) CS2 (l) rxn rxn C(graphite) + O2(g) CO2(g)DH0 = -393.5 kJ + CO2(g) + 2SO2(g) CS2(l) + 3O2(g)DH0 = +1072 kJ rxn DH0 = -393.5 + (2x-296.1) + 1072 =_____ kJ rxn Calculate the standard enthalpy of formation of CS2 (l) given that: 1. Write the enthalpy of formation reaction for CS2 2. Add the given rxns so that the result is the desired rxn. 6.6

  14. C6H12O6 (s) + 6O2 (g) 6CO2 (g) + 6H2O (l) DH = -2801 kJ/mol Chemistry in Action: Fuel Values of Foods and Other Substances 1 cal = ______ J 1 ____ = 1000 cal = 4184 J

  15. The enthalpy of solution (DHsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent. DHsoln = Hsoln - Hcomponents Which substance(s) could be used for melting ice? Which substance(s) could be used for a cold pack? 6.7

  16. The Solution Process for NaCl DHsoln = Step 1 + Step 2 = 788 – 784 = 4 kJ/mol 6.7

  17. First Law of Thermodynamics Energy can be converted from one form to another but energy cannot be created or destroyed. Second Law of Thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. __________ process: DSuniv = DSsys + DSsurr > 0 __________process: DSuniv = DSsys + DSsurr = 0 18.4

  18. The standard entropy of reaction (DS0 ) is the entropy change for a reaction carried out at 1 atm and 250C. rxn aS0(A) bS0(B) - [ + ] cS0(C) dS0(D) [ + ] = aA + bB cC + dD - S S0(reactants) S S0(products) = DS0 DS0 DS0 DS0 rxn rxn rxn rxn What is the standard entropy change for the following reaction at 250C? 2CO (g) + O2(g) 2CO2(g) = 2 x S0(CO2) – [2 x S0(CO) + S0 (O2)] = 427.2 – [395.8 + 205.0] = ?????J/K•mol Entropy Changes in the System (DSsys) S0(CO) = 197.9 J/K•mol S0(CO2) = 213.6 J/K•mol S0(O2) = 205.0 J/K•mol 18.4

  19. What is the sign of the entropy change for the following reaction? 2Zn (s) + O2(g) 2ZnO (s) Entropy Changes in the System (DSsys) When gases are produced (or consumed) • If a reaction produces more gas molecules than it consumes, DS0? 0. • If the total number of gas molecules diminishes, DS0 ? 0. • If there is no net change in the total number of gas molecules, then DS0 may be positive or negative BUT DS0 will be a _______number. The total number of gas molecules goes down, DS is ______. 18.4

  20. nonspontaneous spontaneous Spontaneous Physical and Chemical Processes • A waterfall runs downhill • A lump of sugar dissolves in a cup of coffee • Heat flows from a hotter object to a colder object • A gas expands in an evacuated bulb • Iron exposed to oxygen and water forms rust 18.2

  21. Gibbs Free Energy Spontaneous process: DSuniv = DSsys + DSsurr > 0 Equilibrium process: DSuniv = DSsys + DSsurr = 0 For a constant-temperature process: Gibbs free energy (G) DG = DHsys -TDSsys DG ? 0 The reaction is spontaneous in the forward direction. DG ? 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction. DG ? 0 The reaction is at equilibrium. 18.5

  22. DG = DH - TDS 18.5

  23. The standard free-energy of reaction (DG0 ) is the free-energy change for a reaction when it occurs under standard-state conditions. rxn aA + bB cC + dD - [ + ] [ + ] = - DG0 (?) S S = f Standard free energy of formation (DG0) is the free-energy change that occurs when _____of the compound is formed from its elements in their standard states. DG0 DG0 rxn rxn f DG0 of any element in its stable form is ______. f dDG0 (D) DG0 (?) aDG0 (A) bDG0 (B) cDG0 (C) f f f f f 18.5

  24. - DG0 (reactants) S S = f 2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O (l) DG0 DG0 DG0 - [ ] [ + ] = rxn rxn rxn [ 12x–394.4 + 6x–237.2 ] – [ 2x124.5 ] = -_______ kJ = Is the reaction spontaneous at 25 0C? 12DG0 (CO2) 2DG0 (C6H6) f f 6DG0 (H2O) f DG0 (products) f What is the standard free-energy change for the following reaction at 25 0C? DG0 = -_____kJ ?0 ____________ 18.5

  25. Recap: Signs of Thermodynamic Values

  26. The specific heat (s) [most books use lower case c] of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius. Heat (q) absorbed or released: q =_______ 6.5

  27. How much heat is given off when an 869 g iron bar cools from 940C to 50C? s of Fe = 0.444 J/g •0C Dt = tfinal – tinitial = 50C – 940C = -890C q = msDt = 869 g x 0.444 J/g •0C x –890C = ______ J 6.5

  28. Constant-Pressure Calorimetry qsys = qwater + qcal + qrxn qsys = 0 qrxn = - (qwater + qcal) qwater = msDt qcal = CcalDt Reaction at Constant P DH = qrxn No heat enters or leaves! 6.5

  29. 6.5

  30. Phase Changes The boiling point is the temperature at which the __________________is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is _________. 11.8

  31. The critical temperature (Tc) is the temperature above which the gas cannot be made to ________, no matter how great the applied pressure. The critical pressure (Pc) is the minimum pressure that must be applied to bring about ________ at the _______ temperature. 11.8

  32. Can you find… The Triple Point? Critical pressure? Critical temperature? Where fusion occurs? Where vaporization occurs? Melting point (at 1 atm)? Boiling point(at 6 atm)? Where’s Waldo? Carbon Dioxide

  33. H2O (s) H2O (l) The_______ point of a solid or the ________ point of a liquid is the temperature at which the solid and liquid phases coexist in ________ _______ _______ 11.8

  34. H2O (s) H2O (g) Molar heat of sublimation (DHsub) is the energy required to _________ 1 mole of a solid. ___________ ___________ DHsub = DH??? + DH???? ( Hess’s Law) 11.8

  35. Molar heat of fusion (DHfus) is the energy required to ____1 mole of a solid substance. 11.8

  36. 11.8

  37. Sample Problem • How much heat is required to change 36 g of H2O from -8 deg C to 120 deg C? Step 1: Heat the ice Q=mcΔT Q = 36 g x _____ J/g deg C x 8 deg C = ___ J = ___ kJ Step 2: Convert the solid to liquid ΔH fusion Q = 2.0 mol x ______ kJ/mol = ____ kJ Step 3: Heat the liquid Q=mcΔT Q = 36g x 4.184 J/g deg C x 100 deg C = _____ J = ___ kJ

  38. Sample Problem • How much heat is required to change 36 g of H2O from -8 deg C to 120 deg C? Step 4: Convert the liquid to gas ΔH vaporization Q = 2.0 mol x ________kJ/mol = _____ kJ Step 5: Heat the gas Q=mcΔT Q = 36 g x ______ J/g deg C x 20 deg C = _______ J = ___ kJ Now, add all the steps together 0.59 kJ + 12 kJ + 15 kJ + 88 kJ + 1.5 kJ = 118 kJ

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