Unit 4 Thermodynamics

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Unit 4 Thermodynamics

• By Elliot Prizant and Zak Abecassis

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Thermodynamics

• The study of the laws that govern the conversion
  of energy from one form to another, the direction
  in which heat will flow, and the availability of
  energy to do work.

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Specific Heat Capacity (C)

• This is the heat required to produce a given
  temperature change per gram of material.
• Specific heat capacity (C) Quantity of heat
  supplied
• (mass of
  object) (temperature change)
• C(J/g(K) ___q(J)__
• m(g) x ?T(K)

                                                                                   
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• The average specific heat capacity of a human
  body is approximately 3500 J.kg-1.K-1. A certain
  Japanese sumo wrestler has a body mass of 200 kg.
  What is the wrestler's heat capacity?
• The heat capacity of a body is obtained by
  multiplying its specific heat capacity by its
  mass in kilograms. In this case, the heat
  capacity of the wrestler is3500 (J.kg-1.K-1) x
  200 (kg) 700000 (7.0x105) J.K-1

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Heat Transferred (q)

• q mC?T Units mass in grams, C is the
  specific heat capacity in J/g(K), ?T is the
  temperature final minus initial in Kelvins.
• q means heat is transferred to a substance.
• -q means heat is transferred from the system q
  transferred into the surroundings

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Bond Energies

• q S Reactants Bond Energy S Products Bond
  Energy
• Potential energy of the elemental state is ZERO
• Breaking bonds requires energy to be added to the
  system
• Forming bonds requires energy to be released from
  the system

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Bond Energies

• DE (reactant bonds)-(product bonds)
• EX 2N02 H20 ?HNO3 HNO2
• DE (4 N-O 2 H-O) (5 N-O 2 N-H)
• DE (4(201 KJ/mol) 2(463 KJ/mol)) (5(201
  KJ/mol) 2(391 KJ/mol))
• DE -57 KJ/mol
• (note some N-O bonds are probably double)

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Energy and Changes of State

• Heat of Fusion (Hf) quantity of heat required to
  melt a substance
• Heat of Vaporization (Hv) quantity of heat
  required to vaporize a substance
• Takes more energy to vaporize than to melt (need
  to break more bonds to get gas)
• qmHf or qmHv Hfenthalpy of fusion
  Hventhalpy of vaporization units J/g, J/mol

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Energy and Changes of State
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Calorimetry

• mmetalc?T mwaterc?T
• mmetalc(Tf-Ti) mwaterc(Tf-Ti)
• c for water is 4.18 J/gC
• q reaction -(q water q bomb)

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Enthalpy

• Enthalpy (H) Heat transferred into or out of a
  system at a constant pressure.
• ?H S Product Potential Energy - S Reactant
  Potential Energy Units Joules (J) or
  Kilojoules (KJ).
• Negative ?H is Exothermic/Positive ?H is
  Endothermic
• Enthalpy change for a reaction ?Hºrxn S ?Hºf
  products - S ?Hºf reactants
• q n(?H) or q m(?H)

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Enthalpy

• DHrxnS DHproducts SDHreactants
• Ex C3H8 5 O2 3 CO2 4 H2O
• DHrxn3(-393.5 KJ/mol)4(-241.8 KJ/mol)
  (-103.8 KJ/mol)
• DHrxn-2043.91 KJ

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Hesss Law

• Hesss Law If a reaction is the sum of two or
  more other reactions, then ?H for the overall
  process must the sum of ?H values of the
  constituent reactions.
• Reversing an equation, causes the sign on ?H to
  change.
• Multiplying an equation, multiply ?H by same
  number

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Hesss Law

• PbS (s) 3/2 O2 ? PbO (s) SO2 (g)
• ?H -413.7 kJ
• PbO (s) C (s) ? Pb (s) CO (g)
• ?H 106.8 kJ
• PbS (s) C (s) 3/2 O2 (g) ? Pb (s) CO (g)
  SO2 (g) ?H - 325.3 kJ

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Entropy

• Entropy (S) A measure of randomness in a system
• In order of greatest to least random
  GasesgtLiquidsgtSolids.
• ?S S S (products( S S (reactants) Units
  J/K(mol)
• Increasing of moles of gas in reaction from
  reactants to products makes more entropy and vice
  versa
• Product-favored reactions have higher entropy and
  vice versa

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Entropy
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Spontaneity

• If a reaction is spontaneous, then the reaction
  is product favored.
• Is the reaction spontaneous? T ?H / ?S
• ?S lt 0
  ?S gt 0
• ?H gt 0 Never Spontaneous Maybe
  (at high temps)
• ?H lt 0 Maybe (at low temps) Always
  Spontaneous

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Gibbs Free Energy

• ?G ?H - T?S Units T is in Kelvin, S is in
  J/(molK)
• A reaction is SPONTANEOUS when ?G is NEGATIVE.
• A reaction is NOT SPONTANEOUS when ?G is
  POSITIVE.
• A reaction is at Equilibrium when ?G is zero.
• ?G ?G RT lnQ
• ?G -RT lnK (at equilibrium)
• R8.314 J/(molK) TKelvin
• K is the Thermodynamic Equilibrium Constant

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Gibbs Free Energy

• When ?G lt 0 and K gt 1 Products are favored
• When ?G 0 and K 1 It is at equilibrium
  (Rare)
• When ?G gt 0 and K lt 1 reactants are favored

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THE END
Good Luck Just remember you put the Thermo in
Thermodynamics

• Sadi Carnot (1796-1832) the "father" of
  thermodynamics