Ch. 6

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Ch. 6 8.8 lecture notes

• THERMOCHEMISTRY!!!

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Video Exploding Gummy Bear!
Im a wimp, so youre not getting the live
version! ? Please dont try this at home! ?
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Terms to know

• System
• Surroundings
• Heat
• Work
• Energy

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Rvw Endothermic vs. Exothermic Reactions
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1st Law of Thermodynamics

• Nrg of universe is constant
• Nrg is conserved cannot be created nor destroyed
• Internal nrg (E) total nrg of the system.
• can be changed by flow of work (w), heat (q), or
  both ?E q w
• q gt 0
• q lt 0
• W gt 0
• W lt 0

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State functions

• Internal nrg (E) is a state function
• State functions depend only on initial and final
  state of the system
• Independent of path taken to get from start to
  finish.
• Example

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P-V work
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P-V work

• Work in terms of PV
• For expanding gas
• For compressed gas

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Enthalpy (H)

• Relates nrg P-V work H E PV
• At constant pressure (where only P-V work is
  allowed), ?H nrg flow as heat qp
• How is this so??? Derive the equation!

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Enthalpy of rxns (?H )

• ?H qp Hfinal Hinitial Hprods Hrxts
• Characteristics of enthalpy changes (?H )
• Enthalpy is an extensive property
• ?Hforward -?H reverse
• ?H depends on state of rxts and prods
• In terms of potential nrg, .
• Assume everything is at standard conditions (25
  oC, 1 atm) unless stated otherwise

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Practice problem 1

• 2H2O2 (l) ? 2H2O (g) O2 (g)
• ?Hrxn -196 kJ
• What is ?H if 5.00 g of H2O2 is decomposed?

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Calorimetry

• Measures heat flow by measuring . . .
• Uses a calorimeter (see picture)
• Constant-pressure calorimetry
• Patm remains constant
• Used to determine changes in enthalpy (heats of
  rxn) for rxns occuring in solution
• ? ?H qp
• ?

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Coffee Cup Calorimeter

• On the outside

• On the inside

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Constant-volume calorimetry A bomb calorimeter

• No work is done. Why?
• Weighed rxts placed inside rigid steel container
  (the bomb) and ignited
• ??H q w qV

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Heat capacity

• Heat capacity (C) nrg needed to raise temp of a
  body by.. Units ????
• C heat absorbed
• increase in temp
• Molar heat capacity heat capacity of one
  of substance Units ????
• Specific heat capacity (c) heat capacity of one
  of substance Units ????

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Some specific heats
Substance          Specific Heat at 25oC  
(J/goC) Lead                           0.128
Gold                            0.129
Silver                          0.235
Copper                       0.387
Iron                             0.4498 Carbon
(graphite)        0.711 Granite                
        0.803 Olive Oil                    
2.0 Ethyl alcohol                2.45
Water, (liquid)            4.1796
Notice anything about the first 5 substances?
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Heat transfer
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Heat transfer

• Q mc?T ?H if at constant P
• If a hot object touches a cooler object
• Tf Ti for hot object
• Tf Ti for cooler object
• And Tf of hot object Tf of cooler object
• All heat lost by one object is gained by the
  other, so
• Q1
• Tip to avoid dealing with negative

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Practice problem 2

• A 150.0 g sample of a metal at 75.0 oC is added
  to 150.0 g of water at 15.0 oC. The temperature
  of the water rises to 18.3 oC. Calculate the
  specific heat of the metal, assuming that all of
  the heat lost by the metal is gained by the
  water. (specific heat of water 4.18 J/g oC)

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Phase changes enthalpy
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Phase changes enthalpy

• How do you calculate enthalpy for segments BC and
  DE?
• How do you calculate enthalpy for segments AB, CD
  and EF?

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Phase change constants for water

• Specific heat (c) for ice 2.09 J/goC
• Specific heat (c) for water 4.18 J/goC
• Specific heat (c) for vapor 1.84 J/goC
• ?Hfus Heat of fusion 6.01 kJ/mol
• ?Hvap Heat of vaporization 40.67 kJ/mol

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Phase change calorimetry
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Practice Problems

• Do problems on Calorimetry wkhst

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Pre-assessment Lewis Structures

• On your white board, draw the Lewis structures
  for the following molecules
• H2O
• CO2
• HCN

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Bond nrg enthalpy of rxns

• Reactants break bonds and new bonds form to make
  products
• Breaking bonds is.
• Forming bonds is.
• Bond enthalpy (D)
• As bond strength increases, bond enthalpy.
• Use bond enthalpies to calculate ?H without
  having to know ?Hfs for all species

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More on bond enthalpies

• Need to know Lewis structures for compounds.
  Just seeing N2H4 doesnt help us calculate the
  bond enthalpy for it. We need to know HOW it
  bonds

So, there are N-H bonds and N-N bond
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• ?Hrxn S (D of bonds broken) - S (D of bonds
  formed)
• NOTE Sum of RXTS Sum of PRODUCTS!!!
• Multiply bond enthalpy by number of bonds
• If ?Hrxn gt 0, nrg to break bonds is
  nrg released when new bonds form
• Vice-versa if ?Hrxn lt 0.

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Bond enthalpy, bond length and bond strength
CC CC C ? C
Bond length (Å) 1.54 1.34 1.20
Bond enthalpy (kJ/mol) 348 614 839
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Interpreting the chart

• As bond length ?, bond enthalpy
• As bond length ?, of bonds
• As of bonds ?, bond enthalpy
• As bond strength ?, bond enthalpy
• Therefore
• As bond length ?, bond strength
• As of bonds ?, bond strength

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Bond enthalpies for practice probs

• Bond Bond enthalpy (kJ/mol)
• CC 347
• CC 614
• C ? C 839
• HH 432
• N N 160
• N H 391

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• Bond Bond enthalpy (kJ/mol)
• N ? N 941
• Cl Cl 239
• C H 413
• H Cl 427
• C Cl 339

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Practice problem 3

• Calculate ?Hrxn for each of the following
• Cl2 CH4 ? CH3Cl HCl
• N2H4 ? N2 2 H2
• C2H4 HCl ? C2H3Cl H2

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Practice Problem 4

• The standard enthalpy of formation of NH3 is -46
  kJ/mol. Use this information as well as the
  balanced equation below to estimate the N-H bond
  energy. Compare your result with the value in
  previous table.
• N2 (g) 3H2 (g) ? 2NH3 (g)