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Thermochemistry 6. Bond dissociation enthalpies. Dissociate. dis·so·ci·ate  [di sṓshee àyt] ( past dis·so·ci·at·ed , past participle dis·so·ci·at·ed , present participle dis·so·ci·at·ing , 3rd person present singular dis·so·ci·ates )

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thermochemistry 6

Thermochemistry 6

Bond dissociation enthalpies

dissociate
Dissociate

dis·so·ci·ate [di sṓshee àyt]

  • (pastdis·so·ci·at·ed, past participledis·so·ci·at·ed, present participledis·so·ci·at·ing, 3rd person present singulardis·so·ci·ates)
  • vti CHEMISTRY split something into simpler parts:to cause the molecules of a compound to break down into simpler molecules, atoms, or ions usually in a reversible reaction, or to break down in this way
bond dissociation enthalpies
Bond dissociation enthalpies
  • A simple dissociation equation:
  • A hydrogen molecule has dissociated into two hydrogen atoms
  • A H—H bond has been broken
  • Breaking a bond takes energy, like breaking a pencil
  • This energy is the dissociation energy
bond dissociation enthalpies1
Bond dissociation enthalpies
  • The bond dissociation enthalpy; the enthalpy changes, ΔHo, for the corresponding bond-breaking reactions
  • ΔHo = D = bond dissociation energy
  • Always positive, it takes energy to break a bond
  • ΔHorxn = D(bonds broken) – D(bonds formed)
slide5
Use data in table 7.1 (page 246) to find an approximate ΔHo (in kJ) for the production of ammonia by the Haber process
  • Solution:
  • 3 bonds are formed for each NH3 produced.
  • 1 bond is broken for each H2 reacted
  • One bond is broken for each N2 reacted
slide6
Solution:
  • 1 bond is broken for each H2 reacted
  • One bond is broken for each N2 reacted
  • 3 bonds are formed for each NH3 produced.
problem 8 18
Problem 8.18
  • Use data in table 7.1 to calculate an approximate ΔHo (in kJ) for the industrial synthesis of ethyl alcohol from ethylene

C2H4(g) + H2O (g) C2H5OH (g)

problem 8 19
Problem 8.19
  • Use the data in table 7.1 to calculate an approximate ΔHo (in kJ) for the synthesis of hydrazine from ammonia

2 NH3 (g) + Cl2 (g)  N2H4(g) + 2 HCl (g)

homework
Homework
  • Read through section 8.12:

Fossil fuels, fuel efficiency, and heats of combustion on pages 321- 323

  • Then do problem 8.20 (pg 323)
  • By Thursday!!
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