Thermochemistry 6
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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

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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)


Thermochemistry 6

  • 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


Thermochemistry 6

  • 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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