Thermochemistry 6

1. Thermochemistry 6 Bond dissociation enthalpies

2. 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

3. 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

4. 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)

5. 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

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.

7. 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)

8. 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)

9. 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!!