1. Bond dissociation enthalpies
Thermochemistry 6
Bond dissociation enthalpies

2. 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)
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
Then do problem 8.20 (pg 323)
By Thursday!!