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)
• 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
• A simple dissociation equation:
H2 (g)
2 H (g)
• 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 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)
N2 (g) + 3 H2 (g)
2 NH3 (g)
• 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
N2 (g) + 3 H2 (g)
2 NH3 (g)
Bond Energy
•
•
Solution:
1 bond is broken for each H2 reacted
H
H
436 kJ/mol
•
One bond is broken for each N2 reacted
N
N
240 kJ/mol
•
3 bonds are formed for each NH3 produced.
N
H
390 kJ/mol
Horxn = [1 DN-N + 3 DH-H] - [6 DN-H]
Horxn = [(1 mol x 240 kJ/mol) + (3 mol x 436 kJ/mol)] - [(6 mol x 390 kJ/mol)]
Horxn = -792 kJ
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
• 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
• 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!!