Bond dissociation enthalpies 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: 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)

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

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 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!!