Binary Ionic

Compounds

Chapter 8 Part II

Energetics of Ionic Bonding

• There must be a third piece to the puzzle.

• What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.

Energetics of Ionic Bonding

Lattice Energy • This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions.

• The energy associated with electrostatic interactions is governed by Coulomb’s law:

F = K Q1Q2

d

Lattice Energy • Lattice energy, then, increases with the charge on the ions.

• It also increases with decreasing size of ions.

Energetics of Ionic Bonding

By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process.

Energetics of Ionic Bonding

Energetics of Ionic Bonding

• These phenomena also helps explain the “octet rule.”

Energetics of Ionic Bonding

• Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.

• Bond-dissociation energy is the energy required to break one mole of a particular type of covalent bond in a gas-phase compound.

• Energies of some bonds can differ from compound to compound, so we use an average bond energy.

Bond Energy

Bond Energy

The H—H bond

energy is precisely

known …

… while the O—H bond

energies for the two bonds

in H2O are different.

Average Bond Enthalpies

• This table lists the average bond enthalpies for many different types of bonds.

• Average bond enthalpies are positive, because bond breaking is an endothermic process.

Average Bond Enthalpies NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C—H bonds in methane, CH4, will be a bit different than the

C—H bond in chloroform, CHCl3.

Average Bond Enthalpies

Enthalpies of Reaction • Yet another way to estimate H

for a reaction is to compare the bond enthalpies of bonds broken to the bond enthalpies of the new bonds formed.

• In other words, Hrxn = (bond enthalpies of bonds broken)

(bond enthalpies of bonds formed)

For the reaction N2(g) + 2 H2(g) N2H4(g) to occur …

Calculations Involving Bond Energies

… plus 2(436 kJ),

to break bonds of

the reactants.

… we must supply

946 kJ …

When the bonds of the

product form, 163 kJ plus

4(389 kJ) of energy is

liberated.

∆ H = (+946 kJ) + 2(+436 kJ) + (–163 kJ) + 4(–389 kJ)

Enthalpies of Reaction CH4(g) + Cl2(g) CH3Cl(g) + HCl(g)

In this example, one C—H bond and one Cl—Cl bond are broken; one C—Cl and one H—Cl bond are formed.

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Bond

Average H/mol

Bond

Average H/mol

C-H 413 Cl-Cl 242

H-Cl 431 C-Cl 328

C-C 348 C=C 614

Hrxn ≈ (Hbonds broken) - (Hbonds

formed) Hrxn ≈ [(1(413) + 1(242)] –

[1(328) + 1(431)] Hrxn ≈ -104 kJ/mol