CHEM 2060 Lecture 13: Ionic Bonding L13-1

PART THREE: Ionic Bonding In Molecules And Solids So far we have defined several energies of interaction between charged particles.

- Ionization Energy (or Ionization Potential) - Electron Affinity - Coulombic (Electrostatic) Attraction & Repulsion.

We now use these to look at further bonding properties:

Mostly solids Why are solids important? Catalysts Ad & Ab – Sorbents Lasers Fibre Optics Magnetic Memories Optical Switching (Computers) Batteries Fluorescent Lights Superconductors LED’s ……….

CHEM 2060 Lecture 13: Ionic Bonding L13-2

One of the simplest ionic solids is sodium chloride (NaCl)…Rock Salt, Halite

Various depictions of the Rock Salt structure.

CHEM 2060 Lecture 13: Ionic Bonding L13-3

Discrete NaCl molecules can exist in the gas phase in the lab…but not in nature.

NaCl(g) → Na(g) + Cl(g) Bond formation (gas to gas) ΔH for this reaction is called the Bond Dissociation Enthalpy (409 kJ mol-1). We can judge the stability of ionic solids, whether they form or not, by looking at the free energy change for:

M+(g) + X-(g) → MX(s) Lattice formation (gas to solid) ΔG = ΔH - TΔS

• If ΔG is –ve then the reaction is spontaneous (i.e., favorable). • Note: The process of lattice formation (i.e., to make a solid) is very

exothermic at room temperature (ΔS may be neglected). • We will use ΔH (lattice enthalpy) exclusively and ignore entropy.

CHEM 2060 Lecture 13: Ionic Bonding L13-4

Born Haber Cycle

ΔHf° is the enthalpy of formation of NaCl (from Na and Cl under STP conditions to NaCl under STP conditions). U is the lattice enthalpy of NaCl (from gas phase ions to an ionic solid).

Following the Born Haber cycle, we can use experimentally determined values of ΔHf°, IENa, EACl, sublimation enthalpy of Na, bond dissociation enthalpy of Cl2, to find the experimental (i.e., measured) lattice enthalpy, U of NaCl. There is another (simpler) way of calculating lattice enthalpy…

CHEM 2060 Lecture 13: Ionic Bonding L13-5

Sodium Chloride Lattice: Cubic Each Na+ has 6 nearest neighbor Cl- Each Cl- has 6 nearest neighbor Na+ • The attractive energy between Na+ and its 6 nearest neighbor Cl- is offset by repulsion from 12 next nearest neighbor Na+. • We must sum these up.

First 6 nearest neighbour Na+-Cl- (Attractive) 6 rqq-

E ClNa ×⋅

= Second 12 next nearest neighbor Na+-Na+

(Repulsive)

12 2r

qq E NaNa ×⋅+

=

Third 8 next nearest neighbour Na-Cl (Attractive)

(and so on … for ever) 83rqq-

E ClNa ×=

Cl

Cl

Cl

Cl

Na r

Na

Na

Na2r3r

CHEM 2060 Lecture 13: Ionic Bonding L13-6

This is a “conditionally” convergent series. The sum of all the attractive and repulsive terms can be lumped together.

€

E = -q2

r ⋅ A

A is called the Madelung Constant. For NaCl and other cubic structures, A = 1.74756. • NOTE: In this case, the Coulombic term is overall attractive. QUESTION: Where is the repulsion to stop the solid collapsing? Born-Meyer repulsion ER = be-ar

(Recall: van der Waals repulsion!!!) …again, due to overlap of electron clouds.

b is a constant related to compressibility of solid.

CHEM 2060 Lecture 13: Ionic Bonding L13-7

overall we have (a common value for a is 2.899)

r

Coulombic Attraction

Born Meyer Repulsion

r(-Aq q21

be-ar

(dE =dr 0

E

DIST

CHEM 2060 Lecture 13: Ionic Bonding L13-8

We are almost ready to determine whether bonding in NaCl is ionic … (i.e., whether can it be described by Madelung, Coulomb, Born and Meyer). • How?

1 We can Calculate the Lattice Energy, U. 2 We can Measure the Lattice Energy, U.

1 Calculation gives U from ionic bonding only. 2 Measured gives the real value of U. Lattice Energies (actually, Enthalpies) [Def] Lattice energy, U, is the energy released when 1 mole of a substance is formed from its gas phase ions. e.g. Na+(g) + Cl-(g) → NaCl(s) ΔHlat ≈ U First we will determine the lattice energy from experimental values.

CHEM 2060 Lecture 13: Ionic Bonding L13-9

Born Haber Cycle If we go round this cycle we must expend no energy (overall).