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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).