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Lattice energy

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Energy change upon the formation of one mole of ionic solid

Inchemistry, thelattice energy is theenergy change upon formation of onemole of a crystallineionic compound from its constituentions, which are assumed to initially be in thegaseous state. It is a measure of the cohesive forces that bind ionic solids. The size of the lattice energy is connected to many otherphysical properties includingsolubility,hardness, andvolatility. Since it generally cannot be measured directly, the lattice energy is usually deduced from experimental data via theBorn–Haber cycle.^[1]

Lattice energy and lattice enthalpy

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The concept of lattice energy was originally applied to the formation of compounds with structures likerocksalt (NaCl) andsphalerite (ZnS) where the ions occupy high-symmetry crystal lattice sites. In the case of NaCl, lattice energy is the energy change of the reaction

Na⁺ (g) + Cl⁻ (g) → NaCl (s)

which amounts to −786 kJ/mol.^[2]

Some chemistry textbooks^[3] as well as the widely usedCRC Handbook of Chemistry and Physics^[4] define lattice energy with the opposite sign, i.e. as the energy required to convert the crystal into infinitely separated gaseous ions invacuum, anendothermic process. Following this convention, the lattice energy of NaCl would be +786 kJ/mol. Both sign conventions are widely used.

The relationship between the lattice energy and the latticeenthalpy at pressure $P {\displaystyle P}$ is given by the following equation:

\Delta U_{lattice}=\Delta H_{lattice}-P\Delta V_{m}

where $\Delta U_{lattice}$ is the lattice energy (i.e., the molarinternal energy change), $\Delta H_{lattice}$ is the lattice enthalpy, and $\Delta V_{m}$ the change of molar volume due to the formation of the lattice. Since the molar volume of the solid is much smaller than that of the gases, $\Delta V_{m}<0$ . The formation of acrystal lattice from ions invacuum must lower the internal energy due to the net attractive forces involved, and so $\Delta U_{lattice}<0$ . The $-P\Delta V_{m}$ term is positive but is relatively small at low pressures, and so the value of the lattice enthalpy is also negative (andexothermic).

Theoretical treatments

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The lattice energy of anionic compound depends strongly upon the charges of the ions that comprise the solid, which must attract or repel one another viaCoulomb's Law. More subtly, the relative and absolute sizes of the ions influence $\Delta H_{lattice}$ .London dispersion forces also exist between ions and contribute to the lattice energy via polarization effects. For ionic compounds made of molecular cations and/or anions, there may also be ion-dipole and dipole-dipole interactions if either molecule has amolecular dipole moment. The theoretical treatments described below are focused on compounds made of atomic cations and anions, and neglect contributions to the internal energy of the lattice from thermalized lattice vibrations.

Born–Landé equation

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Main article:Born–Landé equation

In 1918^[5]Born andLandé proposed that the lattice energy could be derived from theelectric potential of the ionic lattice and a repulsivepotential energy term.^[2]

\Delta U_{lattice}=-{\frac {N_{A}Mz^{+}z^{-}e^{2}}{4\pi \varepsilon _{0}r_{0}}}\left(1-{\frac {1}{n}}\right),

where

N_A is theAvogadro constant;

M is theMadelung constant, relating to the geometry of the crystal;

z⁺ is the charge number of the cation;

z⁻ is the charge number of the anion;

e is theelementary charge, equal to1.6022×10⁻¹⁹ C;

ε₀ is thepermittivity of free space, equal to8.854×10⁻¹² C² J⁻¹ m⁻¹;

r₀ is the nearest-neighbor distance between ions; and

n is the Born exponent (a number between 5 and 12, determined experimentally by measuring thecompressibility of the solid, or derived theoretically).^[6]

TheBorn–Landé equation above shows that the lattice energy of a compound depends principally on two factors:

as the charges on the ions increase, the lattice energy increases (becomes more negative),
when ions are closer together the lattice energy increases (becomes more negative)

Barium oxide (BaO), for instance, which has the NaCl structure and therefore the same Madelung constant, has a bond radius of 275 picometers and a lattice energy of −3054 kJ/mol, while sodium chloride (NaCl) has a bond radius of 283 picometers and a lattice energy of −786 kJ/mol. The bond radii are similar but the charge numbers are not, with BaO having charge numbers of (+2,−2) and NaCl having (+1,−1); the Born–Landé equation predicts that the difference in charge numbers is the principal reason for the large difference in lattice energies.

Closely related to this widely used formula is theKapustinskii equation, which can be used as a simpler way of estimating lattice energies where high precision is not required.^[2]

Effect of polarization

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For certain ionic compounds, the calculation of the lattice energy requires the explicit inclusion of polarization effects.^[7] In these cases thepolarization energyE_pol associated with ions on polar lattice sites may be included in the Born–Haber cycle. As an example, one may consider the case ofiron-pyrite FeS₂. It has been shown that neglect of polarization led to a 15% difference between theory and experiment in the case of FeS₂, whereas including it reduced the error to 2%.^[8]

Representative lattice energies

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The following table presents a list of lattice energies for some common compounds as well as their structure type.

Compound	Experimental Lattice Energy^[1]	Structure type	Comment
LiF	−1030 kJ/mol	NaCl	difference vs. sodium chloride due to greatercharge/radius for both cation and anion
NaCl	−786 kJ/mol	NaCl	reference compound for NaCl lattice
NaBr	−747 kJ/mol	NaCl	weaker lattice vs. NaCl
NaI	−704 kJ/mol	NaCl	weaker lattice vs. NaBr, soluble in acetone
CsCl	−657 kJ/mol	CsCl	reference compound for CsCl lattice
CsBr	−632 kJ/mol	CsCl	trend vs CsCl like NaCl vs. NaBr
CsI	−600 kJ/mol	CsCl	trend vs CsCl like NaCl vs. NaI
MgO	−3795 kJ/mol	NaCl	M²⁺O²⁻ materials have high lattice energies vs. M⁺O⁻. MgO is insoluble in all solvents
CaO	−3414 kJ/mol	NaCl	M²⁺O²⁻ materials have high lattice energies vs. M⁺O⁻. CaO is insoluble in all solvents
SrO	−3217 kJ/mol	NaCl	M²⁺O²⁻ materials have high lattice energies vs. M⁺O⁻. SrO is insoluble in all solvents
MgF₂	−2922 kJ/mol	rutile	contrast with Mg²⁺O²⁻
TiO₂	−12150 kJ/mol	rutile	TiO₂ (rutile) and some other M⁴⁺(O²⁻)₂ compounds arerefractory materials

Notes

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References

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^^a ^bAtkins; et al. (2010).Shriver and Atkins' Inorganic Chemistry (Fifth ed.). New York: W. H. Freeman and Company.ISBN 978-1-4292-1820-7.
^^a ^b ^cDavid Arthur Johnson,Metals and Chemical Change, Open University, Royal Society of Chemistry, 2002,ISBN 0-85404-665-8
^Zumdahl, Steven S. (1997).Chemistry (4th ed.). Boston: Houghton Mifflin. pp. 357–358.ISBN 978-0-669-41794-4.
^Haynes, William M.; Lide, David R.; Bruno, Thomas J. (2017).CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data. Boca Raton, FL: CRC Press, Taylor & Francis Group. pp. 12–22 to 12–34.ISBN 9781498754293.
^I.D. Brown,The chemical Bond in Inorganic Chemistry, IUCr monographs in crystallography, Oxford University Press, 2002,ISBN 0-19-850870-0
^Cotton, F. Albert; Wilkinson, Geoffrey; (1966). Advanced Inorganic Chemistry (2d Edn.) New York:Wiley-Interscience.
^M. Birkholz (1995)."Crystal-field induced dipoles in heteropolar crystals I: Concept".Z. Phys. B.96 (3):325–332.Bibcode:1995ZPhyB..96..325B.CiteSeerX 10.1.1.424.5632.doi:10.1007/BF01313054.S2CID 122527743.
^M. Birkholz (1992)."The crystal energy of pyrite".J. Phys.: Condens. Matter.4 (29):6227–6240.Bibcode:1992JPCM....4.6227B.doi:10.1088/0953-8984/4/29/007.S2CID 250815717.

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