Lattice Energies in Ionic Solids

8.3 Lattice Energies in Ionic Solids

Learning Objectives

To understand the relationship between the lattice energy and physical properties of an ionic compound.
To use the Born–Haber cycle to calculate lattice energies.

Recall that the reaction of a metal with a nonmetal usually produces an ionic compound; that is, electrons are transferred from the metal (the reductant) to the nonmetal (the oxidant). Ionic compounds are usually rigid, brittle, crystalline substances with flat surfaces that intersect at characteristic angles. They are not easily deformed, and they melt at relatively high temperatures. NaCl, for example, melts at 801°C. These properties result from the regular arrangement of the ions in the crystalline lattice and from the strong electrostatic attractive forces between ions with opposite charges.

While Equation 8.1 has demonstrated that the formation of ion pairs from isolated ions releases large amounts of energy, even more energy is released when these ion pairs condense to form an ordered three-dimensional array . In such an arrangement each cation in the lattice is surrounded by more than one anion (typically four, six, or eight) and vice versa, so it is more stable than a system consisting of separate pairs of ions, in which there is only one cation–anion interaction in each pair. Note that r₀ may differ between the gas-phase dimer and the lattice.

Note the Pattern

An ionic lattice is more stable than a system consisting of separate ion pairs.

Calculating Lattice Energies

The lattice energy of nearly any ionic solid can be calculated rather accurately using a modified form of Equation 8.1:

Equation 8.4

U = - k' \frac{Q_{1} Q_{2}}{r_{0}}, where U > 0

U, which is always a positive number, represents the amount of energy required to dissociate 1 mol of an ionic solid into the gaseous ions. If we assume that ΔV = 0, then the lattice energy, U, is approximately equal to the change in enthalpy, ΔH :

Equation 8.5

MX(s) \to M^{+ n} (g) + X^{−n} (g) Δ H = U

As before, Q₁ and Q₂ are the charges on the ions and r₀ is the internuclear distance. We see from Equation 8.4 that lattice energy is directly related to the product of the ion charges and inversely related to the internuclear distance. The value of the constant k' depends on the specific arrangement of ions in the solid lattice and their valence electron configurations. Representative values for calculated lattice energies, which range from about 600 to 10,000 kJ/mol, are listed in Table 8.1 "Representative Calculated Lattice Energies". Energies of this magnitude can be decisive in determining the chemistry of the elements.

Table 8.1 Representative Calculated Lattice Energies

Substance	U (kJ/mol)
NaI	682
CaI₂	1971
MgI₂	2293
NaOH	887
Na₂O	2481
NaNO₃	755
Ca₃(PO₄)₂	10,602
CaCO₃	2804

Source: Data from CRC Handbook of Chemistry and Physics (2004).

Because the lattice energy depends on the product of the charges of the ions, a salt having a metal cation with a +2 charge (M²⁺) and a nonmetal anion with a −2 charge (X²⁻) will have a lattice energy four times greater than one with M⁺ and X⁻, assuming the ions are of comparable size (and have similar internuclear distances). For example, the calculated value of U for NaF is 910 kJ/mol, whereas U for MgO (containing Mg²⁺ and O²⁻ ions) is 3795 kJ/mol.

Because lattice energy is inversely related to the internuclear distance, it is also inversely proportional to the size of the ions. This effect is illustrated in Figure 8.2 "A Plot of Lattice Energy versus the Identity of the Halide for the Lithium, Sodium, and Potassium Halides", which shows that lattice energy decreases for the series LiX, NaX, and KX as the radius of X⁻ increases. Because r₀ in Equation 8.4 is the sum of the ionic radii of the cation and the anion (r₀ = r⁺ + r⁻), r₀ increases as the cation becomes larger in the series, so the magnitude of U decreases. A similar effect is seen when the anion becomes larger in a series of compounds with the same cation.

Figure 8.2 A Plot of Lattice Energy versus the Identity of the Halide for the Lithium, Sodium, and Potassium Halides

Because the ionic radii of the cations decrease in the order K⁺ > Na⁺ > Li⁺ for a given halide ion, the lattice energy decreases smoothly from Li⁺ to K⁺. Conversely, for a given alkali metal ion, the fluoride salt always has the highest lattice energy and the iodide salt the lowest.

Note the Pattern

Lattice energies are highest for substances with small, highly charged ions.

Example 2

Arrange GaP, BaS, CaO, and RbCl in order of increasing lattice energy.

Given: four compounds

Asked for: order of increasing lattice energy

Strategy:

Using Equation 8.4, predict the order of the lattice energies based on the charges on the ions. For compounds with ions with the same charge, use the relative sizes of the ions to make this prediction.

Solution:

The compound GaP, which is used in semiconductor electronics, contains Ga³⁺ and P³⁻ ions; the compound BaS contains Ba²⁺ and S²⁻ ions; the compound CaO contains Ca²⁺ and O²⁻ ions; and the compound RbCl has Rb⁺ and Cl⁻ ions. We know from Equation 8.4 that lattice energy is directly proportional to the product of the ionic charges. Consequently, we expect RbCl, with a (−1)(+1) term in the numerator, to have the lowest lattice energy, and GaP, with a (+3)(−3) term, the highest. To decide whether BaS or CaO has the greater lattice energy, we need to consider the relative sizes of the ions because both compounds contain a +2 metal ion and a −2 chalcogenide ion. Because Ba²⁺ lies below Ca²⁺ in the periodic table, Ba²⁺ is larger than Ca²⁺. Similarly, S²⁻ is larger than O²⁻. Because the cation and the anion in BaS are both larger than the corresponding ions in CaO, the internuclear distance is greater in BaS and its lattice energy will be lower than that of CaO. The order of increasing lattice energy is RbCl < BaS < CaO < GaP.

Exercise

Arrange InAs, KBr, LiCl, SrSe, and ZnS in order of decreasing lattice energy.

Answer: InAs > ZnS > SrSe > LiCl > KBr

The Relationship between Lattice Energies and Physical Properties

The magnitude of the forces that hold an ionic substance together has a dramatic effect on many of its properties. The melting pointThe temperature at which the individual ions in a lattice or the individual molecules in a covalent compound have enough kinetic energy to overcome the attractive forces that hold them together in the solid., for example, is the temperature at which the individual ions have enough kinetic energy to overcome the attractive forces that hold them in place. At the melting point, the ions can move freely, and the substance becomes a liquid. Thus melting points vary with lattice energies for ionic substances that have similar structures. The melting points of the sodium halides (Figure 8.3 "A Plot of Melting Point versus the Identity of the Halide for the Sodium Halides"), for example, decrease smoothly from NaF to NaI, following the same trend as seen for their lattice energies (Figure 8.2 "A Plot of Lattice Energy versus the Identity of the Halide for the Lithium, Sodium, and Potassium Halides"). Similarly, the melting point of MgO is 2825°C, compared with 996°C for NaF, reflecting the higher lattice energies associated with higher charges on the ions. In fact, because of its high melting point, MgO is used as an electrical insulator in heating elements for electric stoves.

Figure 8.3 A Plot of Melting Point versus the Identity of the Halide for the Sodium Halides

The melting points follow the same trend as the magnitude of the lattice energies in Figure 8.2 "A Plot of Lattice Energy versus the Identity of the Halide for the Lithium, Sodium, and Potassium Halides".

The hardnessThe resistance of ionic materials to scratching or abrasion. of ionic materials—that is, their resistance to scratching or abrasion—is also related to their lattice energies. Hardness is directly related to how tightly the ions are held together electrostatically, which, as we saw, is also reflected in the lattice energy. As an example, MgO is harder than NaF, which is consistent with its higher lattice energy.

In addition to determining melting point and hardness, lattice energies affect the solubilities of ionic substances in water. In general, the higher the lattice energy, the less soluble a compound is in water. For example, the solubility of NaF in water at 25°C is 4.13 g/100 mL, but under the same conditions, the solubility of MgO is only 0.65 mg/100 mL, meaning that it is essentially insoluble.

Note the Pattern

High lattice energies lead to hard, insoluble compounds with high melting points.

The Born–Haber Cycle

In principle, lattice energies could be measured by combining gaseous cations and anions to form an ionic solid and then measuring the heat evolved. Unfortunately, measurable quantities of gaseous ions have never been obtained under conditions where heat flow can be measured. Instead, lattice energies are found using the experimentally determined enthalpy changes for other chemical processes, Hess’s law, and a thermochemical cycle called the Born–Haber cycleA thermochemical cycle that decribes the process in which an ionic solid is conceptually formed from its component elements in a stepwise manner.. Developed by Max Born and Fritz Haber in 1919, the Born–Haber cycle describes a process in which an ionic solid is conceptually formed from its component elements in a stepwise manner.

Let’s use the Born–Haber cycle to determine the lattice energy of CsF(s). CsF is a nearly ideal ionic compound because Cs is the least electronegative element that is not radioactive and F is the most electronegative element. To construct a thermochemical cycle for the formation of CsF, we need to know its enthalpy of formation, ΔH_f, which is defined by the following chemical reaction:

Equation 8.6

Cs (s) + \frac{1}{2} F_{2} (g) \to CsF (s)

Because enthalpy is a state function, the overall ΔH for a series of reactions is the sum of the values of ΔH for the individual reactions. We can therefore use a thermochemical cycle to determine the enthalpy change that accompanies the formation of solid CsF from the parent elements (not ions).

The Born–Haber cycle for calculating the lattice energy of cesium fluoride is shown in Figure 8.4 "The Born–Haber Cycle Illustrating the Enthalpy Changes Involved in the Formation of Solid Cesium Fluoride from Its Elements". This particular cycle consists of six reactions, Equation 8.6 plus the following five reactions:

Figure 8.4 The Born–Haber Cycle Illustrating the Enthalpy Changes Involved in the Formation of Solid Cesium Fluoride from Its Elements

Reaction 1

Cs(s) \to Cs(g) Δ H_{1} = Δ H_{sub} = 76 .5 kJ/mol

This equation describes the sublimationThe conversion of a solid directly to a gas (without an intervening liquid phase). of elemental cesium, the conversion of the solid directly to a gas. The accompanying enthalpy change is called the enthalpy of sublimation(ΔH_sub)The enthalpy change that accompanies the conversion of a solid directly to a gas. (Table 8.2 "Selected Enthalpies of Sublimation at 298 K") and is always positive because energy is required to sublime a solid.

Table 8.2 Selected Enthalpies of Sublimation at 298 K

Substance	ΔH_sub (kJ/mol)
Li	159.3
Na	107.5
K	89.0
Rb	80.9
Cs	76.5
Be	324.0
Mg	147.1
Ca	177.8
Sr	164.4
Ba	180.0

Source: Data from CRC Handbook of Chemistry and Physics (2004).

Reaction 2

Cs(g) \to {Cs}^{+} (g) + e^{–} Δ H_{2} = I_{1} = 375 .7 kJ/mol

This equation describes the ionization of cesium, so the enthalpy change is the first ionization energy of cesium. Recall from Section 7.3 "Energetics of Ion Formation " that energy is needed to ionize any neutral atom. Hence, regardless of the compound, the enthalpy change for this portion of the Born–Haber cycle is always positive.

Reaction 3

\frac{1}{2} F_{2} (g) \to F (g) Δ H_{3} = \frac{1}{2} D = 79.4 kJ/mol

This equation describes the dissociation of fluorine molecules into fluorine atoms, where D is the energy required for dissociation to occur (Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K"). We need to dissociate only $\frac{1}{2}$ mol of F₂(g) molecules to obtain 1 mol of F(g) atoms. The ΔH for this reaction, too, is always positive because energy is required to dissociate any stable diatomic molecule into the component atoms.

Table 8.3 Selected Bond Dissociation Enthalpies at 298 K

Substance	D (kJ/mol)
H₂(g)	436.0
N₂(g)	945.3
O₂(g)	498.4
F₂(g)	158.8
Cl₂(g)	242.6
Br₂(g)	192.8
I₂(g)	151.1

Source: Data from CRC Handbook of Chemistry and Physics (2004).

Reaction 4

{F(g) + e}^{-} \to F^{-} (g) Δ H_{4} = E A = –328 .2 kJ/mol

This equation describes the formation of a gaseous fluoride ion from a fluorine atom; the enthalpy change is the electron affinity of fluorine. Recall from Section 7.3 "Energetics of Ion Formation " that electron affinities can be positive, negative, or zero. In this case, ΔH is negative because of the highly negative electron affinity of fluorine.

Reaction 5

{Cs}^{+} {(g) + F}^{-} (g) \to CsF(s) Δ H_{5} = – U

This equation describes the formation of the ionic solid from the gaseous ions. Because Reaction 5 is the reverse of the equation used to define lattice energy and U is defined to be a positive number, ΔH₅ is always negative, as it should be in a step that forms bonds.

If the enthalpy of formation of CsF from the elements is known (ΔH_f = −553.5 kJ/mol at 298 K), then the thermochemical cycle shown in Figure 8.4 "The Born–Haber Cycle Illustrating the Enthalpy Changes Involved in the Formation of Solid Cesium Fluoride from Its Elements" has only one unknown, the quantity ΔH₅ = −U. From Hess’s law, we can write

Equation 8.7

ΔH_f = ΔH₁ + ΔH₂ + ΔH₃ + ΔH₄ + ΔH₅

We can rearrange Equation 8.7 to give

Equation 8.8

−ΔH₅ = ΔH₁ + ΔH₂ + ΔH₃ + ΔH₄ − ΔH_f

Substituting for the individual ΔHs, we obtain

Substituting the appropriate values into this equation gives

Equation 8.9

U = 76.5 kJ/mol + 375.7 kJ/mol + 79.4 kJ/mol + (−328.2 kJ/mole) − (−553.5 kJ/mol) = 756.9 kJ/mol

U is larger in magnitude than any of the other quantities in Equation 8.9. The process we have used to arrive at this value is summarized in Table 8.4 "Summary of Reactions in the Born–Haber Cycle for the Formation of CsF(s)".

Table 8.4 Summary of Reactions in the Born–Haber Cycle for the Formation of CsF(s)

	Reaction	Enthalpy Change (kJ/mol)
(1)	Cs(s) → Cs(g)	ΔH_sub = 76.5
(2)	Cs(g) → Cs⁺(g) + e⁻	I₁ = 375.7
(3)	½F₂(g) → F(g)	½D = 79.4
(4)	F(g) + e⁻ → F⁻(g)	EA = −328.2
(5)	Cs⁺(g) + F⁻(g) → CsF(s)	−U = −756.9
	Cs(s) + ½F₂(g) → CsF(s)	ΔH_f = −553.5

Predicting the Stability of Ionic Compounds

Equation 8.7 may be used as a tool for predicting which ionic compounds are likely to form from particular elements. As we have noted, ΔH₁ (ΔH_sub), ΔH₂ (I), and ΔH₃ (D) are always positive numbers, and ΔH₂ can be quite large. In contrast, ΔH₄ (EA) is comparatively small and can be positive, negative, or zero. Thus the first three terms in Equation 8.7 make the formation of an ionic substance energetically unfavorable, and the fourth term contributes little either way. The formation of an ionic compound will be exothermic (ΔH_f < 0) if and only if ΔH₅ (−U) is a large negative number. This means that lattice energy is the most important factor in determining the stability of an ionic compound. Another example is the formation of BaO:

Equation 8.10

Ba (s) + \frac{1}{2} O_{2} (g) \to BaO (s) Δ H = Δ H_{f}

whose Born–Haber cycle is compared with that for the formation of CsF in Figure 8.5 "Comparison of the Enthalpy Changes Involved in the Formation of Solid CsF and BaO from Their Elements".

Figure 8.5 Comparison of the Enthalpy Changes Involved in the Formation of Solid CsF and BaO from Their Elements

The lattice energy of BaO, with a dipositive cation and a dinegative anion, dominates the Born–Haber cycle.

Reaction 1

Ba(s) \to Ba(g) Δ H_{1} = Δ H_{sub} = 180 .0 kJ/mol

More than twice as much energy is required to sublime barium metal (180.0 kJ/mol) as is required to sublime cesium (76.5 kJ/mol).

Reaction 2

Ba(s) \to {Ba}^{2+} (g) + 2 e^{–} Δ H_{2} = I_{1} + I_{2} = 1468 .1 kJ/mol

Nearly four times the energy is needed to form Ba²⁺ ions (I₁ = 502.9 kJ/mol, I₂ = 965.2 kJ/mol, I₁ + I₂ = 1468.1 kJ/mol) as Cs⁺ ions (I₁ = 375.7 kJ/mol).

Reaction 3

\frac{1}{2} O_{2} (g) \to O (g) Δ H_{3} = \frac{1}{2} D = 249.2 kJ/mol

Because the bond energy of O₂(g) is 498.4 kJ/mol compared with 158.8 kJ/mol for F₂(g), more than three times the energy is needed to form oxygen atoms from O₂ molecules as is required to form fluorine atoms from F₂.

Reaction 4

O(g) + 2 e^{–} \to O^{2–} (g) Δ H_{4} = E A_{1} + E A_{2} = 603 kJ/mol

Forming gaseous oxide (O²⁻) ions is energetically unfavorable. Even though adding one electron to an oxygen atom is exothermic (EA₁ = −141 kJ/mol), adding a second electron to an O⁻(g) ion is energetically unfavorable (EA₂ = +744 kJ/mol)—so much so that the overall cost of forming O²⁻(g) from O(g) is energetically prohibitive (EA₁ + EA₂ = +603 kJ/mol).

If the first four terms in the Born–Haber cycle are all substantially more positive for BaO than for CsF, why does BaO even form? The answer is the formation of the ionic solid from the gaseous ions (Reaction 5):

Reaction 5

{Ba}^{2+} (g) + O^{2–} (g) \to BaO(s) Δ H_{5} = – U

Remember from Equation 8.4 that lattice energies are directly proportional to the product of the charges on the ions and inversely proportional to the internuclear distance. Although the internuclear distances are not significantly different for BaO and CsF (275 and 300 pm, respectively), the larger ionic charges in BaO produce a much higher lattice energy. Substituting values for BaO (ΔH_f = −548.0 kJ/mol) into the equation and solving for U gives

\begin{array}{l} U & = Δ H_{sub} (Ba) + [I_{1} (Ba) + I_{2} (Ba)] + \frac{1}{2} D (O_{2}) + [E A_{1} (O) + E A_{2} (O)] - Δ H_{f} (BaO) \\ = 180.0 kJ/mol + 1468 .1 kJ/mol + 249 .2 kJ/mol + 603 kJ/mol - (−5 48.0 kJ/mol) \\ = 3048 kJ/mol \end{array}

Thus U for BaO is slightly more than four times greater than U for CsF. The extra energy released when BaO forms from its ions more than compensates for the additional energy required to form Ba²⁺(g) and O²⁻(g) ions from Ba(s) and $\frac{1}{2} O_{2} (g) .$

If the formation of ionic lattices containing multiply charged ions is so energetically favorable, why does CsF contain Cs⁺ and F⁻ ions rather than Cs²⁺ and F²⁻ ions? If we assume that U for a Cs²⁺F²⁻ salt would be approximately the same as U for BaO, the formation of a lattice containing Cs²⁺ and F²⁻ ions would release 2291 kJ/mol (3048 kJ/mol − 756.9 kJ/mol) more energy than one containing Cs⁺ and F⁻ ions. To form the Cs²⁺ ion from Cs⁺, however, would require removing a 5p electron from a filled inner shell, which calls for a great deal of energy: I₂ = 2234.4 kJ/mol for Cs. Furthermore, forming an F²⁻ ion is expected to be even more energetically unfavorable than forming an O²⁻ ion. Not only is an electron being added to an already negatively charged ion, but because the F⁻ ion has a filled 2p subshell, the added electron would have to occupy an empty high-energy 3s orbital. Cesium fluoride, therefore, is not Cs²⁺F²⁻ because the energy cost of forming the doubly charged ions would be greater than the additional lattice energy that would be gained.

Note the Pattern

Lattice energy is usually the most important energy factor in determining the stability of an ionic compound.

Example 3

Use data from Figure 7.13 "Electron Affinities (in kJ/mol) of the ", Table 7.5 "Successive Ionization Energies (in kJ/mol) for the Elements in the Third Row of the Periodic Table", Table 8.2 "Selected Enthalpies of Sublimation at 298 K", Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K", and Chapter 25 "Appendix A: Standard Thermodynamic Quantities for Chemical Substances at 25°C" to calculate the lattice energy of MgH₂.

Given: chemical compound and data from figures and tables

Asked for: lattice energy

Strategy:

A Write a series of stepwise reactions for forming MgH₂ from its elements via the gaseous ions.

B Use Hess’s law and data from the specified figures and tables to calculate the lattice energy.

Solution:

A Hess’s law allows us to use a thermochemical cycle (the Born–Haber cycle) to calculate the lattice energy for a given compound. We begin by writing reactions in which we form the component ions from the elements in a stepwise manner and then assemble the ionic solid:

\begin{array}{l} (1) & Mg (s) & \to Mg (g) & Δ H_{1} & = Δ H_{sub} (Mg) \\ (2) & Mg (g) & \to {Mg}^{2 +} (g) + {2e}^{-} & Δ H_{2} & = I_{1} (Mg) + I_{2} (Mg) \\ (3) & H_{2} (g) & \to 2H (g) & Δ H_{3} & = D {(H}_{2}) \\ (4) & 2H (g) + {2e}^{-} & \to {2H}^{-} (g) & Δ H_{4} & = 2 E A (H) \\ (5) & {Mg}^{2 +} (g) + {2H}^{-} (g) & \to {MgH}_{2} (s) & Δ H_{5} & = −U \\ Mg (s) + H_{2} (g) & \to {MgH}_{2} (s) & Δ H & = Δ H_{f} \end{array}

B Table 7.5 "Successive Ionization Energies (in kJ/mol) for the Elements in the Third Row of the Periodic Table" lists the first and second ionization energies for the period 3 elements [I₁(Mg) = 737.7 kJ/mol, I₂(Mg) = 1450.7 kJ/mol]. First electron affinities for all elements are given in Figure 7.13 "Electron Affinities (in kJ/mol) of the " [EA(H) = −72.8 kJ/mol]. Table 8.2 "Selected Enthalpies of Sublimation at 298 K" lists selected enthalpies of sublimation [ΔH_sub(Mg) = 147.1 kJ/mol]. Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K" lists selected bond dissociation energies [D(H₂) = 436.0 kJ/mol]. Enthalpies of formation (ΔH_f = −75.3 kJ/mol for MgH₂) are listed in Chapter 25 "Appendix A: Standard Thermodynamic Quantities for Chemical Substances at 25°C". From Hess’s law, ΔH_f is equal to the sum of the enthalpy changes for Reactions 1–5:

\begin{array}{l} Δ H_{f} & = Δ H_{1} + Δ H_{2} + Δ H_{3} + Δ H_{4} + Δ H_{5} \\ = Δ H_{sub} (Mg) + [I_{1} (Mg) + I_{2} (Mg)] + D (H_{2}) + 2 E A (H) - U \\ - 75.3 kJ/mol & = 147.1 kJ/mol + (737.7 kJ/mol + 1450 .7 kJ/mol) \\ + 436.0 kJ/mol + 2 (−72.8 kJ/mol) - U \\ U & = 2701.2 kJ/mol \end{array}

For MgH₂, U = 2701.2 kJ/mol. Once again, lattice energy provides the driving force for forming this compound because ΔH₁, ΔH₂, ΔH₃ > 0. When solving this type of problem, be sure to write the chemical equation for each step and double-check that the enthalpy value used for each step has the correct sign for the reaction in the direction it is written.

Exercise

Use data from Figure 7.11 "First Ionization Energies of the ", Figure 7.13 "Electron Affinities (in kJ/mol) of the ", Table 8.2 "Selected Enthalpies of Sublimation at 298 K", Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K", and Chapter 25 "Appendix A: Standard Thermodynamic Quantities for Chemical Substances at 25°C" to calculate the lattice energy of Li₂O. Remember that the second electron affinity for oxygen [O⁻(g) + e⁻ → O²⁻(g)] is positive (+744 kJ/mol; see Equation 7.13).

Answer: 2809 kJ/mol

Summary

Ionic compounds have strong electrostatic attractions between oppositely charged ions in a regular array. The lattice energy (U) of an ionic substance is defined as the energy required to dissociate the solid into gaseous ions; U can be calculated from the charges on the ions, the arrangement of the ions in the solid, and the internuclear distance. Because U depends on the product of the ionic charges, substances with di- or tripositive cations and/or di- or trinegative anions tend to have higher lattice energies than their singly charged counterparts. Higher lattice energies typically result in higher melting points and increased hardness because more thermal energy is needed to overcome the forces that hold the ions together. Lattice energies cannot be measured directly but are obtained from a thermochemical cycle called the Born–Haber cycle, in which Hess’s law is used to calculate the lattice energy from the measured enthalpy of formation of the ionic compound, along with other thermochemical data. The Born–Haber cycle can be used to predict which ionic compounds are likely to form. Sublimation, the conversion of a solid directly to a gas, has an accompanying enthalpy change called the enthalpy of sublimation.

Key Takeaway

The lattice energy is usually the most important energy factor in determining the stability of an ionic compound.

Key Equation

Lattice energy

Equation 8.4: $U = −k' \frac{Q_{1} Q_{2}}{r_{0}}$

Conceptual Problems

If a great deal of energy is required to form gaseous ions, why do ionic compounds form at all?
What are the general physical characteristics of ionic compounds?
Ionic compounds consist of crystalline lattices rather than discrete ion pairs. Why?
What factors affect the magnitude of the lattice energy of an ionic compound? What is the relationship between ionic size and lattice energy?
Which would have the larger lattice energy—an ionic compound consisting of a large cation and a large anion or one consisting of a large anion and a small cation? Explain your answer and any assumptions you made.
How would the lattice energy of an ionic compound consisting of a monovalent cation and a divalent anion compare with the lattice energy of an ionic compound containing a monovalent cation and a monovalent anion, if the internuclear distance was the same in both compounds? Explain your answer.
Which would have the larger lattice energy—CrCl₂ or CrCl₃—assuming similar arrangements of ions in the lattice? Explain your answer.
Which cation in each pair would be expected to form a chloride salt with the larger lattice energy, assuming similar arrangements of ions in the lattice? Explain your reasoning.
1. Na⁺, Mg²⁺
2. Li⁺, Cs⁺
3. Cu⁺, Cu²⁺
Which cation in each pair would be expected to form an oxide with the higher melting point, assuming similar arrangements of ions in the lattice? Explain your reasoning.
1. Mg²⁺, Sr²⁺
2. Cs⁺, Ba²⁺
3. Fe²⁺, Fe³⁺
How can a thermochemical cycle be used to determine lattice energies? Which steps in such a cycle require an input of energy?
Although NaOH and CH₃OH have similar formulas and molecular masses, the compounds have radically different properties. One has a high melting point, and the other is a liquid at room temperature. Which compound is which and why?

Numerical Problems

Arrange SrO, PbS, and PrI₃ in order of decreasing lattice energy.
Compare BaO and MgO with respect to each of the following properties.
1. enthalpy of sublimation
2. ionization energy of the metal
3. lattice energy
4. enthalpy of formation
Use a thermochemical cycle and data from Figure 7.13 "Electron Affinities (in kJ/mol) of the ", Table 7.5 "Successive Ionization Energies (in kJ/mol) for the Elements in the Third Row of the Periodic Table", Table 8.2 "Selected Enthalpies of Sublimation at 298 K", Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K", and Chapter 25 "Appendix A: Standard Thermodynamic Quantities for Chemical Substances at 25°C" to calculate the lattice energy (U) of magnesium chloride (MgCl₂).
Would you expect the formation of SrO from its component elements to be exothermic or endothermic? Why or why not? How does the valence electron configuration of the component elements help you determine this?
Using the information in Problem 4 and Problem 5, predict whether CaO or MgCl₂ will have the higher melting point.
Use a thermochemical cycle and data from Table 8.2 "Selected Enthalpies of Sublimation at 298 K", Table 8.3 "Selected Bond Dissociation Enthalpies at 298 K", and Chapter 25 "Appendix A: Standard Thermodynamic Quantities for Chemical Substances at 25°C" to calculate the lattice energy of calcium oxide. The first and second ionization energies of calcium are 589.8 kJ/mol and 1145.4 kJ/mol.

Answers

Lattice energy is directly proportional to the product of the ionic charges and inversely proportional to the internuclear distance. Therefore, PrI₃ > SrO > PbS.
U = 2522.2 kJ/mol
Despite the fact that Mg²⁺ is smaller than Ca²⁺, the higher charge of O²⁻ versus Cl⁻ gives CaO a larger lattice energy than MgCl₂. Consequently, we expect CaO to have the higher melting point.