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Wize University Chemistry Textbook > Chemical Bonding

Lattice Energy of Ionic Bonds

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Lattice Energy

Lattice energy: is the energy required to break ionic bonds into cation anion pairs, or the amount of energy released by forming the ionic compound from the constituent ions.

MX(s)→M(g)n++X(g)n−\boxed{MX_{(s)} \rightarrow M^{n+}_{(g)} + X^{n-}_{(g)}}
The lattice energy for an ionic compound depends on the strength of the electrostatic interactions between the cation and anion which can be calculated using the equation below.

Elattice=kQ1Q2d\boxed{\text{E}_{lattice}=\text{k}\frac{\text{Q}_1\text{Q}_2}{\text{d}}}

Elattice is the lattice energy
k is a constant 8.99x109J m/C2
Q1 is the charge of ion 1
Q2 is the charge of ion 2
d is the bond length

Wize Tip
On an exam, it is very unlikely that you would see a question that asks you to calculate the lattice energy.

A more typical exam question will ask you to compare the lattice energies between two different ionic compounds and determine which of the ionic compounds has the higher lattice energy.

If an ionic bond has a higher lattice energy, it means that ionic bond is (stronger/weaker)
stronger
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Elattice=kQ1Q2d\text{E}_{lattice}=\text{k}\frac{\text{Q}_1\text{Q}_2}{\text{d}}

According to the equation for lattice energy, answer the following questions.

  1. If the ions that are forming the ionic bond have greater charges, will the lattice energy be higher or lower?
    Higher
  2. If the atomic radii of the atoms forming the ionic bond is larger, the bond length will be larger and the lattice energy will be larger or smaller?
    Smaller


Wize Tip
Charge is more important than size when ranking compounds based on their lattice energy.


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Example: Lattice Energy

Which of the following pairs of ionic compounds will have stronger ionic bonding (i.e. higher lattice energy)


a) LiF vs BeO

LiF is made up of Li+ and F-
BeO is made up of Be2+ and O2-.

Both ions in BeO have double the charge compared to LiF, and thus the ionic bond strength is roughly quadrupled for BeO vs LiF.
BeO has the higher lattice energy and is the stronger ionic bond!

b) LiF vs CsF

LiF is made up of Li+ and F-
CsF is made up of Cs+ and F-

Both compounds have ions with a +1 or -1 charge, so the charges are the same

However, the sizes of the ions are different.
Cs+ is much larger than Li+, and therefore, since Lattice Energy is inversely proportional to bond length, CsF will have a longer bond and smaller ionic bond energy.
Therefore, LiF has the higher lattice energy and is the stronger ionic bond!

c) LiF vs MgS

MgS is made up of Mg2+ and S2-
LiF is made up of Li+ and F-.

Since MgS is made up of ions with greater charges, it has the higher lattice energy and is the stronger bond!
Recall that ion charges are more important than bond length when it comes to lattice energy :)


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Example: Ranking Lattice Energy

Rank the following compounds in order of increasing lattice energy, RbCl, CaI2, BaSe.

RbCl → Rb+ + Cl-

(+1)(-1) = -1 smallest charges = smallest lattice energy

CaI2 → Ca2+ + 2I-

(+2)(-1) = -2

BaSe → Ba2+ + Se2-

(+2)(-2) = -4 largest charges = greatest lattice energy

RbCl < CaI2 < BaSe


Practice: Ionic Bond Strength

Consider two compounds: CaS and LiCl. Which ionic compound would have a larger Lattice Energy released upon forming their respective ionic compounds. Why?

Practice: Highest Lattice Energy

Choose the option below with the highest lattice energy.
Extra Practice
Lattice energy
Select the correct order in terms of decreasing lattice energy for the following compounds:
NaBr, GaP, LiCl, NaI, LiF and CaO.
Highest melting point
Ionic Bonds
For each of the following pairs of ionic compounds, determine which will have the stronger ionic bonds.
Intermolecular forces: Ionic Bond Strength
Consider two compounds: CaS and NaCl. Which ionic compound would have a larger Coulomb Energy released upon forming their respective ionic compounds. Why?
Ionic Bonding and Lattice Energy
Ionic Bonding and Lattice Energy
Ionic Compounds
Formed between oppositely charged species (cations and anions) and held together by electrostatic attraction
Lattice Energy: Strength
Which of the following has the highest lattice energy?
Highest melting point
Lattice Energy Example 2
Does KCl or MgO have the larger lattice energy?
Lattice Energy Example 1
Does KCl or KBr have the larger lattice energy?
KCl→\rightarrow K+ and Cl KBr → K+ and Br-
Intermolecular forces: Lattice energy
Which of the following has the lowest lattice energy?
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Lattice Energy Example 2
Does KCl or MgO have the larger lattice energy?
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Lattice Energy: Strength
Which of the following ionic solids has the highest lattice energy?
BeO CaSe MgS
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Does KCl or KBr have the larger lattice energy?
"The Periodic Table", image retrieved from Chemistry 2e (https://d3bxy9euw4e147.cloudfront.net/oscms-prodcms/media/documents/Chemistry2e-OP_QCzV58S.pdf) by Flowers, P et al., is licensed under CC BY.
Lattice Energy: Strength
Rank the following in order of increasing lattice energy
LiF, LiCl, Li2S
Intermolecular Forces: Lattice Energy
Which of the following compounds has the largest lattice energy?
Lattice Energy Practice Problem
Rank the following compounds in order of increasing lattice energy, RbCl, CaI2, BaSe.
Lattice Energy: Strength
Which of the following compounds has the highest lattice energy
Lattice Energy: Strength
Rank the following compounds in order of increasing lattice energy:
LiF, LiCl, Li2S
Lattice Energy: Strength
Choose the option below which has the compound with the largest lattice energy listed first
Intermolecular bonds: Highest Lattice Energy
Choose the option below with the highest lattice energy.
Lattice Energy
Which of these three species has the highest lattice energy?
MgO
NaCl
Intermolecular forces: Ionic Bonds
Which of these three species has the lowest lattice energy?
LiF
CaCl
Lattice Energy: Strength
Which of the following relationships is/are CORRECT when comparing lattice energies?
i. CaF2>BaF2i.\ CaF_2>BaF_2
ii. CsBr>RbBr>NaClii.\ CsBr>RbBr>NaCl
Lattice Energy: Strength
Which of these ionic compounds has the highest lattice energy?
Lattice Energy: Strength
Which of the following ionic compounds has the lowest lattice energy?
BaCl2
NaCl