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General Chemistry

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Lattice Energy: Basic Concepts

CHEM-NMW7RB

The following are listed in order of increasing lattice energy:

$$NaCl<{ MgCl }_{ 2 }<{ AlCl }_{ 3 }$$

Recall that one way to define lattice energy is that it is the energy required to break ionic attractions in a solid to form gaseous ions. In other words, if AB is an ionic solid, the lattice energy is the enthalpy change for the following reaction:

$$AB\ (s)\rightarrow { A }^{ n+ }\ (g)\ +\ { B }^{ m- }\ (g)$$

Given this information, which of the statements below would explain this ordering?

Select ALL that apply.

A

Because the size of the cation increases going from ${ Na }^{ + }\rightarrow { Mg }^{ 2+ }\rightarrow { Al }^{ 3+ }$, the larger size of ${ Al }^{ 3+ }$ makes aluminum chloride have the largest lattice energy. Larger cations will be closer to the anions and require more energy to pull apart.

B

Because the size of the cation decreases going from ${ Na }^{ + }\rightarrow { Mg }^{ 2+ }\rightarrow { Al }^{ 3+ }$, the smaller size of ${ Al }^{ 3+ }$ makes aluminum chloride have the largest lattice energy. Smaller cations will be closer to the anions and require more energy to pull apart.

C

Because the charge of the cation increases going from ${ Na }^{ + }\rightarrow { Mg }^{ 2+ }\rightarrow { Al }^{ 3+ }$, the higher charge of ${ Al }^{ 3+ }$ makes aluminum chloride have the lowest lattice energy because of electrostatic repulsion between neighboring cations.

D

Because the charge of the cation increases going from ${ Na }^{ + }\rightarrow { Mg }^{ 2+ }\rightarrow { Al }^{ 3+ }$, the higher charge of ${ Al }^{ 3+ }$ makes aluminum chloride have the highest lattice energy because electrostatic attractions between cations and anions will be larger.