Ch.8 - Basic Concepts of Chemical Bonding

Brown - Chemistry: The Central Science 14th Edition

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Brown 14th Edition

Ch.8 - Basic Concepts of Chemical Bonding

Problem 21c

Chapter 8, Problem 21c

(c) Would you expect salts like NaCl, which have singly charged ions, to have larger or smaller lattice energies compared to salts like CaO which are composed of doubly-charged ions?

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insert step 1> Consider the concept of lattice energy, which is the energy required to separate one mole of an ionic solid into its gaseous ions. It is influenced by the charges of the ions and the distance between them.

insert step 2> Recall that lattice energy is directly proportional to the product of the charges of the ions. This means that higher charges on the ions will result in a larger lattice energy.

insert step 3> Compare the charges of the ions in NaCl and CaO. NaCl is composed of Na^+ and Cl^- ions, each with a charge of ±1. CaO is composed of Ca^2+ and O^2- ions, each with a charge of ±2.

insert step 4> Analyze the effect of ion charges on lattice energy. Since CaO has ions with higher charges compared to NaCl, the lattice energy of CaO is expected to be larger.

insert step 5> Conclude that salts like CaO, with doubly-charged ions, generally have larger lattice energies compared to salts like NaCl, which have singly charged ions.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lattice Energy

Lattice energy is the amount of energy released when gaseous ions combine to form an ionic solid. It is a measure of the strength of the forces between the ions in an ionic compound. Higher lattice energy indicates stronger ionic bonds, which typically results in higher melting points and greater stability of the compound.

Charge of Ions

The charge of ions significantly influences lattice energy. Ions with higher charges create stronger electrostatic attractions between them, leading to greater lattice energy. For example, doubly charged ions like Ca²⁺ and O²⁻ will interact more strongly than singly charged ions like Na⁺ and Cl⁻, resulting in a higher lattice energy for compounds formed from doubly charged ions.

Ionic Size

The size of the ions also affects lattice energy. Smaller ions can get closer together, increasing the electrostatic attraction and thus the lattice energy. In contrast, larger ions have a greater distance between them, which can reduce the strength of the ionic bond. Therefore, the combination of ion charge and size is crucial in determining the overall lattice energy of a salt.

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Related Practice

Textbook Question

NaCl and KF have the same crystal structure. The only difference between the two is the distance that separates cations and anions. (a) The lattice energies of NaCl and KF are given in Table 8.1. Based on the lattice energies, would you expect the Na─Cl or the K─F distance to be longer?

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Textbook Question

NaCl and KF have the same crystal structure. The only difference between the two is the distance that separates cations and anions. (b) Use the ionic radii given in Figure 7.8 to estimate the Na─Cl and K─F distances.

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Textbook Question

(a) Is lattice energy usually endothermic or exothermic?

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Textbook Question

(b) Write the chemical equation that represents the process of lattice energy for the case of NaCl.

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Textbook Question

The substances NaF and CaO are isoelectronic (have the same number of valence electrons). (b) What are the charges of each of the anions in each compound?

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Textbook Question

Write the electron configuration for each of the following ions, and determine which ones possess noble-gas configurations: (a) Be2+, (b) Mn2+, (c) Cd2+, (d) Fe3+, (e) Tl+, (f) At-.

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