The table lists the lattice energies of some compounds.
| Compound | Lattice Energy (kJ/mol) |
| :------- | :---------------------- |
| LiF | -1,036 |
| LiCl | -853 |
| NaF | -923 |
| KF | -821 |
| NaCl | -786 |
Which statement about crystal lattice energy is best supported by the information in the table?
A. The lattice energy increases as cations get smaller, as shown by LiF and KF.
B. The lattice energy increases as the cations get larger, as shown by LiF and LiCl.
C. The lattice energy decreases as cations get smaller, as shown by NaCl and NaF.
D. The lattice energy decreases as the cations get smaller, as shown by NaF and KF.
Answer (1)
The lattice energy increases as cations get smaller, as shown by LiF and KF, because LiF has a more negative lattice energy than KF, and Li+ is smaller than K+.
The answer is: A
Explanation
Understanding Lattice Energy We are given a table of lattice energies for different compounds and asked to determine which statement about crystal lattice energy is best supported by the data. Lattice energy is the energy required to separate one mole of a solid ionic compound into gaseous ions. Generally, lattice energy increases with increasing charge and decreasing ionic size.
Analyzing the Statements Let's analyze the given statements:
Statement 1: The lattice energy increases as cations get smaller, as shown by LiF and KF.
LiF has a lattice energy of -1036 kJ/mol, and KF has a lattice energy of -821 kJ/mol. Li+ is smaller than K+. Since -1036 is more negative than -821, the lattice energy increases (becomes more negative) as the cation gets smaller. This statement is supported by the data.
Statement 2: The lattice energy increases as the cations get larger, as shown by LiF and LiCl.
LiF has a lattice energy of -1036 kJ/mol, and LiCl has a lattice energy of -853 kJ/mol. Here, the cation (Li+) is the same in both compounds. The anion changes (F- to Cl-). This statement is about the effect of cation size on lattice energy, but the cation is the same in both compounds, so this statement is not directly supported by comparing LiF and LiCl.
Statement 3: The lattice energy decreases as cations get smaller, as shown by NaCl and NaF.
NaCl has a lattice energy of -786 kJ/mol, and NaF has a lattice energy of -923 kJ/mol. Here, the cation (Na+) is the same in both compounds. The anion changes (Cl- to F-). This statement is about the effect of cation size on lattice energy, but the cation is the same in both compounds, so this statement is not directly supported by comparing NaCl and NaF.
Statement 4: The lattice energy decreases as the cations get smaller, as shown by NaF and KF.
NaF has a lattice energy of -923 kJ/mol, and KF has a lattice energy of -821 kJ/mol. Na+ is smaller than K+. Since -923 is more negative than -821, the lattice energy increases (becomes more negative) as the cation gets smaller. This statement contradicts the data.
Conclusion Based on the analysis, the first statement is best supported by the data. The lattice energy increases (becomes more negative) as the cation gets smaller, as shown by LiF and KF.
Examples
Understanding lattice energy helps predict the stability and properties of ionic compounds. For example, compounds with high lattice energies are generally less soluble in water. In the construction industry, materials like cement rely on the formation of strong ionic bonds, and thus high lattice energies, to create durable structures. By understanding these principles, engineers can select appropriate materials for different applications, ensuring structural integrity and longevity.
