Question

For each pair, choose the compound with the larger lattice energy, and explain your choice: (a) \(\mathrm{CaO}\) or CaS (b) \(\mathrm{BaO}\) or \(\mathrm{SrO}\)

Short answer

The compounds with larger lattice energies are \(\text{CaO}\) for pair (a) and \(\text{SrO}\) for pair (b).

Step-by-step solution

Understanding Lattice Energy

Lattice energy is a measure of the strength of the forces between the ions in an ionic solid. It is affected by the charges on the ions and the distances between them. Higher charges and shorter distances result in larger lattice energy.

Compare the Charges of the Ions

Examine the charges of the ions in both pairs. For (a) \(\text{CaO}\) has \(\text{Ca}^{2+}\) and \(\text{O}^{2-}\). CaS has \(\text{Ca}^{2+}\) and \(\text{S}^{2-}\). For (b) \(\text{BaO}\) has \(\text{Ba}^{2+}\) and \(\text{O}^{2-}\). \(\text{SrO}\) has \(\text{Sr}^{2+}\) and \(\text{O}^{2-}\).

Compare the Sizes of the Ions

Smaller ions result in shorter distances between them, leading to a larger lattice energy. In (a), \(\text{S}^{2-}\) is larger than \(\text{O}^{2-}\), making \(\text{CaO}\) have a larger lattice energy than CaS. In (b), \(\text{Sr}^{2+}\) is smaller than \(\text{Ba}^{2+}\), making \(\text{SrO}\) have a larger lattice energy than \(\text{BaO}\).

Key concepts

ionic bonds

Ionic bonds are a fundamental concept in chemistry. They involve the transfer of electrons from one atom to another. This transfer creates two oppositely charged ions.
The positive ion (cation) and the negative ion (anion) attract each other through electrostatic forces.
This attraction forms the ionic bond. Ionic bonds usually occur between metals and nonmetals.
For example, in the exercise you are dealing with compounds like \(\text{CaO}\), \(\text{CaS}\), \(\text{BaO}\), and \(\text{SrO}\).All these compounds are formed through ionic bonds.

ionic compounds

When ionic bonds form, they create ionic compounds. These compounds are typically solid at room temperature and have high melting points and boiling points.
The strong electrostatic force between the oppositely charged ions makes these points very high.
For example, \(\text{CaO}\) and \(\text{CaS}\) mentioned in the exercise are both ionic compounds.
Moreover, ionic compounds tend to dissolve in water and conduct electricity in molten or dissolved state because the ions are free to move and carry an electrical charge once they are in solution or melted state.

ion size

Ion size, also known as ionic radius, impacts many properties of ionic compounds, particularly lattice energy.
Smaller ions can get closer together, which increases the strength of the attraction between them. This proximity leads to larger lattice energies.
In the given exercise, for pair (a), \(\text{O}^{2-}\) is smaller than \(\text{S}^{2-}\), so \(\text{CaO}\) has a larger lattice energy compared to \(\text{CaS}\).
Similarly, for pair (b), \(\text{Sr}^{2+}\) is smaller than \(\text{Ba}^{2+}\), making \(\text{SrO}\) have a larger lattice energy than \(\text{BaO}\).

charge of ions

The charge of ions is another critical factor in determining lattice energy. Higher charges result in stronger electrostatic forces between the ions.
For example, all mentioned compounds in the exercise contain ions with charges of \(\text{2+}\) and \(\text{2-}\), but it is essential to consider both the charge and size to determine the lattice energy.
More precisely, when combining smaller ions with high charges, the resulting lattice energy will be significantly higher than that of larger ions with the same charges.
This understanding helps explain why \(\text{CaO}\) has a larger lattice energy than \(\text{CaS}\) and why \(\text{SrO}\) has a larger lattice energy than \(\text{BaO}\).