Question

Which would you expect to have a higher melting point: sodium chloride, \(\mathrm{NaCl}\), or aluminum oxide, \(\mathrm{Al}_{2} \mathrm{O}_{3}\) ?

Short answer

Aluminum oxide (\(\mathrm{Al}_{2} \mathrm{O}_{3}\)) has a higher melting point than sodium chloride (NaCl) due to its higher lattice energy.

Step-by-step solution

Understand the Compounds

First, identify the compounds involved: Sodium chloride (NaCl) and Aluminum oxide (\(\mathrm{Al}_{2} \mathrm{O}_{3}\)). Recognize that both are ionic compounds.

Consider Lattice Energy Factors

Lattice energy is a key factor in determining the melting point of ionic compounds. It depends on the charges of ions and the distance between them. Higher charges and smaller distances increase lattice energy.

Analyze Ionic Charges

NaCl consists of \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\) ions. \(\mathrm{Al}_{2} \mathrm{O}_{3}\) consists of \(\mathrm{Al}^{3+}\) and \(\mathrm{O}^{2-}\) ions. Since \(\mathrm{Al}_{2} \mathrm{O}_{3}\) has ions with higher charges than NaCl, it generally has a larger lattice energy.

Consider Ion Sizes

Though the ionic radii also affect lattice energy, the more significant factor here is the charge magnitude. \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\) are larger than \(\mathrm{Al}^{3+}\) and \(\mathrm{O}^{2-}\), but the higher charge in \(\mathrm{Al}_{2} \mathrm{O}_{3}\) outweighs the size differences.

Conclude by Comparing Melting Points

Given the higher lattice energy due to increased ionic charges in \(\mathrm{Al}_{2} \mathrm{O}_{3}\), it is expected to have a higher melting point than NaCl.

Key concepts

Lattice Energy

Lattice energy is a crucial concept when discussing the stability and properties of ionic compounds. It is the energy required to separate one mole of an ionic solid into its gaseous ions. The magnitude of lattice energy provides insights into the strength of the bonds within the compound. Higher lattice energies indicate stronger attractions between the ions.

Factors affecting lattice energy include:

• **Ionic charges:** Higher charges on ions result in greater attraction and hence a higher lattice energy.
• **Interionic distances:** This is the center-to-center distance between the ions. Shorter distances increase lattice energy because it enhances the electrostatic force between the ions.

Understanding lattice energy helps in predicting various properties such as melting points and solubility of ionic compounds.

Ionic Charges

In ionic compounds, atoms transfer electrons to achieve stability, resulting in ions having positive or negative charges. The magnitude of these charges significantly influences properties like lattice energy and melting point.

For example:

• **Sodium chloride (NaCl):** Composed of \(\mathrm{Na}^{+}\) (single positive charge) and \(\mathrm{Cl}^{-}\) (single negative charge) ions.
• **Aluminum oxide(\mathrm{Al}_{2} \mathrm{O}_{3}):** Consists of \(\mathrm{Al}^{3+}\) (triple positive charge) and \(\mathrm{O}^{2-}\) (double negative charge) ions.

Higher charges in ions like those in \(\mathrm{Al}_{2} \mathrm{O}_{3}\) result in greater attractions between ions, contributing to a higher lattice energy compared to compounds like \(\mathrm{NaCl}\).

Melting Point

The melting point of a compound is the temperature at which it changes from a solid to a liquid. In ionic compounds, the melting point is heavily influenced by lattice energy. This is because a higher lattice energy means stronger forces holding the ions together, which requires more heat energy to overcome.

When comparing compounds, consider:

• **Charge of ions:** Higher ionic charges generally lead to higher melting points.
• **Size of ions:** Smaller ions can pack more tightly together, potentially increasing melting point, though charge difference is a more dominant factor.

Therefore, a compound with higher ionic charges, like \(\mathrm{Al}_{2} \mathrm{O}_{3}\), will generally have a higher melting point than one with lower charges, such as \(\mathrm{NaCl}\).

Sodium Chloride

Sodium chloride, also known as table salt, is a classic example of an ionic compound. It is formed by the transfer of an electron from a sodium atom to a chlorine atom, resulting in the formation of \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\) ions. These ions form a cubic crystal lattice structure.

Some key characteristics of sodium chloride include:

• **Moderate lattice energy** due to its singly charged ions.
• **Fairly high melting point** because of the strong electrostatic forces between ions, even though it is generally lower than many other ionic compounds with higher charges.
• **Solubility in water,** which allows it to dissolve readily due to the charges of its constituent ions attracting water molecules.

Sodium chloride's properties make it an excellent example to study basic ionic compound principles.

Aluminum Oxide

Aluminum oxide is a compound with a more complex ionic structure compared to sodium chloride. It is composed of aluminum ions with a \(3+\) charge and oxide ions with a \(2-\) charge. This results in a much higher lattice energy due to the larger charges.

Noteworthy aspects of aluminum oxide include:

• **Very high melting point:** Far exceeding that of sodium chloride, due to its strong ionic bonds and high lattice energy.
• **Low solubility in water:** Unlike sodium chloride, the strong attractions between the aluminum and oxide ions make it less likely to dissolve in water.
• **Usage in industry:** Often used as a refractory material because of its ability to withstand high temperatures.

These properties illustrate why aluminum oxide is often used in applications requiring durable and heat-resistant materials.