Question

In Chapter \(14,\) you learned that the bonding forces in ionic solids such as \(\mathrm{NaCl}\) are very strong, yet many ionic solids dissolve readily in water. Explain.

Short answer

Ionic solids like NaCl have strong electrostatic forces called lattice energy that hold their ions together. However, when NaCl is put into water, which is a polar solvent, the water molecules surround the ions and attract them through solvation. The solvation energy released can compensate for the energy required to break the ionic bonds, and if it is equal to or greater than the lattice energy, the ionic solid will dissolve in water. This is why many ionic solids dissolve readily in water despite their strong bonding forces.

Step-by-step solution

Introduction

First, let's understand the basic properties of ionic solids, such as NaCl, and water which help us explain the solubility of ionic solids in water. Ionic solids have a crystalline structure with oppositely charged ions held together by strong electrostatic forces, while water is a polar solvent with slightly positive Hydrogen atoms and slightly negative Oxygen atom.

Ionic Solids - Lattice Energy

In an ionic solid, the ions are held together by strong electrostatic forces called lattice energy. This energy is the amount of energy required to separate the ions in one mole of the solid into gas-phase ions. For example, in sodium chloride (NaCl), the positively charged sodium ions (Na+) are attracted to the negatively charged chloride ions (Cl-). The high lattice energy is the reason for the strong bonding forces in these solids.

Water - Polarity and Hydrogen Bonding

Water is a polar molecule due to the presence of electronegative Oxygen atom and electropositive Hydrogen atoms. The Oxygen atom attracts the electrons in the covalent bond, making it slightly negatively charged, while the Hydrogen atoms become slightly positively charged. Consequently, water molecules can form hydrogen bonds with each other and other polar molecules. This ability to form hydrogen bonds makes water an excellent solvent, especially for ionic compounds.

Dissolving Process

When ionic solids like NaCl are put into water, the water molecules surround the ions present in the solid. The slightly negatively charged Oxygen end of the water molecule attracts the positively charged sodium ions (Na+), while the slightly positively charged Hydrogen end of the water molecules attracts the negatively charged chloride ions (Cl-). This process is known as solvation.

Comparing Energy Changes

Dissolving ionic solids in water involves competing energy changes. The lattice energy of the ionic solid must be overcome to separate the ions. However, the energy released by solvation, known as solvation energy, can compensate for the energy required to break the ionic bonds. If the solvation energy is equal to or greater than the lattice energy, the ionic solid will dissolve in water. This is the reason many ionic solids dissolve readily in water, despite having strong bonding forces.

Key concepts

Lattice Energy

Lattice energy refers to the measure of cohesive forces that hold the ions together in an ionic crystal. It's the energy required to separate one mole of an ionic solid into its constituent ions in the gas phase. Consider sodium chloride (NaCl). Here, the lattice energy is significant due to the strong attraction between positively charged sodium ions (Na\(^+\)) and negatively charged chloride ions (Cl\(^{-}\)).

Some important aspects of lattice energy include:

• Magnitude: Greater lattice energy means stronger ionic bonds and less tendency to dissolve.
• Influence by Ion Size and Charge: Small ions with higher charges typically exhibit high lattice energy.

Therefore, the energy it takes to separate these ions is high, but it plays a crucial role in understanding solubility when compared to solvation energy.

Polar Solvents

Polar solvents like water are particularly effective at dissolving ionic compounds. Water's polar nature is due to the uneven distribution of electron density, creating a dipole moment with the oxygen carrying a slight negative charge and the hydrogen atoms carrying slight positive charges.

Factors making water an excellent polar solvent include:

• Polarity: The polar structure allows water to interact effectively with various substances.
• Hydrogen bonding: Water molecules form hydrogen bonds with both themselves and other polar molecules, facilitating dissolution.

Ultimately, these characteristics allow water to break the ionic bonds within a solid by surrounding and stabilizing the separated ions, promoting solubility.

Solvation Process

The solvation process is what occurs when ionic compounds dissolve in a solvent. This involves the attraction and association of solvent molecules with the ions of the solute, leading to separation and stabilization of the ions in the solution.

Key aspects of the solvation process include:

• Ionic Interaction: Water molecules orient themselves so that the positively charged end is near negative ions and the negative end is near positive ions.
• Solvation Energy: The energy released during this process, solvation energy, can counteract the lattice energy.

The ability of water to solvate ions helps overcome the electrostatic forces exerted by the lattice energy, allowing the ionic solid to dissolve. If the solvation energy compensates adequately for lattice energy, dissolution occurs efficiently.